[Fwd: LF: ground loss problems]
Fri, 15 May 1998 20:36:23 -0400
Rik Strobbe wrote:
> Since about 1 month I am trying to put up a 136kHz station but I got stuck to
> a low ERP (5mW) because of high ground loss.
> I will give a desciption of the situation and what I already tried, maybe
> someone has been there before (and maybe even found a solution).
> The TX I use has about 35 - 40 Watt output (hope to get a 200 Watt PA soon).
> The antenna is a 11 meter vertical with 6 horizontal wires at the top. The
> top wires are between 5 and 20 meter long (total 60 meter) and more or less
> equally divided over 180 degrees (I cannot use the other 180 degrees because
> of trees). The antenna is completely surrounded by trees that are up to 18
> meters high. As ground system I have an galvanized iron earth rod of 1.5m and
> 6 radials (more or less equally divided over 360 degrees) each between 20 and
> 30 meter long.
> The antenna is in the back yard so I have about 35 meters of coax (RG58)
> between the TX and the antenna. At the feeding point of the antenna I have
> the loading coil and (between coax and loading coil) a 4/1 autotransformer to
> match the TX more or less to the antenna system (ground loss is 230 Ohm !).
> The radiation resistance was calculated with 2 different methods, both came
> close to 35 milliOhms.
> What I tested so far :
> 1. Loss of the loading coil is about 25 Ohm (can be reduced by replacing the
> 1mm CuL wire on the coil by litz). I measured this value by putting the
> coil in series with a appropiate C (to get resonance on 136kHz) and also
> with a 50 Ohm dummyload. This was fed by the TX and the current compared
> to the current I could get direct into the dummy.
> 2. I measured the ground loss in 3 different ways :
> A. I used the 'capasitance meter' I built. This is a batterysupplied
> device with a 136kHz oscillator weakly coupled into a resonance circuit
> with variable C. The voltage over the LC circuit is measured and used
> bring the circuit to resonance. From the difference in C-value between
> resonance with antenna attached (parralel to C) and without antenna the
> capasitance of the antenna can de determined.
> In order to determine the loss resistance I noted not only the position
> of the variable C but also the value of the voltage over the circuit.
> By replacing the antenna by an RC-circuit (series) that had the same
> maximum voltage at the same position of the variable C I could
> determine the loss resistance (= the R of the RC-circuit).
> The advantage of this method is that you can measure the loss
> resistance without having to use the loading coil.
> B. I measured the resistance of the tuned antenna (= with loading coil)
> with a wheatstonebridge (ground loss = resistance - coil loss).
> C. Based on the values I found in methods A and B I built an
> autotransformer to match the TX to the total loss resistance (ground
> loss + coil loss) and measured the antenna current. Assuming good power
> matching I can calculate the total loss resistance from the TX power
> and the antenna current.
> The results found with these 3 different methods matched well (difference
> less than 10%). So I have good hope I really measured the ground loss and not
> something else.
> Results of the measurement :
> - ground loss with only the earth rod = 320 Ohm
> - ground loss with all 6 radials = 230 Ohm
> - ground loss with only 1 radial = 250-280 Ohm (depending on the radial)
> - ground loss with only 2 radials = 230-250 Ohm (same)
> - ground loss with > 3 radials = 230 Ohm
> - ground loss with only the shield of the 35m coax as 'ground' = 230 Ohm
> - ground loss with anything I could connect connected = 230 Ohm
> For the last test I really connected anything that could act as counterweight
> (waterpipes, fences). But this 230 Ohm seems to be a 'magical limit'.
> The ground at my QTH is dry and sandy. I know this is not the best, but the
> 230 Ohm I got seems very high to me !
> I only live at that place for about 1 year, so I don't know about water-
> levels etc... Maybe drilling a pipe into the earth until it reaches the
> groundwater level will bring solution, but this is a rather drastical method.
> The fact that going from 2 to 6 radials brings NO difference looks odd to me.
> So if anyone experienced something similar (and maybe even has a solution),
> please let me know (I getting pretty desperate).
> 73, Rik ON7YD