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Hi
again, Vic.
Start
with getting an antenna up, and then take it from there. The need for a
preselector is primarily to protect your RX front end from broadcast band
overload. If you have a tuned antenna, you may not need one, at least to get you
started.
The
180ft long wire should be fine. You will need to resonate it with a biggish
coil. It's the height that matters, because most of the radiation is done by the
vertical section. The horizontal bits just act as capacity top-loading. There is
some useful data on G3YXM's site.
I
use one of the late G4FGG's little programs called "TANT136.exe" also
available from Dave's site
Start
listening around, once you have got your antenna set up and resonated. You can
use the regular NAVTEX transmissions on 518 kHz to tweak the tuning, then look
for a weak, steady carrier on 504.0 khz - this is the second harmonic of
Atlantic 252 on 252 kHz, and is usually visible. Most amateur CW activity seems
to be centred around 502 to 502.5 kHz, with slow-speed (QRSS) beacon
transmissions usually taking place between 503 and 503.7 kHz. but there is no
hard and fast bandplan. Some stations send CW beacon transmissions at the bottom
of the band. GW3UEP, for instance, usually transmits a CW beacon around
501.10 kHz.
A
quick e-mail to the reflector will usually elicit a response, and you can also
check out Dave's 500 kHz grabber. There's a bit of activity going on right now.
G3UNT and G3XIZ are having a CW natter on 502 - ish, and G0MRF and G4WGT are
both in beacon mode around 503.6 - 503.7.
I'll
dig out the TX scan, and send it direct. Gary G4WGT is also using a similar
design.
Ah,
yes...you'll need to apply for a NOV as well. Don't leave it too long....I think
the trial period ends in March 2008!
73 and
good luck,
Dave
G3WCB IO91RM
Hi Dave,
Many thanks for your reply and sincere
apologies for this delayed response - too much non radio work :-(
!!!
I would ceratinly appreciate a scan of
your tx design . That would give me a good start. My design skills are rather
limited - my only rf qualification is the RAE which I tookin 1978 but I love
tinkering and homebrew. Having built a K2 and recently a KX1 I have gained a
lot of confidence to have a crack at a 500khz rig. So far the
transverter looks very promising.
Having followed the postings on the
reflector I agree a pre selector on rx looks essential and I think a resonant
antenna. But some stations seem to get quite good results with fairly modest
antennas so there is hope !
My progress on 500 will be slow due to
other comittments but I am determined to put out a signal on that band
!
Many thanks again !
73,
Vic
GW4JUN
----- Original Message -----
Sent: Saturday, May 19, 2007 11:33
PM
Subject: LF: RE: LF 500khz
Hi, Vic.
I've got no experince with the K2, but you'll
probably need a pre-selector to get the best performance out of your rig on
500 kHz. I use a simple design with a fet front end, which can be found
on G3YXM's web-site:- http://www.wireless.org.uk/circuits2.htm
I substituted a J310 fet for the BF245, and used IFTs from a junked
transistor radio for the inductors. My RX is an Icom IC-PCR1000 which is
okay, but the IF filter is a bit wide for close-in CW
work.
For transmitting, I use a design by DF3LP, which
was originally for 137 kHz http://www.qsl.net/df3lp/projects/lftx/index.html It uses an IRF 540 FET as the output stage,
and I have no trouble getting 25W into 50 ohms from a 12VDC supply, although
you may not need this much power to hit the 100 mW ERP limit if you
have a reasonable antenna.
You can scavenge most of the bits for nothing from
scrap PC power supplies, including the case and
fan!
The DF3LP TX design uses a crystal-mixer VFO, but
you could omit this and feed your 1mW straight into the second BC549
transistor. You'd need to re-do the pi-section filters for operation at 500
kHz, but you just need to scale the inductance value by the
ratio 137/500. I've got a scanned copy of my TX circuit if you are
interested.
Quite a few 500 kHz OPS are using valve TXs, with
bottles like ECL82, and 2E26s. It just depends what you are happiest
using!
73, Dave G3WCB
IO91RM
Hi Folks,
Joined the list a couple of weeks
ago amd enjoying reading all the postings but this is my first
posting.
I am planning to use my Elecraft K2
to drive a homebrew transverter to give me a 500khz capability. I have the
transverter interface board in the K2 so have a 1mW output
available.
So far I hace lashed up a 28Mhz
xtal oscillator and a home brew diode ring mixer. Feeding these through a
G0MRF 7 element Low Pass Filter I tested it the other night and it
works ! Wow. Nice clean cw and ssb on 500khz (feeding directly into my
receiver....probably the first and last time I'll hear ssb on 500khz !!)
Amazing carrier suppression on ssb - really impressive for 4 IN4148 diodes
and a couple of FT50-43 cores. Plenty of attenuation of 1000khz and
1500khz harmonics. It's looking good !
Just tried it on receive and it
does work but signals very weak. With the K2 pre amp on I can hear MSF
60Khz at S1 , Loran 100Khz at S4, BBc R4 198Khz at S9, plus a few
other data type sigs below 150khz. Heard nothing on 500Khz . I can hear a
strong intermittent data type signal on about 518Khz Plenty of noise
when I connect the antenna though. Looks like I may need to work on
the antenna and maybe some receive amplification .
I'm thrilled with all this so
far but now I need some help with the rest of the system.
So a few questions for you all, and
any ideas and suggestions gratefully received !
1) Can anyone point me to any
designs for a pa suitable for 500khz. Something around 5 - 10 watts output
from about 1mW input would be great. I am not very knowledgable on
electronics but I love tinkering so something which is easy to get going
would be good. I imagine at 500khz it should not be too
difficult.
2) Has anyone on the list used a K2
with the transverter board to drive a transveter ? I can get a full 1mW tx
output into a 50 ohm dummy load (as per the construction manual) but when
driving my homebrew ring mixer I can only get about 0.3 mW maximum. I
wonder if there is an impedance mismatch which is loading the K2
output. The K2 is connected directly to the mixer. My mixer
has six trifilar turns on a FT50-43 at each end. It clearly works well but
I think I'll need to get the level up a bit at some point.
3) Thinking ahead to
antennas.....I'll need to do something to resonate my 180 ft Inv L . I 'll
probably stick a big loading coil in it somewhere but I have no idea how
much L I'll need . Is there any software out there which will help me with
this ?
Meanwhile I'll press on with the
band pass filter which is not working so well . Time for a tinker
!!
TIA
73,
Vic GW4JUN
Halkyn North Wales
IO83JF
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