VHF / FM aerial and its feeder - UK Vintage Radio Repair and Restoration Discussion Forum

Skywave · 8th Oct 2015, 10:15 pm

The aerial is a 3-element VHF aerial for band II and it is a fairly old one but having overhauled it (rusty nuts & bolts; corroded aluminium connections, etc.), I expect that it will work O.K. The feeder - which will be replaced - is 75 Ohm coax. The dipole itself is a folded dipole, so shouldn't this aerial be fed with 300 Ohm balanced feeder, or are the spacings from the dipole of the reflector and the single director such that the feed-point impedance has been brought down to something closer to 75 Ohms? Or is the impedance mis-match & the balance / unbalance issue with the coax of no real significance for reception?
Note: For VHF / FM reception, this location is not a 'fringe area' and the aerial will be about 40 foot above ground and well in the clear.

Al. / Oct. 8, 2015 //

Nuvistor · 8th Oct 2015, 10:33 pm

From memory the reason for using the folded dipole was to offset reduction in the feed point impedance due to the extra elements, the spacing of the elements will also effect the impedance.
If it is a decent quality aerial it should be in the ballpark of 75 ohm. There still the balun/unbalanced problem unless the aerial as one built in, some did. Try it and see how it works.

Frank

Chris G0EYO · 8th Oct 2015, 10:43 pm

folded dipoles with a theoretical impedance of 300 ohms are often used in Yagis because the addition of reflectors and directors reduces the impedance to nearer 60-70ohms enabling coax to be used. Chris G0EYO

Skywave · 9th Oct 2015, 11:01 am

Thank you, gentlemen. Your comments confirm my understanding on this. The aerial junction box has an outlet for coax only and is clearly a commercially-built product, so 75 Ohm coax it will be. It's simply because it has been such a long time since I had to think about such matters that I needed a quick refresher.

Al.

Herald1360 · 9th Oct 2015, 2:54 pm

If it's a quality aerial there should be a balun inside the connection box- looking something like a small coil on a bracket. If there isn't, and the coax connects direct to the dipole ends, it's not ideal, but the signal level difference will be small though the interference shielding effect of the coax outer will be compromised.

If you're enthusiastic, you could do your own with a suitable ferrite core and a few turns of wire, or add a suitable length matching stub coax dangling from the box. Mr G should provide details easily enough.

Skywave · 10th Oct 2015, 2:24 am

Quote:

Originally Posted by Skywave

Note: For VHF / FM reception, this location is not a 'fringe area' and the aerial will be about 40 foot above ground and well in the clear.

In the light of information received after that remark was made, a correction is required.

► For reference: this particular thread is associated with this one:
https://www.vintage-radio.net/forum/...d.php?t=120163

The stations I listen to most are not available from my 'local' transmitter at Stockland Hill.

It seems that Wenvoe and Mendip are the required transmitters and fortunately, from this location, they are on a similar bearing (for a 3-ele. beam). However, considering the distance from here to Wenvoe, the propagation path and the intervening terrain, I am probably in the fringe area for that TX, even though Wenvoe has an e.r.p of 150 kW.

As is always the case with aerial alignment - especially when you are in an unknown area, TX-wise - some experimentation is required. Moreover, looking around at nearby rooftops, (the usual 'guide'

), I see a lot of TV / UHF aerials, but am yet to see a band II aerial. So, tomorrow, must expand my 'visual search' area!

Al.

Herald1360 · 10th Oct 2015, 9:08 am

From your previous info about an indoor dipole being OK-ish, that's probably good enough for 99% of your neighbours and 90% of the rest are likely happy with one of the DAB freeview or Freesat versions.

I have noticed only a couple of "proper" FM aerials on our 500 odd house estate.

Skywave · 10th Oct 2015, 10:42 pm

Finally chose North Hessary Tor in Devon as my target transmitter using horizontal polarisation. Upon test, I'm now happy with the results. This post is associated to https://www.vintage-radio.net/forum/s...d.php?t=120163, to which further reference should be made.

My thanks to everyone who has contributed to this thread.

At present, I can think of nothing that I need to add to this particular thread.

Al. / Oct. 10, 2015 //

8th Oct 2015, 10:15 pm	#1
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	VHF / FM aerial and its feeder The aerial is a 3-element VHF aerial for band II and it is a fairly old one but having overhauled it (rusty nuts & bolts; corroded aluminium connections, etc.), I expect that it will work O.K. The feeder - which will be replaced - is 75 Ohm coax. The dipole itself is a folded dipole, so shouldn't this aerial be fed with 300 Ohm balanced feeder, or are the spacings from the dipole of the reflector and the single director such that the feed-point impedance has been brought down to something closer to 75 Ohms? Or is the impedance mis-match & the balance / unbalance issue with the coax of no real significance for reception? Note: For VHF / FM reception, this location is not a 'fringe area' and the aerial will be about 40 foot above ground and well in the clear. Al. / Oct. 8, 2015 //

8th Oct 2015, 10:33 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: VHF / FM aerial and its feeder From memory the reason for using the folded dipole was to offset reduction in the feed point impedance due to the extra elements, the spacing of the elements will also effect the impedance. If it is a decent quality aerial it should be in the ballpark of 75 ohm. There still the balun/unbalanced problem unless the aerial as one built in, some did. Try it and see how it works. Frank

8th Oct 2015, 10:43 pm	#3
Chris G0EYO Pentode Join Date: Dec 2008 Location: Redditch, Worcestershire, UK. Posts: 196	Re: VHF / FM aerial and its feeder folded dipoles with a theoretical impedance of 300 ohms are often used in Yagis because the addition of reflectors and directors reduces the impedance to nearer 60-70ohms enabling coax to be used. Chris G0EYO

9th Oct 2015, 11:01 am	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: VHF / FM aerial and its feeder Thank you, gentlemen. Your comments confirm my understanding on this. The aerial junction box has an outlet for coax only and is clearly a commercially-built product, so 75 Ohm coax it will be. It's simply because it has been such a long time since I had to think about such matters that I needed a quick refresher. Al.

9th Oct 2015, 2:54 pm	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: VHF / FM aerial and its feeder If it's a quality aerial there should be a balun inside the connection box- looking something like a small coil on a bracket. If there isn't, and the coax connects direct to the dipole ends, it's not ideal, but the signal level difference will be small though the interference shielding effect of the coax outer will be compromised. If you're enthusiastic, you could do your own with a suitable ferrite core and a few turns of wire, or add a suitable length matching stub coax dangling from the box. Mr G should provide details easily enough. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

10th Oct 2015, 9:08 am	#7
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: VHF / FM aerial and its feeder From your previous info about an indoor dipole being OK-ish, that's probably good enough for 99% of your neighbours and 90% of the rest are likely happy with one of the DAB freeview or Freesat versions. I have noticed only a couple of "proper" FM aerials on our 500 odd house estate. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

10th Oct 2015, 10:42 pm	#8
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: VHF / FM aerial and its feeder Finally chose North Hessary Tor in Devon as my target transmitter using horizontal polarisation. Upon test, I'm now happy with the results. This post is associated to https://www.vintage-radio.net/forum/s...d.php?t=120163, to which further reference should be made. My thanks to everyone who has contributed to this thread. At present, I can think of nothing that I need to add to this particular thread. Al. / Oct. 10, 2015 //