75 ohm antenna? - UK Vintage Radio Repair and Restoration Discussion Forum

sheriff076 · 29th Nov 2009, 8:25 pm

Hi
I've got a vintage Bush VHF54 radio which has an 80ohm balanced input for FM.

Can anyone suggest an antenna design i could make using some 75ohm balanced feeder I've got?

Thanks,

Herald1360 · 30th Nov 2009, 12:13 am

Just split the feeder down about 2'6", stick a tyrap or bit of tape to stop it splitting further and connect the other end to the radio. Instant dipole. Any mismatch between 80ohm and the dipole won't be worth worrying about.

sheriff076 · 1st Dec 2009, 2:19 pm

Thanks for the reply, that's simple enough for me to do but the cable is not as i thought

I've just dug out the reel and its 300 ohm not 75 ohm. So apart from ordering a couple of meters of the 75 ohm i thought i had can anyone suggest an aerial idea for a 300 ohm fed to 75 ohm balanced input solution?

Is it worth worrying about, the radio is in the corner of the lounge and FM is rubbish there so i wanted a solution for an indoors which was decent.

Regards

geofy · 1st Dec 2009, 2:43 pm

While you could make a 1:4 ratio balum transformer or is it 4:1, chances are just doing the same and stripping out about two foot six inches in each direction and trying it in various positions will work.

g

Ian - G4JQT · 1st Dec 2009, 3:12 pm

You need a folded dipole if the feed point and feeder are 300 ohm.

See http://amateur-radio-wiki.net/index.php?title=Dipole

Regards,

Ian

kalee20 · 1st Dec 2009, 3:51 pm

If the radio is designed for 80 ohm, then a normal dipole will work. Strictly, the intervening feeder being 300 ohm is wrong, but as you have the cable you might as well try it.

You can make a folded dipole which will then suit the feeder exactly, but won't match the radio. But again, you might as well try it! Post the results here, let us all know!

Skywave · 1st Dec 2009, 6:27 pm

Just an observation following Kalee20's comment.

I note that you will be situating the aerial in a corner of the lounge, downstairs. On that basis, the walls of the room, the ceiling and other close objects are likely to have a far more detrimental effect on received signal strength (and directivity - not that there is a great deal of this with a dipole, anyway) than a mis-match in any of the scenarios outlined above. Nevertheless, I would expect some improvement over what you have been using - and it is easy to construct and erect (and in the warm, too!

).
So - give it a whirl - but don't expect a dramatic improvement.

And yes - please let us know how it all works out.

Al. / Skywave.

Oldtimer74 · 1st Dec 2009, 6:34 pm

I would certainly make a folded dipole out of the 300 ohm ribbon you have. Simply cut to the length already suggested. Then connect together the two wires at each end. Split one wire in the middle and attach a short length of the cable to this and connect to the set. Dont worry about the mismatch, on such a short length you wouldn`t be able to measure the difference.
Pat G3IKR

Herald1360 · 1st Dec 2009, 11:17 pm

If you've got a local 99p shop or similar, it may be worth a look. I just bought an indoor antenna consisting of a plastic base with two telescopic aerials connected to a length of coax terminated in a Belling Lee (ish) coax plug. No idea how they do it but couldn't resist it (or the similarly priced pack of assorted cable cleats). Didn't find what I was looking for, though

In the same vein as others, ordinary twisted flex or even the more modern two side by side in a sheath isn't a bad approximation to 75 ohm feeder. Then same as before..... or two bits of stiff wire (coathanger, perhaps) in a choc block.

Have you tried just connecting it to the TV aerial- it won't be a match or anything, but it's at least high up and outside!

Chris

G8HQP Dave · 3rd Dec 2009, 2:15 pm

Ordinary thick (79-strand) loudspeaker cable is about 80 ohms impedance as a transmission line. Would be OK for short runs at Band 2 FM. No indoor antenna is going to work particularly well, or be a good match to the cable, so just mess about until you find something that works. Each side of the dipole should be about 75cm long, or maybe 70cm is you prefer the local and independent stations in the upper part of the band.

