Coax as a balanced feeder?
Greg Newberry
Hi,
I apologize in advance if this gets posted twice.
I want to put up an antenna using balanced feeders but getting them in
the house isn't an options. Metal roof, metal siding, patio's, etc. A
few years back I remember an article using 2 equal runs of very low loss
coax to act as a balanced feed system for an antenna.
What I'd like to know is:
Has anyone done this?
What would be the best coax?
How would you handle the braid? Tie together at each end and ground the
transmitter end?
What would be the impedance of something like this?
Would the spacing of the coax pair even be an issue?
If I used about a 30 ft run I could get it into the clear for connection
to real ladder line. I know the losses are there, but with a high grade
of coax and a short run would this have acceptable losses? I run QRP and
don't want to lose very much.
Thanks
--
Greg Newberry - WB7DUO QRP-L #760
newb...@cyberhighway.net
newberrg{dhwtowers/regional/newberrg}@dhw.state.id.us
C. J. Hawley
Greg Newberry wrote:
> I want to put up an antenna using balanced feeders but getting them in
> the house isn't an options. Metal roof, metal siding, patio's, etc. A
> few years back I remember an article using 2 equal runs of very low loss
> coax to act as a balanced feed system for an antenna.
>
> What I'd like to know is:
> Has anyone done this?
Yes, lots of times.
> What would be the best coax?
Large coax...lowest loss.
> How would you handle the braid? Tie together at each end and ground the
> transmitter end?
Yes. Both ends if possible.
> What would be the impedance of something like this?
Twice the impedance of the coax
> Would the spacing of the coax pair even be an issue?
No issue. Tie them together
> If I used about a 30 ft run I could get it into the clear for connection
> to real ladder line. I know the losses are there, but with a high grade
> of coax and a short run would this have acceptable losses?
You can switch from twinlead to double coax to ladder line to open wire
line, whatever it takes to get to the antenna. In your case you could go
to the outside with double coax and then switch to open wire line. That
would probably be the lowest loss.
Chuck, KE9UW
Cecil Moore
C. J. Hawley wrote:
> Twice the impedance of the coax
Hi Chuck, just to be sure I understand. Two parallel pieces of 90 ohm
coax would have a characteristic impedance of 180 ohms?
> You can switch from twinlead to double coax to ladder line to open wire
> line, whatever it takes to get to the antenna.
Does the double coax have to be a certain percentage of a wavelength
before it develops a characteristic impedance? Say I stuck a 10 ft
length of double coax in the middle of 300 ohm ladder-line on 75m.
Would the signal actually see twice the impedance of the coax?
Thanks and 73, Cecil, W6RCA, OOTC (not speaking for my employer)
Marty Gulseth
Greg Newberry wrote:
>
> I want to put up an antenna using balanced feeders but getting them in
> the house isn't an options. Metal roof, metal siding, patio's, etc. A
> few years back I remember an article using 2 equal runs of very low loss
> coax to act as a balanced feed system for an antenna.
Hi Greg,
I *might* still have a copy of the original article. Mid 70's "73 Mag" as I recall. Let
me know if you are interested. The concept fascinated me also, but I HAVE NOT tried this
setup (yet.) By the way, the author also claimed much higher immunity to local noise due
to the coax shielding. YMMV.
>
> How would you handle the braid? Tie together at each end and ground the
> transmitter end?
As I recall, the author tied the braid together on one end only. Connected the tied
together braids to ground at the station (tuner) end.
> What would be the impedance of something like this?
I struggled with this for a long time, then finally drew a "schematic" of the compound
line, assuming lumped constants. The two braids are essentially at the same potential. If
you want to get really detailed, stick in some minimal "R" to account for ohmic losses.
Bottom line - I convinced myself that the impedance really is 2X the characteristic
impedance of the cable.
> Would the spacing of the coax pair even be an issue?
The author contended that exact same length was more critical than spacing. Seems right to
me, as the impedance of the coax is determined by the spacing between center conductor and
braid of the coax, plus dielectric properties of the spacer (inner insulation.) He even
claimed the two could be separated if needed. Given the above, makes sense to me.
