metal roof as antenna?
Daniel Friedan
I'm going to be using a shortwave receiver (a SONY 2010) in a house
which has a metal roof and siding. I believe that the metal is not
aluminum, probably steel (a traditional house siding in downtown
Reykjavik).
I guess that an attic antenna (my previous plan) is unlikely to work
well under a metal roof.
Could I use the roof itself as an antenna?
thanks for any advice,
--
Dan Friedan
internet: fri...@physics.rutgers.edu
bitnet: friedan@ruthep
E. Michael Smith
>
>I'm going to be using a shortwave receiver (a SONY 2010) in a house
>which has a metal roof and siding. I believe that the metal is not
>aluminum, probably steel (a traditional house siding in downtown
>Reykjavik).
>
>I guess that an attic antenna (my previous plan) is unlikely to work
>well under a metal roof.
>
>Could I use the roof itself as an antenna?
Yes, you can. It may or may not work well, depending on far too many
things to worry about. Just clip onto it with an 'aligator' clip and
see what you get... I'd try cliping on at a corner opposite the direction
I was most interested in ... Think of it as being like 1/2 of a big
'bow tie' type antenna (of the kind that are sometimes used indoors on
UHF TV ...)
Also try any metal window frames that might be on the side of the house
you want to listen from ... and gutters and downspouts and...
If all else fails, string a wire from your window to something outside
and above the roof, like a tree or a nearby house.
Mike
--
E. Michael Smith e...@apple.COM
'Whatever you can do, or dream you can, begin it. Boldness has
genius, power and magic in it.' - Goethe
I am not responsible nor is anyone else. Everything is disclaimed.
sch...@auvax1.adelphi.edu
> In article <Jun.17.16.56....@central.rutgers.edu> fri...@central.rutgers.edu (Daniel Friedan) writes:
>>
>>I'm going to be using a shortwave receiver (a SONY 2010) in a house
>>which has a metal roof and siding. I believe that the metal is not
>>aluminum, probably steel (a traditional house siding in downtown
>>Reykjavik).
>>
>>Could I use the roof itself as an antenna?
>
> Yes, you can. It may or may not work well, depending on far too many
> things to worry about. Just clip onto it with an 'aligator' clip and
> see what you get... I'd try cliping on at a corner opposite the direction
> I was most interested in ... Think of it as being like 1/2 of a big
> 'bow tie' type antenna (of the kind that are sometimes used indoors on
> UHF TV ...)
>
> Also try any metal window frames that might be on the side of the house
> you want to listen from ... and gutters and downspouts and...
>
> If all else fails, string a wire from your window to something outside
> and above the roof, like a tree or a nearby house.
>
> Mike
> --
>
> E. Michael Smith e...@apple.COM
>
Yes, it probably will work to a degree, as will any metal object above the
ground but there are a couple of caveats:
1) Safety.. Make sure the metal roof is not likely to become live with power
line voltage. It might be wise to put a small capacitor, lets say .01mfd/1kv,
between the roof and your lead in. This way if the electric service falls on
the roof, or the wire to the attic light gets pinched behind a rafter, you are
only going to get a "buzz", not fireworks or worse. (don't forget they use 220
there) Also, don't leave your wire connected (or dangling over your bed) when
there might be an electric storm. You don't want to play "Ben Franklin"!! I
have seen induced voltage from nearby lightning bolts jump several inches from
large ungrounded metal objects to reach ground. Again, your nose doesn't want
to be the ground.
In the USA, all metal roofs and siding are required to be "grounded" by the
National Electric Code. My visits to the Caribbean seem to indicate this is
often not done there. Even if it is "grounded" with a wire to a water pipe or
ground rod, the inductance of the wire would not ground the roof for RF, so
there should be a good signal there, BUT....
There probably would be quite a lot of interference from stuff (tv's,
flourescent lights, dimmers) inside the house coupling to the roof, AND..
