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LF: Re: RE: LF 500khz

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Subject: LF: Re: RE: LF 500khz
From: "Vic@VicsFloridaVilla" <[email protected]>
Date: Tue, 17 Jul 2007 19:51:24 +0100
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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 -----
From: Dave G3WCB
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
 
 
 
-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Vic@VicsFloridaVilla
Sent: 19 May 2007 21:05
To: [email protected]
Subject: LF: LF 500khz

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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