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