Hi Steve, Having another go at the Cobweb, the one to your design using the double balun. Got it all together yesterday but no matching point at all using MFJ 259 analyser. Found the box was full of water!! which didn't help but after drying out still no matching freqs at all. Checked the balun with 12.5 ohm resistor on output but analyser gave a flat response over the whole coverage with swr at around 3. Just how do you test a balun? Am I doing it wrong. Advice from you would be appreciated.
Thanks, 73 de Ken G3RFH

Ken,

I put a 12.5 Ohm resistor across the dipole end of the balun and measure the input SWR; it's typically below 1.2:1 from 20m thru 10m. If you're seeing 3:1 something's seriously wrong!

Steve G3TXQ

That's what I did Steve. Think I'll start again. I used rg174 for the windings and as you know it is very thin and may have a short in it somewhere, though a continuity check showed no sign of a short. Thanks once again.
73 de Ken G3RFH

Ken,

One intermediate step you could try is to test each of the individual chokes separately as a 25 Ohm 1:1 balun. Terminate the coax pair with a 25 Ohm resistor and check for a flat 25 Ohms at the other end using the MFJ.

Steve G3TXQ

I'll bear that in mind Steve. Meanwhile, no matter which way I connect the baluns I can't get the swr lower than 3. I've run out of rg174 now so it's on hold again!! till I get some more. rg58 is too thick.
73 de Ken G3RFH

If your "box was full of water" Id say your coax is duffed !!!! Maplin sell some thin coax, but its silly expensive.... but if youre desperate !

A good sauce of short lengths of both types (3m to 10m) is wifi extension cables. Not expensive either

Thats a cracking idea.... 6M for £1.99 on ebay - bet the cable is chinese copy - but worth two quid to find out ! Excellent tip !

Hi Steve, Got some brand new RG174 mil spec yesterday and this morning wound the baluns exactly as per your instructions and the photos from your website.
Tested each balun separately as you suggested, into 25ohm load and both showed swr 1.9 across the band 14 to 29 mhz.
Putting them together as per instructions and I get the following readings (using MFJ 259 analyzer) into 12.5 ohm load:
29 mhz swr 2.6
24 mhz 2.5
15 mhz 2.0
14 mhz 1.9
10 mhz 1.6
4 mhz 1.3
1.8 mhz 1.2

I'm hoping you can tell me where I'm going wrong, cos it is driving me a bit daft as it is so simple.Hi.
73 de Ken G3RFH

Ken,

Sound as if the individual test proves they are OK as individual chokes.

Are you sure you got the 4 lengths of coax the same - within about 1/4 inch?

If all else fails, pop them in a jiffy bag a post them to me - I'll take a look.

Steve G3TXQ

Steve, I have re-checked everything and tested again with the same results. So I don't know what is wrong. With my original windings I discovered shorts where the outer cable had been stripped and it had compressed the braid into the inner insulation so that when the inner conductor was soldered, the insulation broke down. Have not used that method with the new cable and so have no shorts - all connections are as they should be.
So I will take advantage of your kind offer and I'll pop them in the post on Monday morning. I assume you are QTHR as in RSGB callbook?
Many thanks again Steve,
73 de Ken G3RFH

Yes - QTHR

Steve

I had a similar problem (somewhere in a previous thread)

It worked OK after I changed to RG316 (killed my wrists winding the stuff).

Just got mine out the box, waiting to finish the antenna, and measured it. Seems like it is like wine, gets better with age!




I included the low frequencies as well to give the bigger picture.

If I measure just the individual bands, it is just about completely flat at 1.3:1 on 20m, 1.55:1 on 10m and 1.8:1 on 6m. Not perfect or as good as the original, but OK to use I should think

OK Steve, thanks.
Other Steve in Coventry, I used RG316 for something last year but forget what it was and where I got it from. It has ptfe insulation which probably makes it more robust than rg174. Will see what 'TXQ Steve says.
73 de Ken G3RFH

This is what I just measured on one of mine:



That one uses RG316

Steve G3TXQ