Hi all...

I've got one of these "squid pole" telescopic fibreglass fishing poles
that I presently use for supporting a vertical 20m dipole. Being the
highest point on the property, I thought if I could build a nice
lightweight antenna to go high up the top of it, it'd work well as a
mast for a 2m antenna.

I'm looking for a lightweight antenna, and feedline, suitable for this
purpose. I wish to operate around 144.300MHz USB... so horizontal
radiation pattern would be better. I get heard perfectly well on a
quarter-wave vertical at present, so I think the gain of a loop will
be sufficient... but a bit extra height would help a great deal.

The full-wave loop looks almost ideal... nice and simple, apparently
good gain over a dipole for an omnidirectional antenna. I have some
300ohm ribbon I can feed it with (RG58 makes the pole bend over... and
of course anything low-loss will be unworkable).

The catch is, I'm not sure how to go about impedance matching, with
the loop having a theoretical impedance (so I hear) of 100ohms, the
ribbon being 300ohms, and a radio that expects 50ohms. I'm not sure
how the feedline characteristic impedance will impact on things, being
300ohm... there's no such thing as 50ohm ribbon in the shops AFAIK.
I'm not sure this has to match though. The antenna itself though,
does need some sort of transformation back to 50ohms.

Giving it some thought... another way to achieve this would be to use
two loops in-phase. 100 ohms in parallel with 100 ohms gives 50
ohms. I'm just not certain of the spacing, and how close they'd need
to be. I've got the wire cut to make two loops (no idea what gauge...
approx 1.5mm diameter solid copper) to a length of 2.08m.

I'd like to know if anyone has an idea how far apart these need to be
(vertically) on the pole so that they work together in-phase (or
alternatively, 180° out-of-phase... I just swap the connections on one
loop... I suspect this will halve the required distance). I could
just experiment, but if someone's been there done that, then I'd
appreciate their insight.

Regards,
Stuart Longland VK4MSL

Hi Stuart,

"Stuart Longland (VK4MSL)" wrote:
The catch is, I'm not sure how to go about impedance matching, with
the loop having a theoretical impedance (so I hear) of 100ohms, the
ribbon being 300ohms, and a radio that expects 50ohms. I'm not sure
how the feedline characteristic impedance will impact on things, being
300ohm... there's no such thing as 50ohm ribbon in the shops AFAIK.
I'm not sure this has to match though. The antenna itself though,
does need some sort of transformation back to 50ohms.

One trick I saw on a ham's web site (I can't find the URL sorry) was to
make home-made ribbon cable, running conductors through regular spacers.
That could be a good option for you simply because you can make it super
light-weight, but it also gives you flexibility with the spacing and
therefore the impedance.

I'm not qualified to say how loop antennas are normally matched, but I
thought that particular idea might be helpful in your case.

Long time no chat! Cheers,

Tom VK7NML

--

SDF Public Access UNIX System - http://sdf.lonestar.org

On Sat, 25 Sep 2010 14:57:41 +0000 (UTC), Thomas Karpiniec
wrote:

Hi Stuart,

"Stuart Longland (VK4MSL)" wrote:
The catch is, I'm not sure how to go about impedance matching, with
the loop having a theoretical impedance (so I hear) of 100ohms, the
ribbon being 300ohms, and a radio that expects 50ohms. I'm not sure
how the feedline characteristic impedance will impact on things, being
300ohm... there's no such thing as 50ohm ribbon in the shops AFAIK.
I'm not sure this has to match though. The antenna itself though,
does need some sort of transformation back to 50ohms.

One trick I saw on a ham's web site (I can't find the URL sorry) was to
make home-made ribbon cable, running conductors through regular spacers.
That could be a good option for you simply because you can make it super
light-weight, but it also gives you flexibility with the spacing and
therefore the impedance.

I'm not qualified to say how loop antennas are normally matched, but I
thought that particular idea might be helpful in your case.


Chapter 17 of the 1962 ARRL Handbook describes how to homebrew
balanced tuners for VHF. I use the 6M version with either a 6M Sterba
curtain or a fullwave 160M loop, works FB. I also built a 2M version
that works well but doesn't get much use as I don't use that band very
much.