I would expect a big problem with rectification in the joints between the
roofing sheets. They undoubtedly aren't bonded together. If there is a strong
local station, you probably will get it all over the dial, due to intermod
created by the rectifying joints. This would be a problem with an indoor
antenna too, as the roof will "re-radiate" inside. (When I drive by a
particular telephone pole near my house, a nearby radio station [AM] comes in
on top of every other AM signal on my car radio, because of a rectifying joint
in the stuff on the pole.)
I have had some luck using metal casement window frames as antennas, but a 30
foot or so length of wire tossed out the window into a tree, or such seems to
work the best of anything. Take a spool of Radio Shack hook-up wire with you.
Don't forget the safety concerns, don't throw it into the tree with the
electric service running through it, and if you leave it up when you aren't
using it, you might want to find a place to ground it to between use, or get a
lightning arrester (which will, of course, need a ground).
Let us know what works when you get back!
--
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
John H. Schmidt |Internet: sch...@auvax1.adelphi.edu
Technical Director, WBAU |Phone--Days (212)456-4218
Adelphi University | Evenings (516)877-6400
Garden City, New York 11530 |Fax-------------(212)456-2424
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
John Pierce
>>Could I use the roof itself as an antenna?
>
>things to worry about. Just clip onto it with an 'aligator' clip and
>see what you get... I'd try cliping on at a corner opposite the direction
>I was most interested in ... Think of it as being like 1/2 of a big
>'bow tie' type antenna (of the kind that are sometimes used indoors on
>UHF TV ...)
This, of course, assumes that the roofing and siding are not grounded.
If they are (and they should be, in case of lightning), your antenna
will be a good short to ground.
Now, if you want to use it as a ground plane for a 1/4 wave vertical
whip, it would work GREAT.
---
################################################################
John Pierce
Atlanta Technology Center
j...@itpg004.atl.hp.com
(404) 850-2492
#################ANY#OPINONS#HERE#ARE#MY#OWN####################
sch...@auvax1.adelphi.edu
>>>
>>>Could I use the roof itself as an antenna?
>>
>>Yes, you can. It may or may not work well, depending on far too many
>>things to worry about. Just clip onto it with an 'aligator' clip and
>>see what you get... I'd try cliping on at a corner opposite the direction
>>I was most interested in ... Think of it as being like 1/2 of a big
>>'bow tie' type antenna (of the kind that are sometimes used indoors on
>>UHF TV ...)
>
> This, of course, assumes that the roofing and siding are not grounded.
> If they are (and they should be, in case of lightning), your antenna
> will be a good short to ground.
>
the ground wire has a fairly high impedance at RF frequencies, this is why a
"ground" wire often makes a good antenna! Try connecting the antenna of a
radio to the well grounded (at DC) water pipe, particularly on the second floor
of a wood frame house. You will probably be surprised how well it may work.
The pipe works something like a vertical antenna.
there is no ground connected, but see my previous posting
concerning rectification with metal roofs.
John Schmidt
> Now, if you want to use it as a ground plane for a 1/4 wave vertical
> whip, it would work GREAT.
>
> ---
> ################################################################
> John Pierce
> Atlanta Technology Center
> j...@itpg004.atl.hp.com
> (404) 850-2492
> #################ANY#OPINONS#HERE#ARE#MY#OWN####################
Gary Coffman
>>>
>>>Could I use the roof itself as an antenna?
>>
>>Yes, you can. It may or may not work well, depending on far too many
>>things to worry about. Just clip onto it with an 'aligator' clip and
>>see what you get... I'd try cliping on at a corner opposite the direction
>>I was most interested in ... Think of it as being like 1/2 of a big
>>'bow tie' type antenna (of the kind that are sometimes used indoors on
>>UHF TV ...)
>
>This, of course, assumes that the roofing and siding are not grounded.
>If they are (and they should be, in case of lightning), your antenna
>will be a good short to ground.
It's probably better if the roof is *not* grounded as far as lightning
protection is concerned. A grounded metal roof is a more attractive
target for lightning than one that is floating at atmospheric potential.
