Semi-related: FM Dipole Total Length

[thekma...@gmail.com]

Semi-related: FM Dipole Total Length:


I recently took to building a simple FM
dipole antenna out of 20gaugd speaker
wire, and aside from one issue, have
had generally satisfactory results.


That issue? The length of the dipole -
the horizontal section that does the
actual receiving! Most on-line calculators
for this sort of thing suggest a total
length of around 4 1/2 - to 5'. So
I carefull built and hung it, and was
consistently able to receive most
stations in my area ABOVE 98mHz
on the FM dial.


One site's formula led me to build
a dipole over 6'(!) which improved things
dramatically in the lower half of the
band. Reception of stations 99mHz or
higher did not seem to be affected.

My old(broken) retail dipole was nearly
6' in length overall, and actually still
works fine, despite one end being
separated and just tied onto the vertical
pair(!!)


Question: Why would the majority of
antenna building sites recommend
such a short overall dipole length if
it pulled in only the upper half of the
FM band reliably?

[analogdial]

thekma...@gmail.com wrote:


>
> Question: Why would the majority of
> antenna building sites recommend
> such a short overall dipole length if
> it pulled in only the upper half of the
> FM band reliably?

How long are the antennas on your other FM radios?

[Michael Black]

It's not a precise thing, since the FM band is 20MHz wide.

I suppose there is different philosophy about it, and maybe one side
thinks that "in the middle" makes more sense than measuring to the top or
bottom half. If it makes no difference, then no fussing needed.

YOu can get fancier. You can make something with wire that isn't too long
but which gives gain. Magazine articles used to detail this.

Michael

[thekma...@gmail.com]

Michael Black wrote: "I suppose there is different philosophy about it, and maybe one side

thinks that "in the middle" makes more sense than measuring to the top or
bottom half. If it makes no difference, then no fussing needed. "

Exactly: Most of the build-your-own sites all
base their measurements of the main(horizontal)
element on the middle of the FM dial(98mHz).
That ususually results in the dipole being
between 4.5 to 4.8 feet wide. One site works
that out to just over 6ft, which has given me the
best performance yet across the whole band.

I really wanted to build this thing out of 300
ohm twin-lead, but refuse to buy it because
it is available only online. No electronics or
electrical retailer or hardware store physically
stocks it! :(

[thekma...@gmail.com]

analogdial:


My other radios are portable, with whip
antenna. Most are just under 3ft fully
extended.

The dipole project is for a stereo receiver
with speaker style push connectors for
external aerials.

[analogdial]

Antenna length shouldn't be critical with a receiver. I've cut dipoles
to exact resonance with a grid dip meter and used clip leads and random
lengths of wire. Resonant antennas do work better but a healthy
reciever in a good signal area has enough gain to make up the
difference.

Any dipole cut with the usual formulas should work well enough with a
normal receiver. You might have a problem with your radio. If you're
OK with how it's working with the longer dipole, you don't have to worry
about it.

Does your radio have a S meter?

[analogdial]

thekma...@gmail.com wrote:


> The dipole project is for a stereo receiver
> with speaker style push connectors for
> external aerials.

Does your radio have a 300 ohm antenna input? A folded dipole will give
a better impedance match than a simple dipole.

You can convert your simple 60" dipole to a folded dipole by
connecting another 60" wire to each end of the simple
dipole.

To keep things simple, use a transmission line some multiple of 60" if
you aren't using 300 ohm twin lead.

[thekma...@gmail.com]

analogdial:

Folded dipole already. I just built one 80"
wide, and am starting to lose stations!! I'll
probably put the 76"wide one back in a
couple of days if things don't improve.

My vertical(feed) is also very long - 6 or
more feet. Should I shorten that to just
reach the back of the stereo receiver,
plus some slack if I need to pull the
rack out?

A friend who repairs vintage electronics
suggested replacing the speaker wire
vertical feed with 300ohm twin-lead for
better separation down to the receiver
connection. Problem is you can't buy
twin lead by walking into a store any
more! It's so rarely used it's an online
only item now.

[Michael Black]

On Sat, 25 Mar 2017, thekma...@gmail.com wrote:

There was a time when people made their own 300ohm line, though
it was open-line, two wires with spacers in between.

