Shorten FM aerial elements?

[stevehertz]

My roof mounted, vertically orientated single dipole FM elements are 75cm each (each half). I want to shorten them to obtain better performance in the higher frequencies (100 - 108MHz) as stations in that frequency band are generally quite weaker than the BBC ones that are positioned at 88.3 to 92.7MHz and perform very well (I don't count BBC R1!). I also use the dipole for DAB transmissions and so they too will also be helped a tad by shortening the elements. All my feeders, distribution etc are all in place and fine. How much should each element be shortened? I'm not looking for a 100% 'spot on' perfect answer (if only aerials were so predictable) just something that will, in general terms help achieve my stated aim, so please keep it simple and to the point guys.

[paulsherwin]

Just calculate the wavelength from the frequency and halve it. I don't think it will make a lot of difference though, so may not be worth the hassle, especially if it involves getting on the roof.

It won't make any difference at the DAB frequencies.

[Herald1360]

You won't notice any improvement less than 3dB or so and you won't see anything like that from a bit of element trimming.


If you fancy a bit of aerial building and rigging, you could try a 5/8 wave design. Popular for mobile two metres back in the day.


http://www.qsl.net/w4sat/five8th.htm

[Nuvistor]

Each leg being 75cm would mean it is already cut for 100Mhz, I cannot see it making a lot of difference moving it up a few MHz.

Frank

[Philips210]

Hi.

I wondered about building a tunable active aerial for the VHF broadcast band so that weaker stations could be received more clearly. Has anyone gone down this route and can recommend a suitable aerial and active amplifier circuit?

Regards
Symon.

[stevehertz]

Ok, I tend to agree. All said and done I'll leave it as it is. Thanks guys.

A J pole might be worth a thought.

[G8HQP Dave]

Shortening the elements to get a small improvement above 100MHz would have made it worse for DAB, so good that you have decided not to do it. This is because improving the quarterwave match at, say 105MHz, would worsen the halfwave match at 210MHz.

[stevehertz]

Brilliant deduction, I hadn't thought of it in 'wavelength terms', just length!

[ms660]

Quarter wave?

Lawrence.

[Nuvistor]

Steves FM aerial is indeed a half wave dipole, this will have an impedance of around 75ohms at 100 MHz, at 200 MHz the impedance will high, i.e a bad match but as these things usually are it will receive some signal on 200Mhz, just not produce the best results but if good enough that's ok.
An interesting artefact, if that's the correct term, is the 3rd harmonic will also present a reasonable match, this was useful on the amateur band 2 meters, a half wave dipole for reception works quite well on 70Cms.

Possibly a reason why some Band 1 aerials worked quite well on Band 3, depending on the relationship between the channels used.
Frank

[VT FUSE]

By definition,a roof mounted VHF Yagi will have sufficient gain and fair bandwidth for receiving duties in most circumstances,I note your desire to improve signals at the top end of the FM band however.
Especially important at VHF is feeder loss,maybe worth making sure that you use the best quality possible to minimise feeder loss especially if it is a fair lengthI wonder how old your present feeder is?
Also a suitable masthead amplifier may yield results but unless carefully designed and implemented may just serve to raise noise.

[VT FUSE]

Sorry,just re-read your post and note your comments of feeders which I failed to note.

[G8HQP Dave]

Originally Posted by ms660

Quarter wave?

Sorry, I forgot its a dipole. Halfwave (FM) and fullwave (DAB).

Originally Posted by VT FUSE

By definition,a roof mounted VHF Yagi will have sufficient gain and fair bandwidth for receiving duties in most circumstances

A Yagi is an inherently narrowband antenna. It takes a lot of work and compromise to design a wideband Yagi; the usual result is low gain. Fortunately, broadcast reception is often uncritical.

[GMB]

What will be much more significant is how the feeder is attached to a vertical dipole. If you don't have a balun then I suggest that what you consider to be the aerial is probably not doing so much, especially as it is a terrible match for DAB.

I am guessing that you are (as so often happens) just receiving using the coax outer. The dipole sort-of joins it up, rather badly. If so then the size of it will not be well defined.

[Biggles]

I have used a masthead amp on an FM antenna at my location as some of the stations are a bit weak here too. It is a Velleman kit, available from ESR, and you can configure it to be supplied with power via the coax. Makes quite a worthwhile improvement. I think Bowood also do a similar sort of thing.
Alan.

[stevehertz]

Originally Posted by GMB

What will be much more significant is how the feeder is attached to a vertical dipole. If you don't have a balun then I suggest that what you consider to be the aerial is probably not doing so much, especially as it is a terrible match for DAB.

I am guessing that you are (as so often happens) just receiving using the coax outer. The dipole sort-of joins it up, rather badly. If so then the size of it will not be well defined.

My coax is indeed connected directly to the aerial - as is the practical norm - for better or worse. Please could you provide a link to a suitable balun for my application (as described in the first post) as the word 'balun' is used to describe and sell a multitude of different products. Thanks.

[GMB]

At VHF a balun can be done quite trivially.

So a clamp-on ferrite near the feed point is a simple solution.
Better is to make a small loop of the coax so it runs more than once through the ferrite clamp (now needs a bigger hole). But not too small a loop or you damage the coax.

I usually put another clamp balun a bit further along the coax, where it has got well away from the aerial itself.

If you have any doubts about the tuner's balance requirements (as some were meant not for coax but twin line) then put another one down near the tuner.

This should then ensure that it is the aerial and not the feeder that is doing the work. But of course, if the aerial is inappropriate as yours sounds it is (on DAB) then this may just make it worse as the coax outer acts as a great big vertical wire.

[ms660]

A folded dipole has a wider bandwidth, maybe one for FM and one for DAB, I'm sure they're available with transformers.

If you want the best reception then you need a proper antenna system.

Lawrence.

[julie_m]

I made a1.5-wave (0.75 + 0.75) folded dipole for FM reception, which worked better here (deep in a valley) than an earlier, 0.25 + 0.25- wave attempt.