Homemade VHF 88-108MHz FM dipole antenna

[brian_mk]

I posted this earlier under another thread, but I suspect this may be a better place for it...

I have a Technics FM tuner from the 70s/80s connected into my HiFi system.

I live in Milton Keynes, so I used to get analogue TV + FM radio through an underground coax cable.
After the analogue TV switch off, the cable system was ripped out to recover the copper. I installed a digital TV aerial in the loft, which works fine.
However I also lost my FM radio signal.

There is not enough room in the loft to install a VHF Yagi array and I didn't fancy climbing a ladder to fix an external aerial to the gable end of the house.
So I decided to try a simple FM dipole based on the article here:-
http://www.northcountryradio.com/Articles/fmdip.htm

It seemed to work reasonably well when I tested it in the living room connected to the tuner with about 3m of 75 Ohm coax.

To install it in the loft meant running about 12->15m of coax up the outside wall.
I mounted the dipole high up in the loft, just below the ridge.

I found the signal level was actually worse than when it was located downstairs.

I was also surprised to find the signal level was higher when I disconnected the dipole leaving just the 15m coax!

By repositioning the dipole lower down in the loft below the rafters, I managed to get about the same signal level as I had downstairs.

I know that installing an aerial in a loft is not ideal, but I did expect a higher signal compared with downstairs even when taking into account cable loss.
I believe 15m of RG6 coax has a loss of about 1dB at 100 MHz.

Can anyone explain why I get a reasonable signal from just the feeder coax with the dipole disconnected?

Note: I also tried adding a balun, made by coiling a few turns of the feeder cable close to the dipole. It made little difference.

[paulsherwin]

You are getting some signal from the coax screening.

It isn't clear why the dipole is behaving so poorly, assuming you have connected it correctly. If it is horizontal, is it aligned correctly for the transmitter you want to use?

Loft aerials have their advantages (I use one) but it's usually necessary to experiment with location and alignment to get a good signal. Lots of things in lofts can interfere with reception. Some houses are roofed with ceramic tiles which screen radio very effectively.

[ms660]

What's the antenna input specs for the tuner? Balanced, unbalanced, 300 ohms, 75 ohms...

Lawrence.

[brian_mk]

The Technics tuner is an ST-S6. It has a 75 Ohm Unbalanced coax input socket (using an F-type plug).

Interestingly, the tuner was supplied a short indoor wire dipole that includes a 75->300 Ohm balun in the plug. This seems a bit strange to me since it is not a folded dipole, so the impedance will be about 75 Ohms, not 300 Ohms. I would not have thought a 1:4 balun would not be the right thing to use.

[brian_mk]

I mounted the dipole on a short tubular mast so I can align it (at least to some extent) for maxiumum signal strength indicated on the tuner signal level meter.

Lack of space in the loft means the angle I can move it through is a bit limited. However, I have a good idea of the ideal orientation from my initial tests downstairs.

[paulsherwin]

Have you tried mounting it vertically?

MK doesn't have particularly good national FM reception. I imagine you are trying to use Oxford or Sandy.

[brian_mk]

When I tried orienting the dipole vertically downstairs, it made little difference.
I've not tried it in the loft yet.

I assume the VHF transmitter (Sandy Heath?) broadcasts both horizontal and vertically polarised signals to acommodate FM antennas in vehicles.

[paulsherwin]

Thinking about it, you are probably using Peterborough. Sandy is a TV transmitter.

The advantage of using VP is that it is omnidirectional, so you will have a wider range of transmitters to try, and there is no need to align it. All the national services use mixed polarisation.

[brian_mk]

I may have to rely on the digital radio channels from the TV or invest in a DAB radio.
It seems a shame to ditch the Technics tuner.

Before I moved to MK, I had a large Galaxie 17 VHF aerial mounted on a rotator:-
http://www.ronsmithaerials.com/catalogue/G17.php

That gave over 15dB gain. On good days, I could receive French FM stations.
I had to sell when I moved as it's a bit too big and obtrusive.

[julie_m]

The length of each arm should be 299792458 / (4 * f). You will need a balun to connect it to a coax socket.

[brian_mk]

Just been looking at the transmitter maps:-

https://ukfree.tv/transmitters/analogueradio

I believe Sandy broadcasts Three Counties (95.5 MHz) and Heart (96.9 MHz).
My dipole is aligned in that direction.

I'm not sure where I'm receiving the BBC stations from - it could be 180 degrees in the opposite direction i.e. Oxford

[fetteler]

You will be able to work out which transmitter you are listening to by noting the frequency of the BBC radio stations and comparing them to this list - the link should take you to a pdf document...

https://www.google.co.uk/url?sa=t&rc...,d.ZGg&cad=rja


Steve.

[G8HQP Dave]

Without a balun much of the signal will be coming from the coax outer. This may favour a short coax. The balun, if present, should either be right at the dipole or about a halfwave (1.5m) away. The instructions on that web page don't seem to make this clear. They are also unclear about the ends of the dipole; you need good spacing from any other conductor so the typical HF wire antenna insulator might not work too well. Better to use a rigid rod for the dipole elements.

Your best FM signal probably comes from either Peterborough or Oxford.

A thin dipole will be best only over a limited frequency range, so for FM a folded dipole may be better - although this will need a 4:1 balun.

Originally Posted by julie_m

The length of each arm should be 299792458 / (4 * f).

This only applies to an infinitely thin dipole, with zero feedpoint capacitance. Everyone else can use 300/4f (f in MHz) to get a rough answer - a thicker dipole will have slightly lower resonance frequency and wider bandwidth.

[brian_mk]

I disagree with the length formula "299792458 / (4 * f)".

That doesn't take into account end effects or the thickness of the wire/tubing.
If you use the formula above for 98MHz (middle of VHF band II), you get a total length of 1.53m.

The actual required length is less than this as descibed here:-
http://www.radio-electronics.com/inf...on-formula.php

It works out to be about 1.46m.

[brian_mk]

Cheers G8HQP Dave.

I am actually using rigid rod - about 8mm dia.

I am also using a balun close to the dipole. I tried 3 turns of thin coax though a 25mm ferrite toriod. However this came from my junk box, so I was unsure of the frequency characteristics of the material. It may not work well at 100 MHz. Hence I also tried 4/5 turns of the feeder cable (air spaced) near the dipole. It made virtually no difference.

[paulsherwin]

Try using Peterborough with a vertical dipole. I would expect the Peterborough signal to be better than Oxford, but Peterborough will be nulled if you have a horizontal dipole aligned for Sandy / Oxford. You should still be able to get the local stations from Sandy.

In my loft I use a home made folded dipole and a cheap 300-75 balun. It's just made with wire and a stick.

Don't ignore my comment about screening from roofing tiles though - it could well explain what's going on here. An external dipole would be the only solution in that case.

[brian_mk]

I'm pretty sure my roof tiles are made from concrete.
My TV antenna works ok in the loft. OK it's a higher frequency, but I doubt if the tiles would attenuate VHF signals more than UHF.

[G8HQP Dave]

Just found that Bow Brickhill is your local FM transmitter. This appears to be a fairly high power (10kW) relay fed from Peterborough.

[paulsherwin]

The present alignment may well null that out as well.

[colourking]

Originally Posted by brian_mk

I was also surprised to find the signal level was higher when I disconnected the dipole leaving just the 15m coax!

Check that the coax screen is making good contact on the plug at the receiver end (and at any joins / sockets if present).

As others have suggested, try vertical polarisation in the loft (also ensure the coax cable comes away from the dipole at a right angle for at least 80cms or so.