Aerial attenuator? - UK Vintage Radio Repair and Restoration Discussion Forum

spycatcher · 25th Jun 2021, 7:49 pm

Hi Guys, I'm quite new to amateur radio and have bought some 70's equipment i intend refurbishing in my retirement the item in the pic was with the rest of the gear and I'm not quite sure exactly what it is, i think its an aerial attenuator but not sure. i would appreciate any comments, cheers

Station X · 25th Jun 2021, 8:08 pm

Looks more like an aerial matching unit to me.

Radio Wrangler · 25th Jun 2021, 8:23 pm

It's a variable inductor of a type called a "Roller-Coaster" and is used in aerial matching units on the outputs of transmitters. It's a medium sized one likely to be 20 to 50uH at the maximum end and suited to a transmitter of up to a few hundred watts, though the power depends on the ATU settings.

They're quite sought after. Good ones have silver plated wire and a silver plated contact wheel. Very good ones have a sintered metal contact block like yours.

It looks to have been made into a low power aerial tuning unit. Those transistor radio variable capacitors will limit the whole thing to a few watts.

David

spycatcher · 26th Jun 2021, 10:34 am

Hi David, Thanks for the info. It came in a bundle with a Codar AT5 and power supply, a K.W. communications KW 204 and a Trio 9R-59DS receiver. Does it make sense that the modified inductor fits in with this rig?

Graham G3ZVT · 26th Jun 2021, 10:06 pm

Quote:

Originally Posted by spycatcher

Hi David, Thanks for the info. It came in a bundle with a Codar AT5 and power supply, a K.W. communications KW 204 and a Trio 9R-59DS receiver. Does it make sense that the modified inductor fits in with this rig?

Not really, I think who ever built that ATU intended it for receive only. Even the AT5 at maybe 6W RF out would probably create a challenge for those miniature solid-dielectric VC's

The roller-coaster itself is a good one, I had the exact same one many years ago, and I no doubt bought it from one of the government surplus stores in and around Shudehill Manchester, but other than that, I can't tell you what original equipment it was made for.

By coincidence I have just re-discovered another roller-coaster in in the garage that is part of an original ATU of some kind, does anyone recognise it?

Terry_VK5TM · 27th Jun 2021, 12:15 am

This little poly VC's will handle up to 10W in most cases - although personally I don't think they should be used in any transmitting application.

As for is it an attenuator - definitely - if it is adjusted incorrectly.

M0AFJ, Tim · 27th Jun 2021, 11:35 am

Quote:

Originally Posted by spycatcher

Hi David, Thanks for the info. It came in a bundle with a Codar AT5 and power supply, a K.W. communications KW 204 and a Trio 9R-59DS receiver. Does it make sense that the modified inductor fits in with this rig?

Hi, just so you know there is a very active KW group on groups.io, they have a lot of knowledge on the 204 if you need any assistance

G6Tanuki · 27th Jun 2021, 6:25 pm

The 'roller-coaster' looks rather like a 'tunable HF choke' that I came across in a few valve-based military transmitters late last century [RACAL cunningly combined the anode-choke and first-stage RF matching into one component!].

Polyvaricons - yes they can deal with 10 Watts or so of RF in a pi-tank ATU provided that the circulating currents [which get greater with higher impedance-matching ratios] are not too high. Wouldn't want to push a Polyvaricon beyond 10 Watts of RF, though we happily used 'close-spaced' 500+500pF broadcast-radio-type airspaced capacitors in the pi-net of two-6146-in-parallel linear amps on 80M and they never flashed-over...

25th Jun 2021, 7:49 pm	#1
spycatcher Diode Join Date: Jun 2021 Location: Selkirk, Selkirkshire, Scotland, UK. Posts: 8	Aerial attenuator? Hi Guys, I'm quite new to amateur radio and have bought some 70's equipment i intend refurbishing in my retirement the item in the pic was with the rest of the gear and I'm not quite sure exactly what it is, i think its an aerial attenuator but not sure. i would appreciate any comments, cheers Attached Thumbnails

25th Jun 2021, 8:08 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Aerial attenuator? Looks more like an aerial matching unit to me. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

25th Jun 2021, 8:23 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: Aerial attenuator? It's a variable inductor of a type called a "Roller-Coaster" and is used in aerial matching units on the outputs of transmitters. It's a medium sized one likely to be 20 to 50uH at the maximum end and suited to a transmitter of up to a few hundred watts, though the power depends on the ATU settings. They're quite sought after. Good ones have silver plated wire and a silver plated contact wheel. Very good ones have a sintered metal contact block like yours. It looks to have been made into a low power aerial tuning unit. Those transistor radio variable capacitors will limit the whole thing to a few watts. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th Jun 2021, 10:34 am	#4
spycatcher Diode Join Date: Jun 2021 Location: Selkirk, Selkirkshire, Scotland, UK. Posts: 8	Re: Aerial attenuator? Hi David, Thanks for the info. It came in a bundle with a Codar AT5 and power supply, a K.W. communications KW 204 and a Trio 9R-59DS receiver. Does it make sense that the modified inductor fits in with this rig?

27th Jun 2021, 12:15 am	#6
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	Re: Aerial attenuator? This little poly VC's will handle up to 10W in most cases - although personally I don't think they should be used in any transmitting application. As for is it an attenuator - definitely - if it is adjusted incorrectly. __________________ Terry VK5TM https://www.vk5tm.com/

27th Jun 2021, 6:25 pm	#8
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Aerial attenuator? The 'roller-coaster' looks rather like a 'tunable HF choke' that I came across in a few valve-based military transmitters late last century [RACAL cunningly combined the anode-choke and first-stage RF matching into one component!]. Polyvaricons - yes they can deal with 10 Watts or so of RF in a pi-tank ATU provided that the circulating currents [which get greater with higher impedance-matching ratios] are not too high. Wouldn't want to push a Polyvaricon beyond 10 Watts of RF, though we happily used 'close-spaced' 500+500pF broadcast-radio-type airspaced capacitors in the pi-net of two-6146-in-parallel linear amps on 80M and they never flashed-over...