The 20,000 plus ampere current of a lightning bolt discharging to ground
through the fairly resistive steel roofing is likely to generate enough
heat to start a fire.
On using a metal roof as an antenna, the joints between panels are likely
not soldered and are likely to form corroded diode junctions that generate
RFI. It would likely make a lousy antenna. Any big hunk of metal is worth
a try though. It might work fine with a good tuner.
Gary KE4ZV
sch...@auvax1.adelphi.edu
> In article <1346...@hpuerca.atl.hp.com> j...@hpuerca.atl.hp.com (John Pierce) writes:
>>>>
>>>>Could I use the roof itself as an antenna?
paragraph deleted
>>
>>This, of course, assumes that the roofing and siding are not grounded.
>>If they are (and they should be, in case of lightning), your antenna
>>will be a good short to ground.
>
> It's probably better if the roof is *not* grounded as far as lightning
> protection is concerned. A grounded metal roof is a more attractive
> target for lightning than one that is floating at atmospheric potential.
> The 20,000 plus ampere current of a lightning bolt discharging to ground
> through the fairly resistive steel roofing is likely to generate enough
> heat to start a fire.
>
I disagree! First, a direct strike is probably big trouble, grounded or not,
but the probability of a strike, which is very small unless you're the tallest
thing around by some margin, is not particularlt affected by whether its
grounded or not. When it's raining, the wet building exterior discharges much
of the static charge, but not induced voltage, see below. This makes the roof
close to ground potential anyway. Also, modern buildings have plumbing,
wiring, etc. fairly close to the roof, so the "ground plane" is only a little
lower even if the roof "floats" In fact a grounded object can apparently even
reduce the chance of a lightning strike, if it has the right shape to "leak"
away the local field. Companies like Lightning Elimination Associates make
various devices, some of which look somewhat like a stainless steel porcupine
or a giant pincushion (sp?) to mount on radio towers or other tall likely
targets. People I know who have installed them claim excellent results.
Anyway, the real danger from the ungrounded roof in the event of lightning is
not the direct strike, but the much more common "near miss", it doesn't even
have to be that near, and it can be a "cloud to cloud" bolt anywhere near
overhead. These bolts can induce thousands of volts in ungrounded objects like
roofs, wires, metal fences, (particularly "electric fences", but even barbed
wire on wooden posts), etc. If your metal roof (ungrounded) is within a few
inches of the electric wiring or the plumbing, (grounded), etc. a brief arc
will jump between the two, discharging the roof. If this arc happens to cross
the path of something flanmmible, like the roof supports, you may well start a
fire. If the roof is grounded, the voltage never builds up to arc, as the
charge is drained off through the ground wire.
It takes about 10,000 volts to jump 1/4 inch, but it is non-linear and
dependent on the shape of the two charged objects, etc.
If you want to play Ben Franklin, stretch 100 feet of wire in your back yard,
insulated with glass insulators, up 20 feet or so. Take one end, run it to
about 1/4 to 1/2 inch from a well grounded object in some location where you
can safely observe from a reasonable distance, preferably not too brightly lit,
and wait for the next reasonably strong thundershower. Count how many times
the induced charge jumps the gap. Just don't touch the wire (Franklin was
lucky, others were killed repeating his experiment, but a direct strike is very
unlikely at 20 feet high) and discharge the wire after the storm by touching it
with a GROUNDED wire, just in case there is a residual charge (probably not
enough to hurt you, but...) Now think what would happen if there was something
flammible in the gap. It proubably wouldn't catch the first time, but sooner
or later... The reason for the glass insulators is that if the insulation on
the wire breaks down before your spark gap, you wont see the arc, it'll be
somewhere else.
> On using a metal roof as an antenna, the joints between panels are likely
> not soldered and are likely to form corroded diode junctions that generate
> RFI. It would likely make a lousy antenna. Any big hunk of metal is worth
> a try though. It might work fine with a good tuner.
>
Very true, in fact they are usually held down by somewhat corroded galvanized
nails.
