HF radio antenna - DIY

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Mike Harding

Joined
Oct 20, 2017
Messages
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Location
South of Melbourne, VIC
I was asked for more information regarding this antenna; whilst not ideal this seems to be the best forum for it - maybe we could have a "General Technical" forum or the like?

An off centre fed antenna intended for the Amateur Radio bands but it should work on most (all?) VKS737 and other "HF radio club" frequencies and it will perform *many* times better than a vehicle mounted whip. There is also a less complex version of this antenna, without the TV ribbon, I can post details of if anyone is interested.

----

Modified 80m off centre fed antenna

A few years ago I switched from centre fed to off centre fed (OCF) antennas. If the 80m OCF is fed with a 4:1 balun it produces a capable and flexible antenna which will cover all HF except 30m and some of 15m at better than 3:1 SWR, it is also a capable, though not ideal, antenna on 6m. In addition to its low SWR experience indicates it is a very capable radiator on all bands.

Conventionally OCFs have a balun, which is fed with co-ax, located at the feed-point and two arms of appropriate length attached to the balun. I use a 12m tall Spiderbeam (posh squid pole) to support my bush antennas and whilst it is a robust support it is not sufficiently strong to support a balun at the top. As a quarter wave OCF is nominally 200R at its fundamental frequency it occurred to me that feeding the two arms with 300R TV ribbon cable for the length of the Spiderbeam and putting the balun at its base may be a solution... and it seems to work very well.

Of course once one tunes away from the 80m fundamental the antenna impedance changes significantly and the TV ribbon is no longer a near match nevertheless the antenna continues to work well right across HF. There is some argument about the polarity of signals produced and/or received by an OCF but I suspect it operates in both vertical and horizontal and this is part of the reason for its efficiency - however this is guesswork on my part.

It is a complex antenna in that the currents are not balanced in either the arms or the TV ribbon and adding the TV ribbon required shortening the arms from their calculated length for a conventional OCF to achieve a good SWR so it's clear the TV ribbon is "antenna-ing" in some fashion. One of these days I'll model it.

The antenna comprises:
Two arms, one of 25m, the other 12m
10m of 300R TV ribbon
A 4:1 balun
RG58 to the radio

Invariably, when I erect the antenna in the bush, the arms are in inverted V configuration, I try to get the ends as high as possible although this is often only 2m or 3m - it doesn't seem to make that much difference. Additionally, the longer arm frequently passes across a tree branch again this doesn't seem to affect performance.

The TV ribbon connects to the arms at the apex and drops down 10m where it connect to the balun. RG58 then connects the balun to the radio.

None of the dimensions (arms or TV ribbon) are critical but they do interact and my experiments suggests the above are around optimum for an 80m fundamental. I have used this antenna all across Victoria and the ground (mineralisation) doesn't seem to make much difference. The antenna runs a SWR better than 2:1 across all HF excepting 10MHz and some of 21MHz. A basic antenna tuner will easily produce 1:1 but is not essential.


TV ribbon to antenna arms connection using RCA audio connectors - note strain relief for the arms
1508811244_ribbontoarms.jpg



TV ribbon to the home made 4:1 balun (4:1 baluns are available commercially - about A$80)
1508811283_ribbontobalun.jpg
 
That was an antenna lesson! :)

Does the vertical portion resonate like a G5RV? I have one of those somewhere (not that I even understand how it works!!!) and having experimented with that and a number of homebrew dipoles in the bush I opted for the vertical squid pole version from this page http://www.perite.com/vk7jj/squidpoles.html . Sure it's a bit of a pain changing bands but setting up quick was the good point for me. I have a Terlin whip that developed a fault so on our last trip I relied on putting up the squidpole to get on the Traveller's Net each day and to play at night.
 
Hi Shane

>Does the vertical portion resonate like a G5RV?

An excellent question.

I am sure the vertical feed line does radiate (and therefore receive also) and I think this is why this antenna will often out-perform a conventional dipole. I base this assertion on the interaction between the feedline and the length of the arms - there shouldn't be any... but there is. As mentioned, I should model the antenna and see what that says.

Also keep in mind that because of the unbalanced currents this style of antenna can cause RF feedback issues is some situations, the Yaesu FT857/897 are particularly susceptible in my experience. A simple choke on the co-ax will do wonders.

>I opted for the vertical squid pole version from this page

I'm not a big fan of it.

Keep in mind that a low SWR does not equate to an efficient antenna... if it did dummy loads would radiate well. Without a decent number of radials (minimum four and preferably eight) this kind of vertical is never going to work well and once you make it multiband with a loading circuit you have the issue of what length should the radials be!?

It depends which bands you wish to use: most bush travellers in Oz want 80m, 40m and maybe 20m. The simplest way to do that is with a linked dipole cut for 80m.
Cut two lengths of wire each 20m and attach them to a piece of plastic where they are connected to co-ax back to the radio.
Cut each length again at the 5m and 10m positions.
Organise a lightweight connector for each of those positions - a simple automotive spade male and female is ideal.
Run that from the top of a 7m squidpole velcro strapped to the bullbar and you have a flexible and capable antenna.