> If I used about a 30 ft run I could get it into the clear for connection
> to real ladder line. I know the losses are there, but with a high grade
Loss is one issue. Matching could be another, but the use of different impedance lines as
in a system is a time-tested technique (or at least used to be) in microwave and RF
circles. End result depends on the antenna in use, and the capability (matching range) of
your tuner.
Two approaches:
1. Get yourself some Smith charts, the Antenna Handbook or equivalent, and hack away. If
needed, drive yourself mad figuring out all the possibilities.
2. Put it all together, give it a try, get on the air and have some QSO's! (I kinda tend
to the latter approach.)
Hope this helps. (I assume) happy hacking. Antennas are fun! 73,
Marty - W7LEJ
Spokane, WA
(std. disclaimers)
C. J. Hawley
Cecil Moore wrote:
>
> C. J. Hawley wrote:
> > Twice the impedance of the coax
>
> Hi Chuck, just to be sure I understand. Two parallel pieces of 90 ohm
> coax would have a characteristic impedance of 180 ohms?
Umm....I'm trying to figure out how to describe it...parallel makes me
think of "in parallel". I meant side by side feeding the center
conductors with the signal and tying the shield together and then to
ground. The characteristic impedance of that would be 180 ohms.
> > You can switch from twinlead to double coax to ladder line to open wire
> > line, whatever it takes to get to the antenna.
>
> Does the double coax have to be a certain percentage of a wavelength
> before it develops a characteristic impedance? Say I stuck a 10 ft
> length of double coax in the middle of 300 ohm ladder-line on 75m.
What I think here is that it doesn't matter exactly what the
characteristic impedance is as long as the feed line remains balanced
with respect to ground. I guess I think that the characteristic
impedance of a short piece of coax is what it is proposed to be no
matter how short the length....I'm thinking of connectors...they can be
short and be measured to have a characteristic impedance of what they
were designed to have.
> Would the signal actually see twice the impedance of the coax?
The signal would see a feedline of twice the impedance of the coax.
I'm trying to figure out why you are interested in this. I found your
stub tuning interesting but could never figure out what to do with the
dangling lengths of ladderline. It wouldn't do to coil them up....but it
seems like the dual coax could be coiled up.
Chuck, KE9UW
Jerry Flanders
newb...@cyberhighway.NET (Greg Newberry) wrote:
>...I want to put up an antenna using balanced feeders but getting them in
>the house isn't an options. Metal roof, metal siding, patio's, etc. A
>few years back I remember an article using 2 equal runs of very low loss
>coax to act as a balanced feed system for an antenna.
>What I'd like to know is:
=================================================
> Has anyone done this?
YES - I HAVE DONE THIS WITH A MULTIBAND DIPOLE WITH ABOUT 120 FEET OF
COAX FOR FEED, FED WITH A MATCHBOX IN THE STATION.
> What would be the best coax?
LOW LOSS. YOU WILL PROBABLY HAVE A HIGH SWR ON SOME BANDS, SO USE
"GOOD" COAX (OR DO AS MOST OF US DO - USE WHATEVER IS AVAILABLE AND
IGNORE THE LOSS).
> How would you handle the braid? Tie together at each end and ground the
>transmitter end?
EXACTLY
> What would be the impedance of something like this?
TWICE THE IMPEDANCE OF EACH LINE. (104 OHMS FOR 52 OHM COAX).
> Would the spacing of the coax pair even be an issue?
NO - BUT KEEP THEM THE SAME LENGTH.
>
>If I used about a 30 ft run I could get it into the clear for connection
>to real ladder line. I know the losses are there, but with a high grade
>of coax and a short run would this have acceptable losses?
I WOULDN'T WORRY ABOUT CHANGING TO LADDER LINE - THAT WILL JUST ADD
ANOTHER DISCONTINUITY IN YOUR FEEDLINE (UNLESS THE IMPEDANCES MATCH,
WHICH IS UNLIKELY).
>I run QRP and don't want to lose very much.
>Thanks
>--
> Greg Newberry - WB7DUO QRP-L #760
THIS WOULD BE AN EXCELLENT APPLICATION FOR SURPLUS COMMERCIAL 75 OHM
CABLE TV COAX (THE 3/4 INCH "SOLID" STUFF THEY RUN DOWN THE STREET).