> Gary KE4ZV
Robb Holmes
Hallicrafters S38-E, one of those cheap and potentially lethal
transformerless 5-tube radios. Apparently, the roof was sufficiently
grounded, and the antenna jack was sufficiently hot to cause sparks
to fly when I attempted to connect the two.
A window-to-tree wire worked much better.
-------
Robb Holmes WUGA-FM Ga. Center for Continuing Ed. Univ. of Georgia
rho...@athena.cs.uga.edu rho...@uga.cc.uga.edu bitnet:rholmes@uga
I have no opinions. Any in this message were caused by an Opinion Virus.
Cobra
>
> I guess that an attic antenna (my previous plan) is unlikely to work
> well under a metal roof.
>
> Could I use the roof itself as an antenna?
>
Yeah, it should work, unless the roof is quite corroded...this will, in all
lielyhood, be one of those "try it and see" propositions.
If you're just trying for a rcvr antenna, even a piece of wire hung out the
window would do better than being inside a Faraday cage and trying to pick
something up... :)
(interesting siding to use for a house...of course, considering the weather
in that part of the world, you use whatever works... :D )
---.sig File 1.0-----------------------------------------------------------
Ken Thompson - Co...@jab.tucson.az.us |"Mr. Science Officer, commence
Alt.address - Co...@coyote.datalog.com | relaxation period!"- Cdr. R. Hoek
Alt.adrs.two - GEnie - K.THOMPSON23 | "Ren and Stimpy" - Space Madness
----------------------------------------.sig Virus Killer Beta Test Ok!----
robert.melchiorre
I don't know. Maybe I'm oversimpflifying, but, it's been my experience that
just a simple piece of wire routed outside a window makes a pretty good
antenna. I would think that connecting it to the roof (at the risk of lighting
hazards) would only be an incremental improvement.
bob melchiorre
allentown, pa
WILLIAMS, STEPHEN EBERT
My comments are a slightly divergent, but oh so important, thread.
>>This, of course, assumes that the roofing and siding are not grounded.
>>If they are (and they should be, in case of lightning), your antenna
>>will be a good short to ground.
>
>It's probably better if the roof is *not* grounded as far as lightning
>protection is concerned. A grounded metal roof is a more attractive
>target for lightning than one that is floating at atmospheric potential.
>The 20,000 plus ampere current of a lightning bolt discharging to ground
>through the fairly resistive steel roofing is likely to generate enough
>heat to start a fire.
>
I really hate to disagree with Gary on the grounding of the roof, but, here
goes...
To lightning, the static potential on an ungrounded roof is trivial when a bolt
is looking for a path to ground. Lighting will readily jump the ceramic
insulators on a 345kV transmission line, and it will do the same to your house.
But to wit, by *solidly* grounding your roof, you stand less of a chance of
seeing those wonderful SW receivers burn up along with your house.
My experience with high voltage lines (7.2kV & up) has taught me that if an
electric arc wants to happen (if it could want), it could go anywhere.
A lightning bolt is an even less predictable critter, so you should guard
against it appropriately.
When you have the roof grounded well, string some wire up on it (i.e. Radio
Shack SW antenna) in whatever lengths and shapes work best. You have 1/2 of a
dipole, with the ground plane roof providing the mirror image.
Theory says it should work well ;-).
_And_, during storms and when not in use, disconnect that lighting rod (antenna
wire) from your electronics.
>
>Gary KE4ZV
Steve
Stephen E. Williams Power System Automation Lab, Texas A&M U.
SEW: "_Just_ How Big of an Arc do you want, Doc ?"
BDR(Doc): "Nothing under 1000Amps or 2ft, whichever's bigger."
Cobra
> I don't know. Maybe I'm oversimpflifying, but, it's been my experience that
> just a simple piece of wire routed outside a window makes a pretty good
> antenna. I would think that connecting it to the roof (at the risk of lighti
> hazards) would only be an incremental improvement.
>
Well, that incremental improvement just *might* be all that's needed in
taking a barely readable signal into one that is tolerable to listen to.
