[Lowfer] thoughts on PA0RDT "mini whip"

[John Andrews]

Doug,

Just a few random thoughts here. At my MA QTH, there is a 5 kW broadcast 
station on 580 kHz, less than 2 miles away. Untuned MF/LF antennas with 
less than "cast iron" preamps have not done well, due to all sorts of 
mixing products. On the other hand, my ME QTH is a good 15 miles from 
the nearest AM broadcast site, and I can use about anything that I want.

Five years ago, I researched the preamp problem, and built the best one 
that I could create with available parts. Considerably more complicated 
than the simple PA0RDT circuit. It had IP2 >65 dBm, and IP3 >75 dBm. I 
used a 1 meter whip, but couldn't raise it much more than 5' above 
ground. Signal levels were less than desired, though I was getting -2 
dBm out on 580 kHz!. Tried all sorts of tricks, and finally gave up in 
favor of loops.

Anyway, the PA0RDT circuit should be fine in many applications, and you 
will probably be OK. Do understand that you're not taking a magic sniff 
of an E-field in mid-air. There is no magic. The total length of the 
antenna will consist of all the cable from the earth up. But as Laurence 
has pointed out, you can find magic combinations of local objects to 
support it and be nearby. Well worth the fun of playing around, should 
things not be to your liking.

I'll look around for the schematic of the best preamp I tried. Not at 
all sure how it would compare to the AMRAD circuit that used the big 
MOSFET. The current and voltage were similar, though, 24V @ 60 mA.

John, W1TAG