SOMETIMES YOU CAN GET THIS STUFF FREE FROM CABLE TV COMPANIES. THIS
SHOULD BE EXTREMELY LOW-LOSS AT HF.
You can also use equal (electrical) lengths of coax in parallel to
reduce the impedance of a balanced line - two 52s in parallel give a
26, or a 75 and a 52 give a ...well - I will let you calculate that
one. This is good to know when you need a quarter-wave matching
transformer for some special matching purpose.
JERRY W4UKU flan...@groupz.net
W8JI Tom
In article <326801...@hp.com>, Marty Gulseth <marty_...@hp.com>
writes:
>>
>> How would you handle the braid? Tie together at each end and ground
the
>> transmitter end?
>
>As I recall, the author tied the braid together on one end only.
Connected
>the tied
>together braids to ground at the station (tuner) end.
>
>
The shields MUST be connected at each end, and should be grounded at
least at the transmitter.
If the shields are not tightly coupled, the shield to shield spacing
becomes part of the conductor spacing and the impedance and loss
increases.
73 Tom
Cecil Moore
C. J. Hawley wrote:
> I found your stub tuning interesting but could never figure out what
> to do with the dangling lengths of ladderline.
I'm thinking of putting cores over two side-by-side pieces of coax to
get a balanced choke and wondering if the resulting 100 ohms will cause
additional reflections on the 300 ohm ladderline.
I experimented with stubs but have replaced them all with discrete
reactances.
W8JI Tom
In article <3269A9...@ccm.ch.intel.com>, Cecil Moore
<Cecil_...@ccm.ch.intel.com> writes:
>I'm thinking of putting cores over two side-by-side pieces of coax to
>get a balanced choke and wondering if the resulting 100 ohms will cause
>additional reflections on the 300 ohm ladderline.
>
>I experimented with stubs but have replaced them all with discrete
>reactances.
>
>73, Cecil, W6RCA, OOTC (not speaking for my employer)
>
>
That would work Cecil, but the short section of line would look like 100
ohms.
There is NO DIFFERENCE between using the coax and using a twisted pair or
a bifilar winding through the cores, that's why I use two centers out of
teflon RG-58 through a stack of large diameter cores. It is actually makes
more effective use of the ferrites since I can use multiple turns and get
common mode flux linkage.
There seems to be some misconception that coax is a magic elixir when
making a choke or current balun. That's exactly the result I was afraid of
in the balun necessary thread.
73 Tom
Mandy Wright
In article <32670...@cyberhighway.net>, Greg Newberry
<newb...@cyberhighway.NET> writes
>Hi,
>
>I apologize in advance if this gets posted twice.
>
>the house isn't an options. Metal roof, metal siding, patio's, etc. A
>few years back I remember an article using 2 equal runs of very low loss
>coax to act as a balanced feed system for an antenna.
We assume the source is balanced as is the load, however you wish to use
a screened feed, presumably to avoid proximity effects and/or radiation.
Use a half-wave dipole, as a balanced load. If this is more than 0.2
wavelengths above the ground and is slung horizontally, it's input
impedance will be 60-70 ohms, a good match for the feeder. To connect a
balanced source we suggest either 75 ohm twinax, or 2 identical lengths,
parralled 135 ohm coax, braid strapped top and bottom. The top braid is
left floating, the lower end is grounded.
The other option, simpler and cheaper, is a 1-1.5 balanced to unbalanced
transformer and standard 50 ohm coax. The di-pole length will be for
the lowest working frequency, parralel tuned traps fitted will be
required for each of the higher bands required.
>
>What I'd like to know is:
> Has anyone done this?
> What would be the best coax?
> How would you handle the braid? Tie together at each end and ground the
>transmitter end?
> What would be the impedance of something like this?
> Would the spacing of the coax pair even be an issue?
>
>If I used about a 30 ft run I could get it into the clear for connection
>to real ladder line. I know the losses are there, but with a high grade
>of coax and a short run would this have acceptable losses? I run QRP and
>don't want to lose very much.
>Thanks
Hope this helps.
Mandy
(Replying on behalf of my husband (G1BCR / G9BZW) who hates computers
despite being a practising RF Engineer!)
