LF: RE: LF 500khz

["Dave G3WCB" <ganymede444@btopenworld.com>]

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: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org]On Behalf Of Vic@VicsFloridaVilla
Sent: 19 May 2007 21:05
To: rsgb_lf_group@blacksheep.org
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