Of course, the inverse may be true, as well, so, as I mentioned previously
(as have others), I'd just try it and see what'll happen; if it does the
job, use it. If it *doesn't*, you haven't lost much for an attempt at using
it.
>
> bob melchiorre
> allentown, pa
John Pierce
this notes string. This is turning out to be a good, interesting discussion
that has given a lot to chew on. But to get those details, I'm afraid I
will have to turn
$BREVITY MODE OFF$
$BANDWIDTH CONSUMPTION APOLOGY MODE ON$
I won't address the lightning safety issue here, as there's another
string already spun off.
To summarize:
>>>
>>>Could I use the roof itself as an antenna?
>>
>>Yes, you can. It may or may not work well, depending on far too many
>>things to worry about.
>
> This, of course, assumes that the roofing and siding are not grounded.
> If they are (and they should be, in case of lightning), your antenna
> will be a good short to ground.
>
*************** John Schmidt:
It will be a good (we hope) short to ground at DC, but probably not at RF, as
the ground wire has a fairly high impedance at RF frequencies, this is why a
"ground" wire often makes a good antenna! Try connecting the antenna of a
radio to the well grounded (at DC) water pipe, particularly on the second floor
of a wood frame house. You will probably be surprised how well it may work.
The pipe works something like a vertical antenna.
There probably would be quite a lot of interference from stuff (tv's,
flourescent lights, dimmers) inside the house coupling to the roof, AND..
I would expect a big problem with rectification in the joints between the
roofing sheets....
*************** Gary Coffman:
On using a metal roof as an antenna, the joints between panels are likely
not soldered and are likely to form corroded diode junctions that generate
RFI. It would likely make a lousy antenna. Any big hunk of metal is worth
a try though. It might work fine with a good tuner.
************* Now my views, intended to spur further thought and discussion:
1) RF Suitablity of plumbing pipes as antennas:
Let's assume that the metal IS grounded, per code (for good reason).
Yes, a water pipe will give surprisingly good performance, but
only at certain freqencies. I assume here that the radio is
"floating" WRT ground. The grounded termination of the antenna
provides a fairly high-Q tuning of the antenna, because integer
multiples of half- and full- wavelengths fall at the ground node.
(I haven't thought through 1/4, 1/8, etc. wavelengths...any ideas?)
However, at frequencies corresponding to other wavelengths, the
antenna is NOT resonant. In fact, the grounding of the far end
of the pipe (antenna) would electrically damp any resonance.
Grounding the far end of the antenna will therefore give a pretty
highly tuned spectral characteristic. It would be similar to
a VHF slot antenna, where a tuned cavity in a conductor is used
to provide a "one-frequency" antenna.
Now for a pipe, depending on where you feed it, you would have
two resonant lengths, one on each side. One will be grounded,
the other probably not. Plus, you have the plumbing trunks
(supplies to different faucets) to give other resonances.
This might work as an antenna, and might not. I see several
variables:
- The length from tap to ground (if close to the true
ground, it would be an effective RF short).
- The length of the unterminated element.
- The number, spacing, and lengths of supply branches.
- The distance from the ground.
- Is the pipe PVC? ( 8-) )
It seems that to use a pipe as an antenna, you would get good
performance at some frequencies, but not others. How do you
find out? The performance would be unpredictable, so you would
just have to use trial and error. If you want to tweak for all
desired frequencies, it might be fun to play around with.
And yes, Gary, a tuner may help.
2) RF Suitablity of metal houses as antennas:
If a pipe's performance is unpredictable, a house would be even
more so. Here we have *multiple* resonant peaks, not just because
of the different electrical elements, but now we don't have a one-
dimensional conductor, but a two-dimensional conductor wrapped
in three dimensions. Where do you tap the house?
I would expect the house to have wild, almost random spectral
characteristics. It might be excellent at one frequency, but
totally nonresonant at another. As several have mentioned,
rectification between different parts would add to the unpred-
ictable nature.