Sussex
UK
Cecil Moore
W8JI Tom wrote:
> There is NO DIFFERENCE between using the coax and using a twisted pair or
> a bifilar winding through the cores, that's why I use two centers out of
> teflon RG-58 through a stack of large diameter cores.
Hi Tom, functionally, it seems the above is not different from running
300 ohm ladder-line through a number of single-turn cores. When you
say "two centers" above, is that with or without the braid? If you
spaced those "two centers" far apart enough to get 300 ohms
characteristic impedance, how far apart would the turns have to be?
Does your "two centers" avoid the problem of finding thermaleze wire?
Did you hear about the guy using 300 ohm twin-lead for the coil on
a base loaded vertical?
W8JI Tom
Hi Cecil,
In article <326BA1...@ccm.ch.intel.com>, Cecil Moore
<Cecil_...@ccm.ch.intel.com> writes:
>Hi Tom, functionally, it seems the above is not different from running
>300 ohm ladder-line through a number of single-turn cores. When you
>say "two centers" above, is that with or without the braid?
No braid.
>If you
>spaced those "two centers" far apart enough to get 300 ohms
>characteristic impedance, how far apart would the turns have to be?
I never worried about it Cecil, so I don't know. Standard transmission
line formulas would apply or be very close unless the wire was spaced
wider than the distance it is away from the cores, you don't want core
material to be in the differential mode fields any more than necessary.
>Does your "two centers" avoid the problem of finding thermaleze wire?
Don't know what that is. I use it because it has high voltage and
temperature breakdown and is weater proof.
>Did you hear about the guy using 300 ohm twin-lead for the coil on
>a base loaded vertical?
No, as a stub? It would be pretty lossy compared to a good coil.
73 Tom
Marty Gulseth
W8JI Tom wrote:
>
> The shields MUST be connected at each end, and should be grounded at
> least at the transmitter.
Hi Tom,
Thanks for filling in the blanks here.
>
> If the shields are not tightly coupled, the shield to shield spacing
> becomes part of the conductor spacing and the impedance and loss
> increases.
Please help me out here a little. are you saying that the proposition that the two
coax lines could be physically separated won't work? As noted, I seem to recall the
original author making that claim. Also as noted, I haven't tried this antenna - only
stored the article in my "fascinating" file.
Thanks and 73,
Marty
W8JI Tom
In article <326D25...@hp.com>, Marty Gulseth <marty_...@hp.com>
writes:
>
>Please help me out here a little. are you saying that the proposition
that
>the two
>coax lines could be physically separated won't work? As noted, I seem to
>recall the
>original author making that claim. Also as noted, I haven't tried this
>antenna - only
>stored the article in my "fascinating" file.
>
>Thanks and 73,
>
>Marty
Hi Marty,
No, the cables can be separated. But it is an absolute requirement they
have the shields connected at both ends.
All bets are off if the line carries significant common mode current. In
that case, you'd have a mess with separated cables.
73 Tom
JF
If you have two pieces of parallel coax the impedance will be half of
the original coax not twice. For example two parallel 75 ohm coaxs
will be 37.5 ohms. The fomula is the same as resistors in parallel.
The length of the coax is not important to the impedance unless it is
so short that the solder connection fanout is a big percentage of the
total.
Good luck,
Jim Ford, N6JF
Fri, 18 Oct 1996 09:00:21 -0700, Cecil Moore
<Cecil_...@ccm.ch.intel.com> wrote:
>C. J. Hawley wrote:
>> Twice the impedance of the coax
>
>Hi Chuck, just to be sure I understand. Two parallel pieces of 90 ohm
>coax would have a characteristic impedance of 180 ohms?
>
>> You can switch from twinlead to double coax to ladder line to open wire
>> line, whatever it takes to get to the antenna.
>
>Does the double coax have to be a certain percentage of a wavelength
>before it develops a characteristic impedance? Say I stuck a 10 ft
>length of double coax in the middle of 300 ohm ladder-line on 75m.
>Would the signal actually see twice the impedance of the coax?
