UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 29th Jun 2021, 12:15 pm   #1
Johnnycosmos
Triode
 
Join Date: Jun 2020
Location: York, North Yorkshire, UK.
Posts: 12
Default Random Wire Antenna-Request to check my thinking

Hi,

I'm re jigging my garden random wire antenna. I've done some reading (accepted a little knowledge is often the worse!). This is my thinking. Please let me know if any thoughts.

I've draped about 60 metres of insulated single copper core wire, in a rhombus shape, around garden perimeter. It is about 3m high. The long sides are twice the length of the short. The long sides are perpendicular to Droitwich. I'm working on the premise that my garden is my garden so have as long a wire as garden will allow.
The wire has an insulator at each end.


To Balun or not to balun?
The radio is an old valve and I understand requires a high impedance signal. The aerial socket is a banana plug.
I've read that some would add a Balun between the aerial wire and the input feed so as to take the input into the house in Co Ax. I understand that co ax will reduce the noise from the house electrical installation. However this will mean that the impedance will be reduced when it enters the radio, due to the balun?.
Given that the distance from aerial wire to receiver is only about 5m my thinking is that I don't bother with a balun and just take the wire to the receiver.
Does this sound sensible? Or should I use a Balun and co ax? If so what ratio?

Earth

The radio has an earth socket. It is also earthed via the plug into the house installation. I plan to take an earth wire from radio earth to an earth rod in garden. If I do this do I disconnect the earth in the plug?
Do I need to take an earth from the aerial wire too?

Hope I've not got to much hold on the wrong end of the stick.

Kindest wishes

John
Johnnycosmos is offline  
Old 29th Jun 2021, 1:05 pm   #2
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Random Wire Antenna-Request to check my thinking

It sounds like your receiver is intended for a random long wire antenna to go straight into the socket.

To use a balun, you need something balanced in the first place. A length of coax would go on the unbalanced end, and what have you got that's balanced? Your antenna is unbalanced. A balun would operate one lump of antenna against a second similar lump of antenna.

So you are fully unbalanced. THe downside is that you could pick up some interference where a balanced arrangement would give you some cancellation.

Long wires are usually short enough to still look high-Z. Your set probably expects this. Coax comes in 50Ohm and 75 Ohm flavours. If you had a long run of coax, the capacitive loading would spoil things for you because the signal and load Z will be a lot higher than 75 Ohms. The unmatched coax capacitance will likely also pull the tuning of the first resonant tank in your receiver, losing more signal. Offsetting this is the job of the antenna trimmer control on better classic sets.

So, plug it straight in.

If too much interference think about ferrite beads on mains leads, think about screening on the feeder near and in the house.

Whatever you do won't be perfect, so don't get fixated on analyses.

Minimising interference may be more important than maximising signal.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 29th Jun 2021, 3:13 pm   #3
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Random Wire Antenna-Request to check my thinking

How would you screen an incoming antenna lead?

Rather like coax. Coax will do it, but coax in made for low impedance, and a long wire into a classic set is high impedance. You want the least number of pF/metre capacitance, so 75 ohms is better than 50 Ohms, but a loose braid spaced out around the wire is better still.

I've pulled the inner out of large diameter coax and threaded a thin insulated wire inside instead to get something a little better.

In general, good reception and bandwidth says screen the least length, low noise says screen the most!

Who said he universe had to be fair?

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 29th Jun 2021, 3:29 pm   #4
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Random Wire Antenna-Request to check my thinking

Quote:
Originally Posted by Radio Wrangler View Post
How would you screen an incoming antenna lead?
They used to supply such systems for domestic radios back in the day, basically antenna>>transformer>>twin (screened) down lead>>transformer>>receiver, screen Earthed.

Lawrence.
ms660 is offline  
Old 29th Jun 2021, 3:34 pm   #5
Johnnycosmos
Triode
 
Join Date: Jun 2020
Location: York, North Yorkshire, UK.
Posts: 12
Default Re: Random Wire Antenna-Request to check my thinking

Many thanks. Much appreciated. That's all helped me a lot.
John
Johnnycosmos is offline  
Old 29th Jun 2021, 5:47 pm   #6
Guest
Guest
 
Posts: n/a
Default Re: Random Wire Antenna-Request to check my thinking

Or two "magnetic blauns", one each and as per post 4

Make your own https://m0ukd.com/homebrew/baluns-an...re-balun-unun/
 
Old 29th Jun 2021, 8:54 pm   #7
Mr 1936
Heptode
 
Join Date: Mar 2017
Location: Romsey, Hampshire, UK.
Posts: 511
Default Re: Random Wire Antenna-Request to check my thinking

Hi Johnnycosmos

If it's UK ground wave reception you want, your aerial primarily needs HEIGHT. Once you have exhausted the practical limits for height, then you can by all means add one horizontal part (Inverted L) or multiple horizontal parts (Tee, capacity hat). But the best these horizontal parts can do is to double the effective height of the vertical bit. Sounds like you need more height and less horizontal, if you can achieve that ?

If your house and its wiring generate interference (most do), place the aerial AND earth rod as far away as you can. I'd then recommend using good 75 ohm screened coax feeder plus a 9:1 impedance transformer at each end (step down, coax, step up), to link the aerial system to the radio. This will work the same as a 675 ohm feeder which will be high enough to suit the radio and the aerial.

Try experimenting - it's part of the fun and not expensive.
Mr 1936 is offline  
Old 29th Jun 2021, 11:35 pm   #8
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Random Wire Antenna-Request to check my thinking

If noise is still a problem then a 'magnetic loop' antenna is well worth considering. The Wellbrook one has been thoroughly discussed here, but it's rather expensive. Several people have investigated designs for building their own and posted on here.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 30th Jun 2021, 12:03 am   #9
majoconz
Heptode
 
majoconz's Avatar
 
Join Date: Jan 2008
Location: Ashhurst, Manawatu, New Zealand
Posts: 570
Default Re: Random Wire Antenna-Request to check my thinking

It shouldn't be too hard to convert your antenna into a 'rhombic'. Then you can mount a balun at the near point, then coax down to the radio and, if necessary, another balun at the radio terminal. Heaps of info here.....

https://en.wikipedia.org/wiki/Rhombic_antenna

If are confident enough to do it, then you can wrap a few turns, say ten, of insulated wire around the antenna RF coil and bring the ends out to the rear panel - there's your low impedance unbalance input that you can join your cax cable to.
__________________
Cheers - Martin ZL2MC
majoconz is offline  
Old 30th Jun 2021, 9:14 am   #10
Mr 1936
Heptode
 
Join Date: Mar 2017
Location: Romsey, Hampshire, UK.
Posts: 511
Default Re: Random Wire Antenna-Request to check my thinking

Hi

A Rhombic responds to horizontal polarization, which might be useful for long distance reception via skywave but won't be much good for short to medium distance via groundwave.

In a domestic situation, typical size constraints will make it more suitable for shortwave.
Mr 1936 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:44 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.