I did a search and this is an article I remember that was in Audio
Magazine:

http://home.earthlink.net/~schultdw/antenna/PlainWireFancyReception.html

It doesn't add much to what's been said about the simple dipole, but I
made and used the Collinear antenna mentioned. I think it helped, but I
can't remember. There were some stations that were not quite distant, and
I think it helped. I just had it tacked to the wall at ceiling level, and
I guess it fell down one time and I stopped using it.

As for not being fussy, I actually use a dipole I put up for FM reception
for TV reception. It works fine for local channels, and I can PBS and
NBC. It's just by chance, but orientation may mean I lose some channels.
On the other hand, at the higher channels it can be multiple wavelengths.

Michael

[analogdial]

thekma...@gmail.com wrote:


> My vertical(feed) is also very long - 6 or
> more feet. Should I shorten that to just
> reach the back of the stereo receiver,
> plus some slack if I need to pull the
> rack out?
>
> A friend who repairs vintage electronics
> suggested replacing the speaker wire
> vertical feed with 300ohm twin-lead for
> better separation down to the receiver
> connection. Problem is you can't buy
> twin lead by walking into a store any
> more! It's so rarely used it's an online
> only item now.

A 6 foot lead in shouldn't be a problem with any normal sort of wire.
The simple answer is that lead ins at an even multiple of a quarter
wavelength (like 1/2 wave) won't change any impedances but, if there's a
mismatch, a lead in at an odd multiple of a 1/4 wave (like 1/4 or 3/4
wave) might change things. 60" is close enough to a 1/2 wave in the FM
band that you don't have to worry about the lead in impedance making a
difference. A foot or two shorter or longer might make a very small
difference, if you feel like experimenting.

I think I've seen the twin lead folded dipoles for FM still stocked at
places like Wal Mart but they were overpriced, especially if you have
some of your own wire to use.

[George Cornelius]

In article <63e2ba46-df63-4d19...@googlegroups.com>, thekma...@gmail.com writes:
> Michael Black wrote: "I suppose there is different philosophy about it, and maybe one side
> thinks that "in the middle" makes more sense than measuring to the top or
> bottom half. If it makes no difference, then no fussing needed. "
>
> Exactly: Most of the build-your-own sites all
> base their measurements of the main(horizontal)
> element on the middle of the FM dial(98mHz).
> That ususually results in the dipole being
> between 4.5 to 4.8 feet wide. One site works
> that out to just over 6ft, which has given me the
> best performance yet across the whole band.

The hams have used this formula for years for
length in feet of a HF dipole:

length = 468 / f

This is less than a half wave in free space
because it includes a correction of maybe 5%
for something called velocity factor. If you
are making a folded dipole (needed for 300 ohm
inputs) this may have to be modified a bit, and
could even need modification depending on the
insulation used and the wire diameter.

For 100 Mhz the formula gives 4.68 feet; for 88
it gives 5.32 feet.

> I really wanted to build this thing out of 300
> ohm twin-lead, but refuse to buy it because
> it is available only online. No electronics or
> electrical retailer or hardware store physically
> stocks it! :(

Bizarre.

But it doesn't matter if you use twinlead. Bare
wire/tubing is even better.

By the way, are you going into 300 ohm inputs?
If you are not impedance matched then you can
have transmission line length issues, with some
line lengths effectively nulling out one station
or another. [If you can't get 300 ohm transmission
line, what _are_ you using to attach to the radio?]

And indoor antennas have problems with vapor
barriers that are required for the use of
air conditioning. My home has insulation with
a metal vapor barrier on the interior face, so
as you can imagine, my only hope for over the
air TV reception from inside is to aim through
a window which has had the metal screen removed.

Also, a horizontal antenna will be directional.
If not resonant, or too close to reflecting/
conducting structures, it may not be directional
in ways that you can predict. Be sure that your
problems are not caused by this, and if they are,
consider a vertical (ground plane? 5/8 wave?)
antenna and a 75:300 ohm transformer/balun.

Finally, in my part of the US at least, the
noncommmercial and low power stations tend to
be clustered at the low frequencey end. Check
with the broadcasters to see what their effective
radiated power is in your direction from their
towers. If necessary, build your antenna for the
weakest stations you want to receive because a
strong station will come in OK regardless.

George

P.S. Plain old rabbit ears may just do the trick
since, after all, the FM band is stuffed right
in between VHF TV channels 6 and 7. In addition,
these are adjustable. They're often used without
a transformer but you may want one to upconvert
to 300 ohms into the transmission line if you
are just using it as a tuned half wave dipole.