>
>Thanks and 73, Cecil, W6RCA, OOTC (not speaking for my employer)
Curt Phillips
Marty Gulseth <marty_...@hp.com> wrote:
>Greg Newberry wrote:
>> I want to put up an antenna using balanced feeders but getting them in
>> the house isn't an options. Metal roof, metal siding, patio's, etc. A
>> few years back I remember an article using 2 equal runs of very low loss
>> coax to act as a balanced feed system for an antenna.
>me know if you are interested. The concept fascinated me also, but I HAVE NOT tried this
>setup (yet.) By the way, the author also claimed much higher immunity to local noise due
>to the coax shielding. YMMV.
>> transmitter end?
>together braids to ground at the station (tuner) end.
I'd like to add my $.02 (probably about what it is worth :-).
I've read about such a configuration, but the impedance mentioned in the
book or article (I don't remember where I read it) was much different that
anyone else here has mentioned.
The theory that I read said that, with the coax shields grounded and tied
together, that the "twin lead conductors" (i.e., the two center conductors
of the two pieces of coax) would appear (electrically) to be infinitely far
apart. Therefore, the impedance would be the same as a ladder line or
twin-lead with, say 20 foot spacing between their conductors. Therefore,
it seems as though they were claiming an impedance of 1000 ohms, or
something in that range... something appropriate for "ladder line" with
extremely wide spacing.
It seemed logical when I read it (which probably why I still remember
it), but it's interesting that no one else here seems to agree with that
theory. Perhaps the author was very wrong, but it "sounded" good. :-)
(Never let actual radio theory interfere with a good sounding explanation?
:-)
Anyway, I'm interested in comments. I've never used two coaxes this way
either, but as a proponent and user of ladder line, someday I may.
=========== Opinions expressed are solely those of the author ==========
Curt Phillips, CEM KD4YU (ex-WB4LHI) | They that can give up essential
Engineer/Writer/Adventurer/Raconteur | liberty to obtain a little
Chairman, Tar Heel Scanner/SWL Group | temporary safety deserve neither
ARRL Life; QCWA; Raleigh ARS; NRA; AEE|liberty nor safety. -Ben Franklin
==== cphi...@pobox.com ====== [Copyright 1996 All rights reserved]====
Cecil Moore
JF wrote:
> If you have two pieces of parallel coax the impedance will be half of
> the original coax not twice.
Hi Jim, seems to be a semantic problem. The "parallel coax" conductors
in a balanced transmission line are in series so the impedances add.
Chuck (Jack) Hawley
Curt Phillips wrote:
> The theory that I read said that, with the coax shields grounded and tied
> together, that the "twin lead conductors" (i.e., the two center conductors
> of the two pieces of coax) would appear (electrically) to be infinitely far
> apart. Therefore, the impedance would be the same as a ladder line or
> twin-lead with, say 20 foot spacing between their conductors. Therefore,
> it seems as though they were claiming an impedance of 1000 ohms, or
> something in that range... something appropriate for "ladder line" with
> extremely wide spacing.
>
> It seemed logical when I read it (which probably why I still remember
> it), but it's interesting that no one else here seems to agree with that
> theory. Perhaps the author was very wrong, but it "sounded" good. :-)
> (Never let actual radio theory interfere with a good sounding explanation?
Think about it, if the impedance from the first one is 50 ohms to the
shield and the shields are tied together and the impedance from the
shield to the second one is 50 ohms.......
Chuck, KE9UW
William E. Sabin
Consider two coaxes whose braids are joined together along their lengths.
The generator and load are connected to the two center leads and the
braids float (not grounded).
For each coax the value of Z0 = sqrt(L/C). For the pair of coaxes the two
C's are in series so that the total capacitance is C/2. The L's are also
in series so that the total inductance is L*2.
The new Z0' = sqrt(4*L/C) = 2*Z0.
Bill W0IYH
W8JI Tom
In article <54rdmc$b...@redstone.interpath.net>, cphi...@pobox.com (Curt
Phillips) writes:
>
> The theory that I read said that, with the coax shields grounded and
tied
>together, that the "twin lead conductors" (i.e., the two center
conductors
>of the two pieces of coax) would appear (electrically) to be infinitely
far
>apart. Therefore, the impedance would be the same as a ladder line or
>twin-lead with, say 20 foot spacing between their conductors. Therefore,
>it seems as though they were claiming an impedance of 1000 ohms, or
>something in that range... something appropriate for "ladder line" with
>extremely wide spacing.