73 de n4jvp
Fritz


Long time no chat! Cheers,

Tom VK7NML

Hi Stuart,

"Stuart Longland (VK4MSL)" wrote in news:010bf74f-
:


Hi all...

I've got one of these "squid pole" telescopic fibreglass fishing poles
that I presently use for supporting a vertical 20m dipole. Being the
highest point on the property, I thought if I could build a nice
lightweight antenna to go high up the top of it, it'd work well as a
mast for a 2m antenna.

Are you intending putting the 2m antenna on top of the pole carrying the
20m vertical dipole? Do you think they might interact?


I'm looking for a lightweight antenna, and feedline, suitable for this
purpose. I wish to operate around 144.300MHz USB... so horizontal
radiation pattern would be better. I get heard perfectly well on a
quarter-wave vertical at present, so I think the gain of a loop will
be sufficient... but a bit extra height would help a great deal.

Not only light weight, but low wind resistance I should think.


The full-wave loop looks almost ideal... nice and simple, apparently
good gain over a dipole for an omnidirectional antenna. I have some
300ohm ribbon I can feed it with (RG58 makes the pole bend over... and
of course anything low-loss will be unworkable).

The catch is, I'm not sure how to go about impedance matching, with
the loop having a theoretical impedance (so I hear) of 100ohms, the
ribbon being 300ohms, and a radio that expects 50ohms. I'm not sure
how the feedline characteristic impedance will impact on things, being
300ohm... there's no such thing as 50ohm ribbon in the shops AFAIK.
I'm not sure this has to match though. The antenna itself though,
does need some sort of transformation back to 50ohms.

The common '300 ohm' TV ribbon is not exactly 300 ohms, or necessarily
close to it. You can expect a loss of about 1dB for a 30m run of matched
TV ribnon on 2m.

One solution to 'match' the load to the line is a quarter wave
transformer... look it up in a handbook or Google for it. It is
relatively easy to construct a quarter wave line section of arbitrary
impedance on 2m.

At the other end, you are still faced with the challenge of transforming
300 ohm balanced to 50 ohm unbalanced. You could use the combination of a
4:1 half wave balun using coax (50:200) and another quarter wave
transformer to 300 ohms.

The thing is that you might not be able to simply design the sections
this simply, you may need to make an adjustable impedance transformer of
some kind at the tx end. You could home brew a little ATU, or use
something like a stub tuner. Even if the TV ribbon operates at VSWR=3,
the loss should be acceptable for a 30m run.

This project throws you into the deep end, a project to drive learning
about and understanding transmission lines.

There are a lot of different potential solutions, the above is one and
others will have other ideas.

Owen

Hi all...

Thanks for the suggestions thus far...

Regarding the tuners in the ARRL handbook... I've got the 1975 edition
here, I'll have a look there.

On Sep 26, 6:47*am, Owen Duffy wrote:
Hi Stuart,

"Stuart Longland (VK4MSL)" wrote in news:010bf74f-
:

Hi all...

I've got one of these "squid pole" telescopic fibreglass fishing poles
that I presently use for supporting a vertical 20m dipole. *Being the
highest point on the property, I thought if I could build a nice
lightweight antenna to go high up the top of it, it'd work well as a
mast for a 2m antenna.

Are you intending putting the 2m antenna on top of the pole carrying the
20m vertical dipole? Do you think they might interact?

I think there probably will be some... but hopefully not a serious
problem since only one will be driven at a time. At the moment I've
got a quarter-wave groundplane and a TV-antenna in _very_ close
proximity (about 40mm apart) so surely any interference I get with the
20m vertical will be minor compared to what I (and the TV viewers at
my QTH) already put up with.

I'm looking for a lightweight antenna, and feedline, suitable for this
purpose. *I wish to operate around 144.300MHz USB... so horizontal
radiation pattern would be better. *I get heard perfectly well on a
quarter-wave vertical at present, so I think the gain of a loop will
be sufficient... but a bit extra height would help a great deal.

Not only light weight, but low wind resistance I should think.

Indeed.

