I have today got my 500KHz station on the air. I am located near
Canterbury, locator JO01md. Here are some technical details.
The aerial is an inverted-L with a 15m vertical and a 35m horizontal
section. This was originally my top band aerial. I have added a 1mH end
loading coil (460 turns of 0.5mm enamelled wire on a 38mm OD white poly
waste pipe occupying 275mm of winding length). The loading coil is 14.7m
from the far end. I trimmed the 14.7m for resonance at 502KHz because I
wanted a roughly resonant aerial to simplfy feeding, but from the way
the resonance wanders about as the wind blows I'm not convinced this was
such a great idea. Standard formulae suggest this aerial has a radiation
resistance of almost exactly 1 ohm.
The aerial is end fed against a reasonable earth system comprising a 20m
x 20m mat of stock fencing with the sections crimped together. I also
have about twenty 30m radials. On top band I can work east coast US
easily with 100W and I have worked VK and W6 with 400W. I was quite
surprised (well, p****d off) to measure the return loss of the aerial
system at 502KHz as 22dB. This suggests the input resistance is a little
over 40 ohms, almost all of which is earth loss resistance! So much for
my "killer" top band earth mat.
I am using a link-coupled series tuned ATU with a loaded Q of 7 placed
at the base of the aerial to get the SWR very close to 1:1. The 2:1 SWR
bandwidth of the aerial system is less than 2 KHz (1KHz either side of
resonance).
I have built a QRP transmitter using an ECL82 triode pentode, as
popularised by G0UPL and others. Output is 4 watts. Cathode keying with
a small choke and 0.5uF capacitor gives a clean 3msec rise and fall on
the keying waveform, but the un-neutralised PA has a backwave that is
only 40dB down. I've added a "half wave" filter after the pi tank in an
effort to avoid QRMing MW broadcast stations. Harmonics are over 50dB
down.
The main HF station FT1000MP Mk5 seems just OK on 500KHz. Band noise is
S 1 in a 500Hz passband on the meter.
The rig is crystal controlled on 502.0KHz, so you know where to find me!
All I need now is some stations to work. "Can you hear me, Mother?".
--
Cheers,
Alan G3XAQ
[email protected]
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