>
>
The center conductor only interacts with the shield, and the shield
interacts with the outside world. If the center is excited and the shield
is floated, the effect is almost exactly the same as if you were feeding
the shields. So if the shields float the line looks FAT and as if it had
the physical spacing of the shields.
If the shields are connected at each end normal transmission line type
currents (differential mode) cancel on the outside of the shields, all
transmission current stays inside each cable, and the impedance is the sum
of the two cables. Loss remains fixed at the value of one cable alone. (If
the SWR is unity in both cases.)
Common mode (unwanted parallel) currents flow down the outside of the
shields and radiate, just as from open wire line. So having the shield
does not help this problem.
A second way it would work is if the coupling (by electromagnectic or
induction fields) between the cables is very very high and there is no
space difference between them to cause time delays.
The common mode surge impedance is the effective diameter of the two outer
shields, and is affected by line spacing. But it only concerns common mode
signals. The differential mode impedance (as a balanced line) is the sum
of the two lines.
73 Tom
Curt Phillips
"Chuck (Jack) Hawley" <c-ha...@uiuc.edu> wrote:
>Curt Phillips wrote:
>> The theory that I read said that, with the coax shields grounded and tied
>> together, that the "twin lead conductors" (i.e., the two center conductors
>> of the two pieces of coax) would appear (electrically) to be infinitely far
>> apart. Therefore, the impedance would be the same as a ladder line or
>> twin-lead with, say 20 foot spacing between their conductors. Therefore,
>> it seems as though they were claiming an impedance of 1000 ohms, or
>> something in that range... something appropriate for "ladder line" with
>> extremely wide spacing.
>> it), but it's interesting that no one else here seems to agree with that
>> theory. Perhaps the author was very wrong, but it "sounded" good. :-)
>> (Never let actual radio theory interfere with a good sounding explanation?
>Think about it, if the impedance from the first one is 50 ohms to the
>shield and the shields are tied together and the impedance from the
>shield to the second one is 50 ohms.......
But (just to advance the discussion, not because I'm going to "fall on my
sword" for this theory, but to make sure it is given a "fair" analysis)...
the radio never sees the shield. I'm talking about a situation where
both center conductors are connected to the balanced input of a radio or
tuner, and the two shields are connected together and grounded, but not
connected to the radio in any way.
So the radio never sees the shield(s), never "knows" that they are there
and doesn't care. All the radio/tuner sees is two (hopefully) balanced
wires which evidence some characteristic impedance, an impedance which is
normally determined by their distance apart. Since each of these two lines
are "totally isolated" from each other (within the limits of practicality),
they are "worlds apart" and their impedance is correspondingly high.
Maybe. :-)
========== Opinions expressed are solely those of the author ===========
Curt Phillips KD4YU (ex-WB4LHI) | "It's wondrous out here, with
Engineer/Writer/Double Naught Spy | treasures to satiate desires, both
Chairman, Tar Heel Scanner/SWL Grp| subtle and gross. But it's not
ARRL Life; QCWA; Raleigh ARS; NRA | for the timid."- "Q" on the universe
W8JI Tom
In article <54u1js$i...@redstone.interpath.net>, cphi...@pobox.com (Curt
Phillips) writes:
> Since each of these two lines
>are "totally isolated" from each other (within the limits of
practicality),
>they are "worlds apart" and their impedance is correspondingly high.
>
> Maybe. :-)
>
>
Nope, not at all. It "sees" the surge impedance total of both lines. Thank
goodness, because it's damn hard to feed the infinite impedance you're
trying to create!
Anyway, the two line are not totally isolated from each other, because the
shields are common.
Now here's the kicker for all the "it takes two wire" enthusists. A single
wire has finite surge impedance without any second conductor. So even a
single wire floating out in space isn't an infinite impedance transmission
line. Come to thing of it, neither is space itself.
73 Tom
altavoz
altavoz: At the risk of wasting NG bw , I must thank you for
this tutorial , its one of the best i've seen , THANK YOU.
SIMPLE,CLEAR,WELL STATED...
