At 04:52 PM 29/01/97 +0000, Kristinn, TF3KX wrote:
>My short/low horizontal wire didn=B4t do a great job
>during the CQ 160/CW last weekend. My next plan was
>to put up a half wave horizontal wire at 15m/50ft
>for the upcoming topband/SSB and the ARRL DX contests.
>
>Several of you guys have advised me against a
>horizontal-only antenna for 160 and suggested an
>inverted-L. Now, the only practical way for me to
>feed the inverted-L from my attic shack location
>is into the horizontal part (the very end or
>somewhere off the end). I would appreciate any
>comments on this, including the questions:
>
>1. Has anyone you know done this?
>
>2. What do I do with the bottom end of the vertical
> part? I would think it should be connected
> directly to the radial wires I would bury in the
> ground. An alternative is leaving the wire just
> suspended a little above the ground, but I suspect
> this will give me low current in the vertical
> section - and correspondingly low radiation there.
>
>3. Any comments on how to feed the horizontal part?
> Tuning out reactances, etc.
>
>4. Any comments on the desired length of each
> section? I guess most would say "the longer the
> vertical part, the better". I would be stuck with
> about 15m/50ft vertical and approx. 25m/82ft
> horizontal to my shack window.
>
>5. I have space to add another top section of almost
> any length, resulting in a "T" rather than the
> inverted "L". Both US/VE and EU would be broadside
> to this "T" (or the horizontal part of the "L").
> Any opinions on the performance of "T" vs. "L"?
> Again, any length suggestions for that one?
>
>6. And finally there is one thing that concerns me a lot.
> Is there an RF hazard with the inverted-L? I can=B4t
> see the vertical leg from my shack and there is no
> way to guarantee that kids won=B4t be curious and tug
> the wire. I would assume that if I can guarantee that
> the part of the wire that is within reach is at
> maximum current, the voltage will be minimal and
> probably harmless. I could also insulate the wire
> and hope it holds during wet weather conditions, etc.
>
>Thanks and appreciations
>73 de Kristinn, TF3KX (kiddi@marel.is)
>
>
Hi Kristinn!
Your idea is simple and appealing, however the performance of such an
antenna is likely to be disappointing because:
1. high RF currents will flow into your rig, house wiring, microphone,
keyer, etc., causing everything in the shack to be "hot";
2. there will be a significant loss of radiated power due to RF currents
flowing in the house wiring, and the RF field passing through the house
wall, both of which are lossy;
3. the antenna is likely to be noisy on receive, due to its tight coupling
to the house wiring and electrical appliances in general.
My suggestion is to use a T antenna, which will cancel out the horizontal
component and maximise the vertical component of radiation.
If you can fit it in, the flat top should be around 130m long (adjust for
resonance), with the vertical part in the middle. This would make the
antenna a half wave long, from the bottom to either end. The impedance at
the bottom end of the T will be very high and resistive, making it easy to
feed at this point through a parallel tuned circuit, with the coax tapped a
few turns up, and the bottom end grounded. You could switch it for 80 and
40, and even 20 if you are willing to sacrifice a small amount of the
vertical pattern.
With this antenna, you can use a modest system of buried radials, without
incurring significant ground loss. An advantage is that the high current
point on 160 is at the top, above most obstructions. Such an antenna was
described in QST a couple of years ago, and although I have the article
somewhere, naturally I can't seem to find it!
If the flat top must be shorter, then you are faced with a non-resonant
antenna, and the complications of loading. I suggest a loading coil at the
top of the vertical part, with link coupling to the coax. This will confine
the antenna currents to the antenna, and prevent them flowing on the outside
of the coax, minimising any additional RF loss. The antenna will also be
quieter on receive, because noise from the house will be unable to flow
along the outside of the coax into the antenna.
For tuning, you could fit a small high voltage tuning capacitor in parallel
with the secondary of the loading coil, although this should not be overdone
because the coil losses are increased due to the higher circulating current.
With such an antenna, the bottom end is ungrounded of course. It will be at
a high RF potential, so you would need to find a way to prevent people
touching the wire. A high value resistor from the primary to the secondary
of the loading coil should be fitted as a static bleed. The coax should come
away at right angles, as much as possible. Unfortunately, multibanding would
be very difficult, but then you can't win them all!
A final suggestion applies to any antenna. I find that clip-on ferrite
"beads", fitted to the coax at several points, go a long way towards
minimising stray RF on transmit and noise on receive.
Hope the above is of some use, and good luck with your antenna experiments.
73s,
Peter VK3APN
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