[thekma...@gmail.com]

George Cornelius:

Thanks for the information!

1. Presently, my folded dipole is all 20AWG
speaker wire. It is connected to a 300ohm
to 75ohm transformer

http://cdn3.volusion.com/zcgcs.pctzl/v/vspfiles/photos/28-2035-2.jpg?1457181788

that connects to the round FM port on the back of my
JVC stereo receiver.

The horizontal folded dipole itself is 80"
wide, and my friend recommends I
replace the vertical section of speaker
wire with old fashioned 300ohm
twin lead, for the sake of separating
those vertical lines going down to the
receiver. He said the speaker wire
vertical feed is probably cancelling
out as many stations as the dipole
is pulling in.

He also said he never heard of any
advantage of a folded dipole(a loop)
over a simple dipole(basically a
single piece of twin lead able to
reach at least 5 feet above the rack,
then split horizontally and be tacked
as straight as possible against
the wall).

[thekma...@gmail.com]

468/98mHz = 4.72 feet.

Is that total dipole width or one
side, from vertical section to tip?

[analogdial]

thekma...@gmail.com wrote:

> George Cornelius:
>
> Thanks for the information!
>
> 1. Presently, my folded dipole is all 20AWG
> speaker wire. It is connected to a 300ohm
> to 75ohm transformer

Have you tried a different transformer? Your transformer may be wonky.

As an experiment, you can also try a simple dipole connected directly to
the input connector, one wire to the connector's ground, one wire to the
center connector. A simple dipole will be a good impedance match with
the radio.

>
> http://cdn3.volusion.com/zcgcs.pctzl/v/vspfiles/photos/28-2035-2.jpg?1457181788
>
> that connects to the round FM port on the back of my
> JVC stereo receiver.
>
> The horizontal folded dipole itself is 80"
> wide, and my friend recommends I
> replace the vertical section of speaker
> wire with old fashioned 300ohm
> twin lead, for the sake of separating
> those vertical lines going down to the
> receiver. He said the speaker wire
> vertical feed is probably cancelling
> out as many stations as the dipole
> is pulling in.

Does he give an explaination for his opinion? If I recall, you've said
your lead in is 60". That is close enough to a half wavelength to make
whatever lead in impedence your wire has non-critical.

If you want to experiment, make your own seperators out of cardboard or
whatever and see what difference it makes.

Or just buy the 300 ohm twin lead to get your answer.

>
> He also said he never heard of any
> advantage of a folded dipole(a loop)
> over a simple dipole(basically a
> single piece of twin lead able to
> reach at least 5 feet above the rack,
> then split horizontally and be tacked
> as straight as possible against
> the wall).

The gain of either of the dipoles is the same but the folded dipole
typically works well over a wider band of frequencies.

[George Cornelius]

In article <a8394f83-8be4-4a93...@googlegroups.com>, thekma...@gmail.com writes:
> George Cornelius:
>
> Thanks for the information!
>
> 1. Presently, my folded dipole is all 20AWG
> speaker wire. It is connected to a 300ohm
> to 75ohm transformer
>
> http://cdn3.volusion.com/zcgcs.pctzl/v/vspfiles/photos/28-2035-2.jpg?1457181788
>
> that connects to the round FM port on the back of my
> JVC stereo receiver.

That would be a 75 ohm coaxial cable connector.

> The horizontal folded dipole itself is 80"
> wide, and my friend recommends I
> replace the vertical section of speaker
> wire with old fashioned 300ohm
> twin lead, for the sake of separating
> those vertical lines going down to the
> receiver. He said the speaker wire
> vertical feed is probably cancelling
> out as many stations as the dipole
> is pulling in.

Probably not. A transmission line that
is matched at both ends (same ratio of
volts to amperes, measured in ohms with
1 ohm being a 1:1 ratio) - i.e., the cable
matching both the source and the load -
has very little loss. It goes up some
with a mismatch such as your cable has
(I suspect it's above 75 ohms and below
300), but it's not a show stopper for
short cables such as what you are
using.

As analogdial has mentioned, cables
close to an odd multiple of 1/4
wavelength should be avoided - even if
it's not loss in the cable specifically
the mismatch will be so bad that it is
basically a tuned circuit that nulls out
your station. I like analogdial's
suggestion of using a cable that is a
half wavelength plus or minus (use 468/f
or slightly shorter), or using a multiple
of that.