--
______End of text from altavoz___________
altavoz
altavoz: Well stated , TOM .
I like the one in the ARRL HB explaining WG as balanced
line supported on stubs and where everyone gets confused is
where the H field of the stubs suddenly points in the
direction of propagation.
Single connductor xmission lines (WG) must have at least one
( H or E field) pointing in the direction of propagation.
All coax and twin lead in fund mode has both H,E
transverse "Tem"
Roy Lewallen
In article <54rdmc$b...@redstone.interpath.net>,
cphi...@pobox.com (Curt Phillips) wrote:
> The theory that I read said that, with the coax shields grounded and
tied
>together, that the "twin lead conductors" (i.e., the two center conductors
>of the two pieces of coax) would appear (electrically) to be infinitely
far
>apart. Therefore, the impedance would be the same as a ladder line or
>twin-lead with, say 20 foot spacing between their conductors. Therefore,
>it seems as though they were claiming an impedance of 1000 ohms, or
>something in that range... something appropriate for "ladder line" with
>extremely wide spacing.
>. . .
With the coax shields tied together, the center conductor-center conductor
impedance is twice the impedance of a single coax line. In this case, the
two cables are effectively in series. If you connect the center conductors
together, the impedance from the center conductors to the shield is 1/2 the
impedance of a single coax line. Here, they're in parallel.
Roy Lewallen, W7EL
Curt Phillips
w8j...@aol.com (W8JI Tom) wrote:
>In article <54rdmc$b...@redstone.interpath.net>, cphi...@pobox.com (Curt
>> The theory that I read said that, with the coax shields grounded and tied
>>together, that the "twin lead conductors" (i.e., the two center
>conductors
>>of the two pieces of coax) would appear (electrically) to be infinitely far
>>apart. Therefore, the impedance would be the same as a ladder line or
>>twin-lead with, say 20 foot spacing between their conductors. Therefore,
>>it seems as though they were claiming an impedance of 1000 ohms, or
>>something in that range... something appropriate for "ladder line" with
>>extremely wide spacing.
>The center conductor only interacts with the shield, and the shield
>interacts with the outside world. If the center is excited and the shield
>is floated, the effect is almost exactly the same as if you were feeding
>the shields. So if the shields float the line looks FAT and as if it had
>the physical spacing of the shields.
>If the shields are connected at each end normal transmission line type
>currents (differential mode) cancel on the outside of the shields, all
>transmission current stays inside each cable, and the impedance is the sum
>of the two cables. Loss remains fixed at the value of one cable alone. (If
>the SWR is unity in both cases.)
One reason people talk about using two coax cables in this way is because
of the "requirement" that ladder-line be kept away from other metal.
Sometimes in routing the ladder-line into your shack, close proximity to
metal is hard to avoid.
This "twin coax" arrangement has been touted as a good way to get around
that. In other words, run ladder-line where you can, switch to "twin-coax"
to get through the trouble areas, then connect into the antenna tuner or
switch back to ladder-line for the remainder of the run (if it is
substantial). It would be an interesting exercise to calculate the
resulting impedance.
From using ladder line fed antennas in far from optimal conditions, I
suspect that this is not as important or crucial as some people may say.
Perhaps if you want the pattern of the dipole to be exactly according to
the book, but then it needs to be at least 1/2 wave high, etc, etc.
In my experience, a good antenna tuner can 'chew up' the results of a few
"less than optimal" conditions. After all, Kurt Sterba has loaded a couple
of shopping carts with his. Of course, the effectiveness and radiation
pattern might be hard to predict.
So I guess a good maxim (or Murphy's 25th law) is:
A good antenna tuner cures all ills. :-)
Thanks to everyone who has contributed to this thread. It has been
interesting and educational. (BTW, I'm a ME, not an EE. :-)
========= Opinions expressed are solely those of the author ========
Curt Phillips, CEM KD4YU (ex-WB4LHI) | Hey, Bill & Hillary!
Engineer/Writer/Hurricane Fran survivor| Where's that middle class
Chairman, Tar Heel Scanner/SWL Group | tax cut you promised
ARRL Life; QCWA; Raleigh ARS; RCMA; NRA| us in 1992?
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