I'm not sure how steel would affect the performance, as compared
to say, an aluminum mobile home or a copper roof. Any ideas?
3) And a plain piece of wire:
A plain piece of wire makes a pretty good broadband antenna,
precisely because it is *not* ground-terminated. Yes, it has
a characteristic frequency, but it does not damp off-frequency
resonances as vigorously as a ground-terminated antenna.
The plain wire has lower-Q than the the "tuned" terminated
antenna. It does not perform well for a single frequency,
but that's not usually what you want. It's the old trade-off
syndrome. But a wire is predictable.
And to improve performance, don't let the receiver "float" WRT
ground. Make a good signal ground at the receiver. For Dan's
ICF-2010, that is the outer conductor of the antenna jack.
Besides, if you want to tune for a specific frequency, you'd want
a folded dipole or something that will let you choose the resonant
frequency. With a pipe or a house, you'd have to explore until you
DISCOVER the optimium frequency.
4) How about some coax?
If you could run some coax to an outside antenna, you would get
even better performance. As John Schmidt pointed out, the metal
house would help to contain and possibly reradiate RF noise from
inside the house (but also shield noise from outside).
Several band-trap antennas are available with 75 Ohm or 50 Ohm
coax connections. I also saw someplace that there is a long-
wire balun (BALanced-UNbalanced transformer) available. Sorry,
I don't know where at the moment. Any ideas?
Some remote active antennas, like the MFJ-1024 and the McKay Dymek
DA-100D use a preamp at the remote antenna, fed through coax to
a control box inside the house. These might be a good choice for
Dan's situation.
John Pierce
>
>To lightning, the static potential on an ungrounded roof is trivial when a bolt
>is looking for a path to ground. Lighting will readily jump the ceramic
>insulators on a 345kV transmission line, and it will do the same to your house.
I agree. I have a book about lightning at home by a Dr. (Mr?) Uman. He
puts it well:
After a lightning bolt travels through 20 miles, what makes you
think that 6 inches of rubber will stop it?
Moral: Always give lightning what it wants: an easy path to ground. Just
make sure that path goes where it won't hurt anything important--like YOU.
Gary Coffman
>In article <1992Jun19.1...@ke4zv.uucp>, ga...@ke4zv.uucp (Gary Coffman) writes:
>>
>> It's probably better if the roof is *not* grounded as far as lightning
>> protection is concerned. A grounded metal roof is a more attractive
>> target for lightning than one that is floating at atmospheric potential.
>> The 20,000 plus ampere current of a lightning bolt discharging to ground
>> through the fairly resistive steel roofing is likely to generate enough
>> heat to start a fire.
>>
>I disagree! First, a direct strike is probably big trouble, grounded or not,
Well the point is moot in the US anyway since the NEC requires metal
roofs to be grounded for protection against falling electrical wires.
However, it's been my experience at mountaintop sites that are struck
frequently that floating structures are both less likely to be struck
and receive less damage when they are struck than structures that are
grounded.
Induced potentials are not a problem as long as they have no where to
*go*. Unless there is a potential difference in the circuit, and the
circuit is complete, or completeable by a short arc, no damaging current
will flow. On a granite mountain, there simply is no good ground. Attempts
to provide one usually result in equipment damage.
Gary KE4ZV
Gary Coffman
>>
>>To lightning, the static potential on an ungrounded roof is trivial when a bolt
>>is looking for a path to ground. Lighting will readily jump the ceramic
>>insulators on a 345kV transmission line, and it will do the same to your house.
>
>I agree. I have a book about lightning at home by a Dr. (Mr?) Uman. He
>puts it well:
>
> After a lightning bolt travels through 20 miles, what makes you
> think that 6 inches of rubber will stop it?
>
>Moral: Always give lightning what it wants: an easy path to ground. Just
>make sure that path goes where it won't hurt anything important--like YOU.
Ahem. Lightning is able to travel 20 miles through air because there exists
a conductive channel formed by the cloud leaders and the ground streamers
that build through corona discharge. The resistance in the bolt channel
is very low. In fact the bolt can't occur *until* the resistance is low.