Because you are using a balanced feed line
into a balanced antenna, I personally
would put the transformer at the radio
input, but what you are doing may not
really be a problem there, either.

Consider using just an ordinary dipole,
though, and skipping the transformer.

> He also said he never heard of any
> advantage of a folded dipole(a loop)
> over a simple dipole(basically a
> single piece of twin lead able to
> reach at least 5 feet above the rack,
> then split horizontally and be tacked
> as straight as possible against
> the wall).

Not much difference. A folded dipole is useful
for a slightly wider range of frequencies and
is a 300 ohm device instead of a 70 ohm device.

George

P.S. Here's an article from QST magazine
written by one of my favorites, the late, great
Lew McCoy. In the first 1 1/2 pages or so he
covers the basics, then gets into the specifics
of antennas for shortwave bands (15, 40, and 80M)
useful to novice class amateurs. Note: receiving
and transmitting principles are the same, although
you may need thicker wire and better insulators
when transmitting.

https://www.arrl.org/files/file/Technology/tis/info/pdf/6201025.pdf

[thekma...@gmail.com]

george cornelius wrote: "Consider using just an ordinary dipole,

though, and skipping the transformer. "

Ok - but remember, my receiver has
a round(75ohm?) FM external
connector on the back. I need
that transformed to convert from
twin lead to something that can
be connected to that FM input.

Now, please elaborate on how the
vertical portion of my dipole is a
multiple of the FM waves, so I
can cut it to a length that won't
null out stations.

[George Cornelius]

In article <5e4bc867-820e-4d91...@googlegroups.com>, thekma...@gmail.com writes:
>George Cornelius wrote: "Consider using just an ordinary dipole,

>though, and skipping the transformer. "
>
> Ok - but remember, my receiver has
> a round(75ohm?) FM external
> connector on the back. I need
> that transformed to convert from
> twin lead to something that can
> be connected to that FM input.

I gather that you are not electronics oriented, and since
you have an odd transmission line (I hesitate to call it
twinlead, which has a specific meaning), you're generally
going to have to rig something up, maybe even doing some
soldering. Of course coax all the way to the antenna would
work better - it would actually be an impedance match. You
would still have to attach the coax (RG-6/U) to the antenna
end, say by drilling (and tapping?) a short vertical section
of plastic (1/2" PVC pipe?) for say #6 binder head screws,
with washers, and putting the connections under those -
shield to one side of the antenna, center conductor to
the other.

What are you doing at the moment? If the the transformer
has screw terminals, and those go to the folded dipole,
how are you terminating your feed line, i.e., the zip cord
between the xformer and the set? Looks like you already
have the same problem, and at both ends, no less.

I'm assuming you are not using the xformer backwards!

Anyway, you do not have to abandon the folded dipole
idea, but if you can't get 300 ohm twinlead it's a
way to get to a solution.

> Now, please elaborate on how the
> vertical portion of my dipole is a
> multiple of the FM waves, so I
> can cut it to a length that won't
> null out stations.

a-d suggested you make the feed line five feet, but says
six is within reason, The same formula applies: 468/f
for a half wavelength.

Note that the feed line is _not_ part of the antenna.

And if it were properly matched (300-300-300 or 75-75-75),
length would not matter.

For a dipole of either type you cut it so the completed
antenna is 1/2 wavelength measured horizontally - see
the article. So each side is 234/f .

George

P.S. I'm beginning to think your zip cord wires are too
close together for a folded dipole, which you generally
expect to have a gap between the upper and lower portions,
and that this explains the strange behavior. Another
reason to move away fom the antenna you built and try
some other variant.

[thekma...@gmail.com]

George Cornelius wrote: "Note that the feed line is _not_ part of the antenna. "

That's what I always thought, so I
figured it's length(height) did not
matter. I think I'm just wayyyy
over-complicating this matter, and
should go with a simple zip-cord
dipole. Like I said, the old one
worked great, even with one half
broken off and tied around the
feed line. It was even pulling in
stations when it was detached from
the stereo receiver and sitting
in a pile in the corner!

[analogdial]

thekma...@gmail.com wrote:

> George Cornelius wrote: "Note that the feed line is _not_ part of the antenna. "
>
> That's what I always thought, so I
> figured it's length(height) did not
> matter. I think I'm just wayyyy
> over-complicating this matter, and
> should go with a simple zip-cord
> dipole. Like I said, the old one
> worked great, even with one half
> broken off and tied around the
> feed line.