The full-wave loop looks almost ideal... nice and simple, apparently
good gain over a dipole for an omnidirectional antenna. *I have some
300ohm ribbon I can feed it with (RG58 makes the pole bend over... and
of course anything low-loss will be unworkable).

The catch is, I'm not sure how to go about impedance matching, with
the loop having a theoretical impedance (so I hear) of 100ohms, the
ribbon being 300ohms, and a radio that expects 50ohms. *I'm not sure
how the feedline characteristic impedance will impact on things, being
300ohm... there's no such thing as 50ohm ribbon in the shops AFAIK.
I'm not sure this has to match though. *The antenna itself though,
does need some sort of transformation back to 50ohms.

The common '300 ohm' TV ribbon is not exactly 300 ohms, or necessarily
close to it. You can expect a loss of about 1dB for a 30m run of matched
TV ribnon on 2m.

Thankfully, I think I'll be lucky if I use more than 10m... 1dB isn't
too bad though, probably better than the equivalent length of RG58.

One solution to 'match' the load to the line is a quarter wave
transformer... look it up in a handbook or Google for it. It is
relatively easy to construct a quarter wave line section of arbitrary
impedance on 2m.

At the other end, you are still faced with the challenge of transforming
300 ohm balanced to 50 ohm unbalanced. You could use the combination of a
4:1 half wave balun using coax (50:200) and another quarter wave
transformer to 300 ohms.

The thing is that you might not be able to simply design the sections
this simply, you may need to make an adjustable impedance transformer of
some kind at the tx end. You could home brew a little ATU, or use
something like a stub tuner. Even if the TV ribbon operates at VSWR=3,
the loss should be acceptable for a 30m run.

By the sounds of things... homebrewing a 2m ATU may not seem so
silly... I did make an air-core autotransformer for use on HF which
I'm able to use to tune a short end-fed vertical (actually, a 6' 27MHz
CB whip) quite successfully on 40m... perhaps the same concept will
work for VHF. I was hoping though, if I could hook two loops in
parallel, that would give me 50 ohms... I guess it'll be time for
experimentation.

This project throws you into the deep end, a project to drive learning
about and understanding transmission lines.

There are a lot of different potential solutions, the above is one and
others will have other ideas.

Yep... A lot of people say "Just buy X brand/Y model antenna"... but I
believe in giving the homebrew route a try first. :-) I live to
learn.

Regards,
Stuart Longland VK4MSL

"Stuart Longland (VK4MSL)" wrote in
:

....
Are you intending putting the 2m antenna on top of the pole carrying
the 20m vertical dipole? Do you think they might interact?

I think there probably will be some... but hopefully not a serious
problem since only one will be driven at a time. At the moment I've
got a quarter-wave groundplane and a TV-antenna in _very_ close
proximity (about 40mm apart) so surely any interference I get with the
20m vertical will be minor compared to what I (and the TV viewers at
my QTH) already put up with.

If you want to learn something, make your experiments simple... and that
means separating your experimental antenna from things that will (and
they surely will) confuse things.

....

By the sounds of things... homebrewing a 2m ATU may not seem so
silly... I did make an air-core autotransformer for use on HF which
I'm able to use to tune a short end-fed vertical (actually, a 6' 27MHz
CB whip) quite successfully on 40m... perhaps the same concept will
work for VHF. I was hoping though, if I could hook two loops in
parallel, that would give me 50 ohms... I guess it'll be time for
experimentation.

Firsly, ask yourself it ATU is a common approach, and if not, why not.

Re the unloaded / base loaded 6'whip on 40m , it is unlikely that it is
very efficient, but there is a lesson there, even inefficient antennas
can be used to make contacts... just less of them. Some people use QSOs
as 'proof' that an antenna 'works', whatever 'works' means.


Yep... A lot of people say "Just buy X brand/Y model antenna"... but I
believe in giving the homebrew route a try first. :-) I live to
learn.

That was what ham radio was about once, a dying part of hobby now.

Some also say "any antenna is better than no antenna at all", but if you
are interested in the technology of antennas, you will do better than
that.