Antennas are a never ending subject and there is still some Black Magic involved in them :)
 
Thanks very much for all of that. It's worth a go so I will do it. I have a G5RV at home and it works well. That reminds me i have a G5RV still in box so i will try that out bush as well.
Jaros :Y:
 
My Father liked to go bush and asked me to design him a
portable Antenna for his CB Radio.
AM Only.
I measured off 11 meters of coax,
Put a 259 connector with the feed end 11 meters long from connector to radio.
Stripped off half the coax and also tuned it via the length of the outer braid.
When ever he wanted to use it for a quick QSO, He would just check it over a small tree.
When he was based for a few days, Used a sling shot and a bit of cord and strung it up.
He managed to talk to quit a few people all over Australia with that and also the Kiwi's
got in on the act a few times. LOL
 
The good old days with Gamma matched drain pipes and
Antenna's hidden in the TV Antenna's Guy wires. LOL
.
Just about any conductor really but being coax and just nylon cord,
Lent itself for portability on a motor bike.
All you then need is a tree and a sling shot to hang it up.
.
I was doing that when I was 9 years old.
Also been pushed for my foundation license.
That will be next year. :Y: :Y: :Y:
 
Tathradj said:
The good old days with Gamma matched drain pipes and
Antenna's hidden in the TV Antenna's Guy wires. LOL
.
Just about any conductor really but being coax and just nylon cord,
Lent itself for portability on a motor bike.
All you then need is a tree and a sling shot to hang it up.
.
I was doing that when I was 9 years old.
Also been pushed for my foundation license.
That will be next year. :Y: :Y: :Y:
Click to expand...

I never had those issues to need to hide the antenna, the 9ft stainless whip off the back of the Torana hatchback with a 250W linear was a successful skip rig for me,
then again I was not silly enough to use the linear around town bothering people, I reserved that for going fishing or going country/bush.
Really miss those days, and I actually dont remember where the stainless whip went, hopefully I sold it and it wasnt stolen. LOL

Might give this coax antenna a go if I find another 27mhz SSB radio, take it camping with the kids and look for some "skip" again,
that would spin them out, maybe get a postcard from another user like in the old days.
Actually spoke to the Aussie Antarctica base on one Christmas Eve, drifted in and out for about 15min. :perfect:

ZELLO seems like a cop out to use for 'sudo' radio communication locally or internationally these days.
 
From the pl259 connector to the end of the braid acts as
a ground plane for the inner that is exposed which is the radiator
element.
Buffs can correct me at the moment as I am mentally flat,
I think it is called a co-linear ground plane.
 
What about a "J" pole ribbon type antenna for the 27mhz ?

I literally just remembered I have 3 or 4 AM 27mhz sets sitting in the shed, I had them serviced and tuned but never used,
a mate of mine is a radio tech, but not seen him for years.
 
468 / MHz = 1/2 wave length (ft)
Measure and cut a length of wire to your half wave length answer
Cut the wire in half and then feed each piece of wire with your coax, + one side, - the other
you now have a half wave dipole which can be run vertical, horizontal or even at 45deg... whatever you like
the higher you can get it the better

Ideally any antenna you build you should check the VSWR to be on the safe side, if ya don't and your SWR is out you can cook your transmitter TX transistors!

If propogation is good and your up on top of a hill, with 10 watts on 27Mhz you can work the world!

CQ CQ CQ.. this is BigL looking for a Big Gal, pshshshhhhh, phshshhhhhhhh..... Oh Hi there honey, show us ya money :lol:
 
by the way, a G5RV, Off Center Fed, End Fed or any antenna that is a half wave in length and not voltage fed or connected at its high current point ie... fare smack in the middle as I outlined how to calc above, is a mismatch for 50 ohm and nessitates a matching balun. So all these fangdangled antennas mentioned are useless unless you fully understand what your doing and can even be disastrous for your radio and if using enough power you too!!!
Feeding the middle of a half wave dipole will give you about 72ohm at the feed point... good enough for a safe VSWR less than 2:1. if you can find someone with a SWR Bridge you can easily tune the antenna without any Balun or Tuned length of 300 ohm ribbon, so all you need is the wire and required length of 50 ohm coax to you radio and your good to go. Believe me, a half wave dipole setup like this works very very well

1628313427_51792a77-5c7d-4a92-b11e-ac52c9d9d348.jpg
 
Tathradj said:
From the pl259 connector to the end of the braid acts as
a ground plane for the inner that is exposed which is the radiator
element.
Buffs can correct me at the moment as I am mentally flat,
I think it is called a co-linear ground plane.
Click to expand...
coaxial ground plane its called. A colinear is a type of phased array, they have what looks like coils in the middle, these coils change the phase and in doing so results in gain.... +dB
 

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