[Premium-Rx] The 'Ideal' Logwire

[Al Klase]

The problem was sort of addressed by the Radio Manufacturers 
Association (RMA) in the 1930's, when all-wave receivers 
were in vogue.  They developed a standard dummy antenna to 
be inserted between a signal generator and a receiver under 
test.  As near as I can tell this was intended to simulated 
a typical inverted-L, say 40ft up and 60 ft out. The circuit 
is a 200pF cap in series with a 20uH inductor.  The inductor 
is shunted by a series combination of 400pF and 430ohms. he 
standard signal generator is terminated by 50 ohms resulting 
in the dummy antenna seeing a 25-ohm source.

The assumption is, that below about 2Mhz, where the antenna 
is well shy of a quarter wavelength, the it is largely 
capacitive, and that 25ohms is a reasonable equivalent of 
radiation resistances and ground losses.

Above 2MHz, the antenna Z will vary between 50-ohmish at odd 
multiples of 1/4 wave and 3000-ohmish at even multiples. 
The dummy assumes 400 ohms at all HF frequencies, the 
geometric mean of the two extreme values.

So the most reasnable compromise is to use a 4:1 or 9:1 
transformer between the antenna and feed line.

4:1 works pretty well with 75-ohm TV coax.  There's a good 
4:1 balun design on my webpage at: 
http://www.webex.net/~skywaves/ANTENNA/antsys.htm

The suggested Amidon BLN-73-202 "binocular" core is a good 
bet for the frequency range you're talking about.

Regards,
Al


NewRadioChap at aol.com wrote:
> Does anyone know if there is an 'equivalent' circuit (or lab set-up) 
> that will simulate a typical logwire (single-ended) antenna working into 
> a co-ax? I don't just mean the theory - I am looking for practical work.
> I have been doing experiments and research on broadband balun designs 
> and construction - particularly VLF and HF (approx 10KHz to 10MHz) to 
> optimise my receive capability using a variety of longwires from 20ft to 
> 200ft. I've found that most commercially made broadband balun 
> transformers for receive do not appear to work very well - if at all - 
> and have no benefit. My homebuilt ones don't fair any better. When 
> frequency swept, all exhibit distinct bandwidth/resonant peculiarities. 
> In fact, by incorrectly connecting certain types of balun, I get a 
> voltage boost (gain) at LF, with high loses at HF! This is confirmed on 
> the antenna sweep. There must a pile of work previously carried out on 
> this that someone can point me to, I'd much appreciate it. If I can find 
> a simulation, it will save me putting life at risk trying to do 
> measurements up a 40ft tree, and instead do them indoors! Every dB 
> counts....
> 
> 


-- 
Al Klase - N3FRQ
skywaves at bw.webex.net
Flemington, NJ 08822
Web Page:  http://www.webex.net/~skywaves/home.htm