[Lf] [Fwd: LF: loop antenna]

[Andre Kesteloot]

-------- Original Message --------
Subject: LF: loop antenna
Date: Mon, 25 Nov 2002 17:58:05 +0100
From: Rik Strobbe <rik.strobbe at fys.kuleuven.ac.be>
Reply-To: rsgb_lf_group at blacksheep.org
To: rsgb_lf_group at blacksheep.org
References: <3DE1338E.2F1E9274 at att.net> 
<000801c29474$a4e23300$1ee086d4 at erica>

Hello group,

I am about to put up a transmitting loop antenna. Dimensions will be 23m
long and 15m high. Thanks to Dick (PA0SE) I could achieve a wheel of 7mm
litz wire, so wire losses should be minimized (about 0.04 Ohm).

Some questions :

- What distance should I keep to the ground ? It's a tradeoff between loop
area (and thus radiation resistance) and loss resistance.
- What distance should I keep to the greenery (birch trees) ? Same tradeoff
as above but also keeping in mind that the trees are used as antenna
support and that the 7mm litz wire is rather heavy stuff (so from the
mechanical point of view it would be best to have the wire just hanging on
the branches).
- How to match the antenna ? As it is an experimental setup I would tend to
keep the resonance tuning (= series capacitor) and impedance matching (=
transformer) separated. Any experience with a 100/1 .. 400/1 transformer
(matching 50 Ohm of the TX to  0.1 .. 0.4 Ohm of the loop - at least I hope
to get losses that low).

Some quick and dirty calculations :
If the lower wire is 1m above ground then the loop area is 322m^2 and
radiation resistance is 0.14 .. 0.28 milli-Ohm (let's assume 0.2 milli-Ohm).
1. if Rloss = 1 Ohm and 450W RF power -> antenna current  = 21A  -> EIRP =
135mW
2. if Rloss = 0.5 Ohm and 450W RF power -> antenna current  = 30A  -> EIRP
= 370mW
3. if Rloss = 0.2 Ohm and 450W RF power -> antenna current  = 47A  -> EIRP
= 675mW
4. to get 1W EIRP with 450W RF power I would need 71A antenna current and
losses as low as 0.09 Ohm

73, Rik  ON7YD