URM59 Feeder: Zo - UK Vintage Radio Repair and Restoration Discussion Forum

russell_w_b · 6th May 2016, 11:12 pm

Does anyone know the Zo of URM59 feeder, and whether it's even co-ax or not? It's 10.3mm outer diameter.

I have an old commercial 4-element Yagi (ex-'MOULD' network) with a driven folded dipole, a reflector, and two directors. It's made by an Irish company called Sigma and is wide-band: 137 - 150MHz. There's a 330mm section of URM59 entering the termination on the dipole, and the other end is grafted and (extremely well) sealed and sleeved to a hacked-off section of 50 Ohm URM67. I suspect modified Zo matching, and I'm curious to know the Zo of the matching section, but can't get to the core or the dipole termination to measure it for thick goo.

Nor can I find anything about URM59.

Restoration73 · 6th May 2016, 11:36 pm

It's very likely to be 75 ohms, the 330mm section will be a quarter wave in coax.

russell_w_b · 6th May 2016, 11:47 pm

It looks to be a quarter-wave (guessing about 0.67 velocity factor) for 152MHz, so yes. I'll stick the VNA up it on Monday and see what it looks like on 144 MHz. It's a shame not to use it.

It's just that I thought it was some esoteric impedance, having not found anything about it!

Radio Wrangler · 7th May 2016, 12:03 am

It depends on how the Yagi design modifies the nominal 300 Ohms of the folded dipole.

If it really is 300 Ohms then a series matching section, a quarter wave of the geometric mean of 300 and 50 Ohms is required so the root of 50*300 is 122.5 Ohms. But I suspect the folded dipole isn't 300 Ohms.

David

russell_w_b · 7th May 2016, 8:28 am

If the URM59 is simply 75 Ohm Zo, this would suggest a DPZ of 112.5 Ohms for a quarter-wave section, which, I guess, is reasonable for a folded dipole with parasitic elements. But then there are loads of common 75 Ohm cables about. Why not use those?

I was even wondering if it was some kind of bi-ax forming a Pawsey balun?

The original plan was to look at the DPZ of the aerial at the box and match it up with a shorted stub, but it's that well-made and weatherproof it seems a shame to rive it all to bits for the sake of it.

The aerial is a wide-band one, and I suspect the matching section is optimised for the frequency range within that band. If it's optimised for 141 - 143 MHz I can get away with trimming the section for 144 MHz. But if it was optimised for the upper MOULD range of 149 - 149.9MHz I'd probably need to strip it out and start again as per plan 'A'.

dodgy-dxer · 7th May 2016, 4:39 pm

http://www.av-iq.com/avcat/images/do..._Datasheet.pdf

Radio Wrangler · 7th May 2016, 5:47 pm

Just amused at how far down the datasheet you have to go before they mention Zo.

David

dsergeant · 7th May 2016, 6:08 pm

At least it is on there. CPC have a habit of not putting Zo in their catalogue data at all, thinking that dc resistance per km is far more important.

Glad this one is solved. I did have a hunt earlier but could find nothing about URM59 anywhere.

Dave

Radio Wrangler · 7th May 2016, 6:30 pm

For cable and connector data one of the best sites/catalogues I've found is Huber & Suhner with data for all the RG series and quite a few others. Belden's site is quite good, too.

David

russell_w_b · 7th May 2016, 7:10 pm

Quote:

Originally Posted by dodgy-dxer

http://www.av-iq.com/avcat/images/do..._Datasheet.pdf

Excellent! Many thanks.

Mystery solved indeed...

6th May 2016, 11:12 pm	#1
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	URM59 Feeder: Zo Does anyone know the Zo of URM59 feeder, and whether it's even co-ax or not? It's 10.3mm outer diameter. I have an old commercial 4-element Yagi (ex-'MOULD' network) with a driven folded dipole, a reflector, and two directors. It's made by an Irish company called Sigma and is wide-band: 137 - 150MHz. There's a 330mm section of URM59 entering the termination on the dipole, and the other end is grafted and (extremely well) sealed and sleeved to a hacked-off section of 50 Ohm URM67. I suspect modified Zo matching, and I'm curious to know the Zo of the matching section, but can't get to the core or the dipole termination to measure it for thick goo. Nor can I find anything about URM59. __________________ Regds, Russell W. B. G4YLI.

6th May 2016, 11:36 pm	#2
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: URM59 Feeder: Zo It's very likely to be 75 ohms, the 330mm section will be a quarter wave in coax.

6th May 2016, 11:47 pm	#3
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: URM59 Feeder: Zo It looks to be a quarter-wave (guessing about 0.67 velocity factor) for 152MHz, so yes. I'll stick the VNA up it on Monday and see what it looks like on 144 MHz. It's a shame not to use it. It's just that I thought it was some esoteric impedance, having not found anything about it! __________________ Regds, Russell W. B. G4YLI.

7th May 2016, 12:03 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: URM59 Feeder: Zo It depends on how the Yagi design modifies the nominal 300 Ohms of the folded dipole. If it really is 300 Ohms then a series matching section, a quarter wave of the geometric mean of 300 and 50 Ohms is required so the root of 50*300 is 122.5 Ohms. But I suspect the folded dipole isn't 300 Ohms. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

7th May 2016, 8:28 am	#5
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: URM59 Feeder: Zo If the URM59 is simply 75 Ohm Zo, this would suggest a DPZ of 112.5 Ohms for a quarter-wave section, which, I guess, is reasonable for a folded dipole with parasitic elements. But then there are loads of common 75 Ohm cables about. Why not use those? I was even wondering if it was some kind of bi-ax forming a Pawsey balun? The original plan was to look at the DPZ of the aerial at the box and match it up with a shorted stub, but it's that well-made and weatherproof it seems a shame to rive it all to bits for the sake of it. The aerial is a wide-band one, and I suspect the matching section is optimised for the frequency range within that band. If it's optimised for 141 - 143 MHz I can get away with trimming the section for 144 MHz. But if it was optimised for the upper MOULD range of 149 - 149.9MHz I'd probably need to strip it out and start again as per plan 'A'. __________________ Regds, Russell W. B. G4YLI.

7th May 2016, 4:39 pm	#6
dodgy-dxer Heptode Join Date: Mar 2008 Location: Newmarket, Suffolk, UK. Posts: 613	Re: URM59 Feeder: Zo http://www.av-iq.com/avcat/images/do..._Datasheet.pdf

7th May 2016, 5:47 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: URM59 Feeder: Zo Just amused at how far down the datasheet you have to go before they mention Zo. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

7th May 2016, 6:08 pm	#8
dsergeant Octode Join Date: Mar 2006 Location: Bracknell, Berkshire,UK. Posts: 1,175	Re: URM59 Feeder: Zo At least it is on there. CPC have a habit of not putting Zo in their catalogue data at all, thinking that dc resistance per km is far more important. Glad this one is solved. I did have a hunt earlier but could find nothing about URM59 anywhere. Dave

7th May 2016, 6:30 pm	#9
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: URM59 Feeder: Zo For cable and connector data one of the best sites/catalogues I've found is Huber & Suhner with data for all the RG series and quite a few others. Belden's site is quite good, too. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done