Ohm's law still holds for lightning as for any other electrical circuit.
The typical lightning bolt has a potential difference of 20 to 60 kilovolts.
That's only sufficient to jump an inch or so in dry air. It's the ionized
air channel that allows the bolt to travel. Break that with any insulator
that is rated for 60 kv or more and you stop the bolt.
Such high voltage engineering in a residence is a challenge beyond most
hobbiests, however, and a good lightning rod and ground system is an
alternative. That's not simple to engineer properly either.
Gary KE4ZV
Gary Coffman
>$BANDWIDTH CONSUMPTION APOLOGY MODE ON$
>
> Let's assume that the metal IS grounded, per code (for good reason).
> Yes, a water pipe will give surprisingly good performance, but
> only at certain freqencies. I assume here that the radio is
> "floating" WRT ground. The grounded termination of the antenna
> provides a fairly high-Q tuning of the antenna, because integer
> multiples of half- and full- wavelengths fall at the ground node.
> (I haven't thought through 1/4, 1/8, etc. wavelengths...any ideas?)
An antenna can't be self resonant unless it is a 1/2 wave electrically,
or a multiple thereof. That's because a resonant antenna has to have
voltage and current standing waves. However, that's not necessary for an
effective *receiving* antenna. Unless you are concerned with maximum power
transfer and a fixed impedance feeder tap, non-resonant antennas can be
just as effective as resonant antennas of the same *capture* area. Tapping
the signal from the antenna can be more complex, since the impedance may
be arbitrary at a given frequency, but that's easily handled with active
matching. What's important is how much of the wavefront the antenna can
intercept. That's basically a size and shape issue. IE bigger and skinnier
is better.
Grounding one end does not make the piece of metal "high Q", though it
can shift the frequency at which the antenna is resonant by the ground
mirror effect. IE an antenna of half the normal resonant length will
be resonant if one end is grounded. That's the basis of 1/4 wave ground
plane antennas. But resonance isn't necessary for an effective receiving
antenna since we aren't concerned with power transfer. All we need is
sufficient voltage to drive a Class A stage. Note a Class A stage requires
*no* driving power and all receiver front end stages operate Class A.
> However, at frequencies corresponding to other wavelengths, the
> antenna is NOT resonant. In fact, the grounding of the far end
> of the pipe (antenna) would electrically damp any resonance.
It's not resonant because it's the wrong length to support standing
waves of voltage and current. The ground has nothing to do with
"damping" resonance since there isn't one anyway. To damp the signal,
the ground connection would have to be in *parallel* with the antenna.
> Now for a pipe, depending on where you feed it, you would have
> two resonant lengths, one on each side. One will be grounded,
> the other probably not. Plus, you have the plumbing trunks
> (supplies to different faucets) to give other resonances.
No, the pipe is resonant solely by it's length, it doesn't matter
where you tap the feeder. A halfwave dipole fed in the center is
resonant at the same frequencies as a half wave fed at one end or
a Windom fed 37% off center. Only the total length matters. The
impedance at the feed point will be *different* but that can be
transformed with a matching network.
The other feeder trunks *will* make a difference because they will
act as tuned stubs off the main line. That will affect the *impedance*
at a particular tap point at particular frequencies, but active matching
can easily deal with this. Note that a stub may have a voltage zero or
a current zero at it's connection point, but it can't have both *at once*.
Therefore a matching device can always transform the voltage or current
maxima to a voltage maxima suitable for driving a Class A stage.
> And yes, Gary, a tuner may help.
It's not a matter of *may*, over a broad frequency range, it's a matter
of *will*.
>2) RF Suitablity of metal houses as antennas:
>
> If a pipe's performance is unpredictable, a house would be even
> more so. Here we have *multiple* resonant peaks, not just because
> of the different electrical elements, but now we don't have a one-
> dimensional conductor, but a two-dimensional conductor wrapped
> in three dimensions. Where do you tap the house?