Good luck Stuart

Owen

On Sun, 26 Sep 2010 08:31:20 GMT, Owen Duffy wrote:

"Stuart Longland (VK4MSL)" wrote in
:


By the sounds of things... homebrewing a 2m ATU may not seem so
silly... I did make an air-core autotransformer for use on HF which
I'm able to use to tune a short end-fed vertical (actually, a 6' 27MHz
CB whip) quite successfully on 40m... perhaps the same concept will
work for VHF. I was hoping though, if I could hook two loops in
parallel, that would give me 50 ohms... I guess it'll be time for
experimentation.

Firsly, ask yourself it ATU is a common approach, and if not, why not.


I doubt a noticeable percentage of amateurs use wire antennas with a
balanced feed for VHF in 2010. The antenna being considered isn't
commonly used so a common approach to matching might not provide the
most appropriate solution.

73 de n4jvp
Fritz

Stuart Longland (VK4MSL) Inscribed thus:

The full-wave loop looks almost ideal... nice and simple,
apparently good gain over a dipole for an omnidirectional antenna.
I have some 300ohm ribbon I can feed it with (RG58 makes the pole
bend over... and of course anything low-loss will be unworkable).

The catch is, I'm not sure how to go about impedance matching, with
the loop having a theoretical impedance (so I hear) of 100ohms, the
ribbon being 300ohms, and a radio that expects 50ohms. Â*I'm not
sure how the feedline characteristic impedance will impact on
things, being 300ohm... there's no such thing as 50ohm ribbon in
the shops AFAIK. I'm not sure this has to match though. Â*The
antenna itself though, does need some sort of transformation back
to 50ohms.

Regards,
Stuart Longland VK4MSL

Try this url.
http://www.xs4all.nl/~pa0fri/Ant/Quad/quadeng.htm

--
Best Regards:
Baron.

Registered User wrote in
:

....
I doubt a noticeable percentage of amateurs use wire antennas with a
balanced feed for VHF in 2010. The antenna being considered isn't
commonly used so a common approach to matching might not provide the
most appropriate solution.

The problem is Fritz, that the common HF technique of operating the
transmission line at high VSWR and transforming the impedance to that
which suits the transmitter at the transmitter is less appropriate at VHF
due to increased transmission line loss.

At 2m and above, the technique used more often is to use low loss lines
operated at low VSWR, save short sections that might form an impedance
transformation function.

That is not to say it can't be done other ways.

Omni directional horizontal antennas aren't very popular on 2m except for
the odd beacon and mobile station, full wave loops aren't very often the
choice in that case anyway. Nevertheless, the project is a vehicle for
learning.

Owen

"Owen Duffy" wrote in message
...
Registered User wrote in
:

...
I doubt a noticeable percentage of amateurs use wire antennas with a
balanced feed for VHF in 2010. The antenna being considered isn't
commonly used so a common approach to matching might not provide the
most appropriate solution.

The problem is Fritz, that the common HF technique of operating the
transmission line at high VSWR and transforming the impedance to that
which suits the transmitter at the transmitter is less appropriate at VHF
due to increased transmission line loss.

At 2m and above, the technique used more often is to use low loss lines
operated at low VSWR, save short sections that might form an impedance
transformation function.

That is not to say it can't be done other ways.

Omni directional horizontal antennas aren't very popular on 2m except for
the odd beacon and mobile station, full wave loops aren't very often the
choice in that case anyway. Nevertheless, the project is a vehicle for
learning.

Owen

I think you have hit on the solution. For VHF and above the antennas are
usually resonant and the bandwidth is small enough you can match the antenna
at one frequency and the swr will be low across the band. There has been an
improvement in the coax over the years also. The 9913/LMR400 types have
about half the loss at vhf and above as the older rg-8 type.

While it is hard to put up, the G-line may be something to think about at
450 and above if one does want to use something other than coax/hardline.

Most ssb/weak signal stations operate horizontal on vhf. While you may be
able to have some fun with a small antenna, a 5 element or more beam will
surely help. I did make a contact of several hundred miles on 432 with a
1/4 wave vertical on ssb one time, but that was a rare condition. My
rotator for the beam was locked up and the beam was in the wrong direction.
Just tried the vertical for fun and it did make a couple of contacts.