[Fwd: LF: Aerials for 136]

[André Kesteloot]

Walter Blanchard wrote:

> I wanted to know what the chances were of radiating the maximum 1 watt from
> a back-garden aerial.
> All following figures are approximate although not actually complete guesses!
> If I've gone wrong somewhere - quite likely - please tell.
> First  assume a 50 ft conducting vertical with six guy wires electrically
> attached at the top and brought down to a 50 ft radius
> tie-down, but terminated electrically at 15 ft above the ground - you'll
> see why later! The coil would of course be in series at
> its base.
>
> Basic calculation:
> Capacity 525 pf; effective height at 136 kHz 7.65m; radiation resistance  =
> 0.0189 ohm.
> Ae current required for 1 watt = 7.28 A.
> Ae reactance = 2230 ohms
> Ae volts = 16.2 kV rms;  23 kV peak !!!!!! (hence the 15 ft)
> Loading coil inductance 2.61 mH
> Coil loss resistance Q = 300  RL =  7.43 ohm
>             ditto                  Q = 150  RL = 14.9 ohm
>
> Reqd power input :
> Coil resistance, ground loss, radiation resistance, mast loss all in series
> with power generator.
> Ignore mast loss (3 inch ali tube). Ground loss is a guess but assume :
> Good            = 1 ohm (very unlikely unless you can get out a lot of radials);
> Average                 = 4 ohms (possible if good soil and 12-16 radials)
> Poor            = 16 ohms (most likely in amateur circumstances)
> Then:
> Ground  Coil Q  Total Loss Res. Reqd Power
> Good    300      8.43             446 W
> Good    150     15.9              841 W
>
> Average 300      11.43            605 W
> Average 150      18.9            1000 W
>
> Poor    300     23.43           1240 W
> Poor    150     30.9            1635 W
>
> Next, a 75 ft vertical wire with a 130ft flat top ("T") that should be at
> least two parallel conductors
> held 4-6 ft apart fed in the middle via a delta section from the vertical
> wire/s.
> (This is NOT a traditional delta match). Assume all wires 10g or thicker
> with negligible loss.
> This sort of aerial is often used professionally for short-range beacons
> etc on 300 kHz or so.
>
> Capacity 597 pf; effective height 17.35 m; radiation resistance = 0.097 ohm
> Ae current for 1 watt = 3.21 A
> Ae reactance = 1960 ohms
> Ae volts = 6.29 kV rms; 8.9 kV peak.
> Coil inductance 2.29 mH
> Coil loss resistance Q=300 RL= 6.53 ohm
>         ditto                      Q=150 RL= 13.1 ohm
>
> Ground  Coil Q  Total Loss Res. Reqd Power
> Good    300     7.53            77.6 W
> Good    150     14.1            145 W
>
> Average 300     10.53           109 W
> Average 150     17.1            176 W
>
> Poor    300     22.53           232 W
> Poor    150     29.1            300 W
>
> Draw your own deductions! And, apart from the kilovolts on the aerial note
> that with a poor earth
> and rather low-Q coil the coil itself is going to have to dissipate
> anything from 150 to 800 W!
> By the way, with anything other than a good earth Litz wire will do nothing
> for you.
>
> Walter G3JKV.