29th Nov 2009, 8:25 pm	#1
sheriff076 Triode Join Date: Oct 2009 Location: South Yorkshire, UK. Posts: 27	75 ohm antenna? Hi I've got a vintage Bush VHF54 radio which has an 80ohm balanced input for FM. Can anyone suggest an antenna design i could make using some 75ohm balanced feeder I've got? Thanks,

30th Nov 2009, 12:13 am	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: 75 ohm antenna? Just split the feeder down about 2'6", stick a tyrap or bit of tape to stop it splitting further and connect the other end to the radio. Instant dipole. Any mismatch between 80ohm and the dipole won't be worth worrying about.

1st Dec 2009, 2:19 pm	#3
sheriff076 Triode Join Date: Oct 2009 Location: South Yorkshire, UK. Posts: 27	Re: 75 ohm antenna? Thanks for the reply, that's simple enough for me to do but the cable is not as i thought I've just dug out the reel and its 300 ohm not 75 ohm. So apart from ordering a couple of meters of the 75 ohm i thought i had can anyone suggest an aerial idea for a 300 ohm fed to 75 ohm balanced input solution? Is it worth worrying about, the radio is in the corner of the lounge and FM is rubbish there so i wanted a solution for an indoors which was decent. Regards

1st Dec 2009, 2:43 pm	#4
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: 75 ohm antenna? While you could make a 1:4 ratio balum transformer or is it 4:1, chances are just doing the same and stripping out about two foot six inches in each direction and trying it in various positions will work. g

1st Dec 2009, 3:12 pm	#5
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,323	Re: 75 ohm antenna? You need a folded dipole if the feed point and feeder are 300 ohm. See http://amateur-radio-wiki.net/index.php?title=Dipole Regards, Ian

1st Dec 2009, 3:51 pm	#6
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: 75 ohm antenna? If the radio is designed for 80 ohm, then a normal dipole will work. Strictly, the intervening feeder being 300 ohm is wrong, but as you have the cable you might as well try it. You can make a folded dipole which will then suit the feeder exactly, but won't match the radio. But again, you might as well try it! Post the results here, let us all know!

1st Dec 2009, 6:27 pm	#7
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: 75 ohm antenna? Just an observation following Kalee20's comment. I note that you will be situating the aerial in a corner of the lounge, downstairs. On that basis, the walls of the room, the ceiling and other close objects are likely to have a far more detrimental effect on received signal strength (and directivity - not that there is a great deal of this with a dipole, anyway) than a mis-match in any of the scenarios outlined above. Nevertheless, I would expect some improvement over what you have been using - and it is easy to construct and erect (and in the warm, too! ). So - give it a whirl - but don't expect a dramatic improvement. And yes - please let us know how it all works out. Al. / Skywave.

1st Dec 2009, 6:34 pm	#8
Oldtimer74 Rest in Peace Join Date: Apr 2008 Location: Worcester, Worcestershire, UK. Posts: 330	Re: 75 ohm antenna? I would certainly make a folded dipole out of the 300 ohm ribbon you have. Simply cut to the length already suggested. Then connect together the two wires at each end. Split one wire in the middle and attach a short length of the cable to this and connect to the set. Dont worry about the mismatch, on such a short length you wouldn`t be able to measure the difference. Pat G3IKR

1st Dec 2009, 11:17 pm	#9
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: 75 ohm antenna? If you've got a local 99p shop or similar, it may be worth a look. I just bought an indoor antenna consisting of a plastic base with two telescopic aerials connected to a length of coax terminated in a Belling Lee (ish) coax plug. No idea how they do it but couldn't resist it (or the similarly priced pack of assorted cable cleats). Didn't find what I was looking for, though In the same vein as others, ordinary twisted flex or even the more modern two side by side in a sheath isn't a bad approximation to 75 ohm feeder. Then same as before..... or two bits of stiff wire (coathanger, perhaps) in a choc block. Have you tried just connecting it to the TV aerial- it won't be a match or anything, but it's at least high up and outside! Chris Last edited by Herald1360; 1st Dec 2009 at 11:22 pm.

3rd Dec 2009, 2:15 pm	#10
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: 75 ohm antenna? Ordinary thick (79-strand) loudspeaker cable is about 80 ohms impedance as a transmission line. Would be OK for short runs at Band 2 FM. No indoor antenna is going to work particularly well, or be a good match to the cable, so just mess about until you find something that works. Each side of the dipole should be about 75cm long, or maybe 70cm is you prefer the local and independent stations in the upper part of the band.