I wasn't paying attention to that before but that indicates your radio
is OK and you don't have oddball unseen problems like wiring or pipes in
the wall which might be causing trouble.

You could try just sticking a couple of feet of wire into the center of
your coax jack just to see what you get. No lead in or transformer to
worry about.

If you have some coax with the right connector, you can use that
for your lead in. Just connect the two wires of a simple dipole to the
two conductors of the coax. Simple dipole, not a folded dipole, when
using coax lead in. No need for a transformer.

>It was even pulling in
> stations when it was detached from
> the stereo receiver and sitting
> in a pile in the corner!

OK, you lost me on that point.

[thekma...@gmail.com]

analogdial: lost point

The broken dipole was simply THAT
GOOD, it improved reception on the
receiver just by being within 4 feet of
it. It even effected reception of this
disaster I have hooked up right now!

[analogdial]

How paranormal. Perhaps it's powering itself with energy it's sucking
up from WWV time signals.

I was recently reflecting on what a great show Art Bell used to have.
Great callers, great interviews. Any moment could be a WTF moment.

If you want a further explaination for your amazing broken antenna, you
could consider calling George Noory.

His show needs some livening up.

[George Cornelius]

Actually, any metal nearby will affect reception,
so that isn't such an odd statement as it sounds.

But, anyway, thanks for sending me a jpeg. I had it
all wrong in my head about how you were set up, and,
yes, with your F-connector to screw terminal balun
plugged directly onto the connector at the back of
the set you would have a pure vanilla installation
for twinlead and, if a-d is right, with about 1/2
wavelength - say five feet - of parallel feedline,
300 ohm or not, to the antenna, then we may be
able to rule out feed line as an issue and that
leaves the antenna. Shortest path between two
points: put another balun at the far end, tie it
to two 2.5" lengths of ordinary wire, and use
five feet of the speaker wire feed line between
them. You can always figure out how to drop the
baluns and use coax later on. Or to beg borrow
or steal five feet of twinlead from someone.

As I mentioned privately, I have the following old
commercially made FM dipole and am willing to ship it
to you:

64" end-to-end
70" feed line
1.75 oz weight by my little handheld postal scale balance

George

[George Cornelius]

In article <$4ugEV...@eisner.encompasserve.org>, corn...@eisner.decus.org (George Cornelius) writes:
> tie it
> to two 2.5" lengths of ordinary wire,

Oops! 2.5 _feet_ on each side.

[thekma...@gmail.com]

George Cornelius wrote: "Oops! 2.5 _feet_ on each side. "

30". My first dipole was about that
length per side. Got in only the upper
half of the band. Once I got up past
35" per side, I started getting in
stations below 97 FM.

Now I'm up to 80" total, and I get the
whole band in, but moderate static
throughout.

[George Cornelius]

In article <6308bb8e-ef08-4d15...@googlegroups.com>, thekma...@gmail.com writes:

> 30". My first dipole was about that
> length per side. Got in only the upper
> half of the band. Once I got up past
> 35" per side, I started getting in
> stations below 97 FM.
>
> Now I'm up to 80" total, and I get the
> whole band in, but moderate static
> throughout.

Oh, if both antennas behave the same way
then we have to look somewhere else for
the discrepancy.

And you can be sure there is one, or we
have done something astounding: we will
have overturned 100 years of experience
with building dipole antennas.

I found two more commercially made FM
dipoles today, and of course they are
all roughly the same length. One was
tacked to a piece of plastic trim -
basically quarter round - and behind my
old Harmon Kardon 330B receiver, which
I no longer use.

Attaching the antenna and moving the
speakers over from some other sound
system I had been trying out, I fired
it up, and what I found was:

o The public radio station at 91.7 I
had been having trouble with on the
other side of town, and closer to
the tower, came booming in and
read 8 points on the signal strength
meter. So low end of the band was
OK.

o Major stations both at low end and
high end of the band came in at roughly
the same reading.

o Did not go into further detail, say
with more distant stations, but from
just a few minutes of testing, the
standard dipole design is working like
a charm with no preference for any
particular part of the band.

I would start looking for problems at
your receiver end, and if that is not
the problem, put the commercial dipole
outside, into a 4:1 xformer, and use
ordinary CATV coax to feed back into the
house and into your F connector.

I expect that one or the other of these
will tell the story.

George