In message <YEXtJ$JYB1$
fFwNF@b-howie.co.uk>, Brian Howie
<
brian@b-howie.co.uk> writes
I was looking around the Internet for some ideas for Top-Band/472KHz
vertical antennas and came across these articles.
Http://alancordwell.co.uk/Legacy/radionavigation/hifix/hifixaerial.html
http://alancordwell.co.uk/Legacy/radionavigation/hifix/reminiscences.html
Hifix was a navigational aid on LF. It was the "jingle bells" noise on >Top-band. They were dotted around the coast. I found the disused
installation at Corswall Point in Galloway a couple of years ago.
It seems a contributor to uram Ian Jackson G3OHX has been here before
and built copies of the antenna,
It is a bit unusual in that not only is there a capacitive top hat, but >another one about halfway up.
Ian points out it that it is not good for skywave ; an inverted L is
better.
I modelled the antenna as is and there is not much radiation upwards,
The elevation angle is about 29 degrees on a typical real ground.
The antenna impedance is about 2,5 -202j ohms and needs about 17uH to
tune it. You need good grounding to get the efficiency up.
The same antenna is 0,14 - 1200j on 472KHz, so the efficiency will
really take a hit, It would actually be a pretty decent low angle
antenna on 80m
I removed the middle capacity hat and found it didn't have much
effect, so a simplification is possible.
Maybe Ian would like to comment on what he found.
Brian GM4DIJ
Not much to add, Brian - except that in my version of the Hifix aerial,
I didn't use an earth. Instead, I used a single 'other half of the
dipole' off-the-ground counterpoise wire. I only used it two or three
times, using the mobile gear in my old Ford Prefect (parked at the base
of the pole), with an ATU feeding the bottom of the pole against the counterpoise.
My aerial was a 28' aluminium pole with a 12' whip on top - making
around 40' vertical. The bottom of the pole was insulated from earth by standing it in a glass jamjar or a plastic bucket.
There were four polypropylene guys from the top of the pole to four
equally spaced pegs in the ground. The four 'top hat' wires were
connected to the top of the whip, and ran to the same four pegs (with,
of course, insulators on the end of each wire).
The counterpoise wire ran from near the base of the pole out to one of
the pegs, then continued to each peg in turn, around the bottom of the
square. I'm not sure how long the counterpoise was, but if the pegs were
(say) 20' from the pole, that would make it a total of around 132' (a
160m quarterwave!).
In case anyone is interested, here are a couple of links to photos from
a 1965 DXpedition to Rutland, which show my car and my Hifix-type aerial (although you can make out very little of it).
https://ibb.co/Kh33TCQ
https://ibb.co/sRgBSJg
As to why the real Hifix aerial had the half-way-up-the-pole top-hat
wires, I suspect that they served no purpose other than being guys - and
if they didn't really affect the electrical performance on 1.9MHz, there
would be no reason to insulate them from the pole. BTW, at Seahouses, I
recall that the original aerial was later replaced by one much taller
(possibly 80' or 90').
-- 1an
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