>
> I would expect the house to have wild, almost random spectral
> characteristics. It might be excellent at one frequency, but
> totally nonresonant at another. As several have mentioned,
> rectification between different parts would add to the unpred-
> ictable nature.
Resonance is still a non-issue for a receiving antenna. What is an issue
is if the house is large with respect to a wavelength. If it is, it will
act as a shorted turn at the desired frequency and it will be impossible
to tap it for a voltage maximum. This is the same problem you have with
an aircraft. Suppose you have a metal aircraft flying through the Earth's
magnetic field. Why can't you tap power from it? It's because to get at
the potential difference between the wing tips you have to run a conductor.
That conductor will have the *same* voltage induced on it and no net
current can flow. That's the case with a metal house as well. If the
house is broad with respect to it's length, it intercepts the wave in
such a way that there is no point where you can tap a potential difference.
What you have to do instead is work the house against ground as a capacitive
probe antenna. That's a totally non-resonant system that's completely
broadband. It also presents a very high impedance that will require active
matching. Since it's a very high impedance, grounding of the house will
defeat it. You could install your safety ground through a choke, however,
and then the house would make a dandy capacitive probe.
> I'm not sure how steel would affect the performance, as compared
> to say, an aluminum mobile home or a copper roof. Any ideas?
Galvanized steel has much higher resistance to RF than copper or aluminum.
This is real skin resistance as opposed to radiation resistance or reactance.
It dissipates real power as heat. If the steel structure offers a low
impedance at a frequency, the losses will be high. If the structure offers
a high impedance, little current flows and losses will be correspondingly
low.
>3) And a plain piece of wire:
>
> A plain piece of wire makes a pretty good broadband antenna,
> precisely because it is *not* ground-terminated. Yes, it has
> a characteristic frequency, but it does not damp off-frequency
> resonances as vigorously as a ground-terminated antenna.
No.
> The plain wire has lower-Q than the the "tuned" terminated
> antenna. It does not perform well for a single frequency,
> but that's not usually what you want. It's the old trade-off
> syndrome. But a wire is predictable.
And no again. The only thing that lowers Q is resistance, real resistance
in parallel with the reactance of the wire, or other structure. The formula
for Q is X/R. Grounding has nothing to do with it.
Gary KE4ZV
John Pierce
> The other point is that all theory aside, it's the end result that matters.
Yes! The point we have all made, one way or another, is it might work and
it might not. Try it. Help it with a tuner. If it doesn't work throw a
wire (or a washbowl, or Yagi, or... :-)) out the window. I read the
consensus (for whatever reason) to be: as an antenna, a metal building would
be predictably unpredictable.
And yes, Gary and John, we ARE talking receivers here. You are right.
Sorry. The concept IS interesting, though.
Jonathan Papai
>It takes about 10,000 volts to jump 1/4 inch, but it is non-linear and
>dependent on the shape of the two charged objects, etc.
>
>If you want to play Ben Franklin, stretch 100 feet of wire in your back
>yard, insulated with glass insulators, up 20 feet or so. Take one end, run
>it to about 1/4 to 1/2 inch from a well grounded object in some location
>where you can safely observe from a reasonable distance, preferably not too
>brightly lit, and wait for the next reasonably strong thundershower. Count
>how many times the induced charge jumps the gap. Just don't touch the wire
>(Franklin was lucky, others were killed repeating his experiment, but a
Franklin was lucky, but also more careful than his contemporaries.
He conducted his experiments by standing inside a house next to a open
window, making sure to keep the first couple of feet of kite string nearest
to him dry. He realized that the silk was conductive when wet and
insulator when dry.
>--
>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>John H. Schmidt |Internet: sch...@auvax1.adelphi.edu
>Technical Director, WBAU |Phone--Days (212)456-4218
>Adelphi University | Evenings (516)877-6400
>Garden City, New York 11530 |Fax-------------(212)456-2424
>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
J.P. Papai
Pa...@KCGL1.ENG.OHIO-STATE.EDU
N 40 36'43" W 81 12'58"
