Radio hams - an aerial question..

[wishbone]

I've asked a few people on here about using CB aerials for fm radio a while back, but had no definite advice as to whether it would work ok.

From what I can gather a CB aerial would need to be shortened by about 3/4 of its length to go from 25mhz to ~100mhz. ( I think? )

So my question is would I be able to use something like this if I chopped it just above the middle coil. I assume that the base loaded coil makes it hard to work out properly.

Reason I ask is I fitted a new mast in the standard place and it has lasted 2 weeks before being killed by a tree - not even off road, just rural..

Cheers for any advice,
Rob.

[tavvi]

If that antenna is a quarter wavelength long it should be about 2.64 metres long without a coil - assuming that it is optimised for 27MHz and that they have allowed for a 5% reduction in the speed of electro-magnetic waves when travelling through the antenna wire as opposed to free space. You could calculate the approximate length of the coil by subtracting the actual length of the antenna from 2.64 metres and use that value when calculating how much to cut off. I would hope that the product spec provides the coil length though.

If, for example, it is only 2.0 metres long then the coil should be providing about 0.64 metres of the length.

A quarter wavelength antenna for 100MHz should be approximately 71 centimetres long without the coil - using the same assumptions as above. In the example above you would cut it so that you are left with the coil plus another 7 centimetres.

However, I would recommend that you call Procom UK (01227 743099) and ask for Matt Luckhurst. They supply FM aerials and will cut to size if you have a specific requirement. I also found that their prices were very reasonable. You might need an adapter to fit their aerial to your mount but they can probably supply these as well. I have no connection with the company other than having bought from them on the recommendation of an acquaintance.

http://www.procom.dk/products/mobile...nas/gps-c-r-fm

[wishbone]

Thanks for the detailed reply - I'm thinking maybe lose the coil on my old springer aerial and cut the whip to the 71cm you stated and see how I get on.

If no joy I'll contact the place you linked to.

Cheers!

Rob.

[wishbone]

Well the coil on my springer just unscrews, so it's a case of unscrewing the coil, cutting a bolt down to join the 2 pieces (replacing where the coil was) and chopping the aerial down a bit. I went with 75cm as that is nicely in the middle of the bbc radio stations frequencies (88 - 100)

Mine was complicated slightly by a corroded grub screw needing drilling out, and not having a bolt with the correct thread - I re-tapped with a metric tap.

Off to look for wing mounts.

[tavvi]

Having refreshed my memory via the ARRL Handbook I was surprised to find that no extra allowance should be made for the coil, i.e. if the coil takes up 15 centimetres of vertical height then that is what should be used in the calculation - not the length of the unwound coil. It might be worthwhile trying to cut to length with the coil included since the coil will provide some additional resilience for the antenna.

In theory the type of mount can also affect the antenna efficiency, i.e. the mount might also have to be included in the length measurement.

With or without the coil, if you get an overall effective length of 75cm it should be fine.

[wishbone]

It might be worthwhile trying to cut to length with the coil included since the coil will provide some additional resilience for the antenna.


With or without the coil, if you get an overall effective length of 75cm it should be fine.

Bit late with the cutting advice (see pics above)

I'm pretty sure the whip will be robust enough for my use.

Thanks,
Rob.

[wishbone]

I've just rigged up a spare stereo in the garage and connected the aerial to it. It's picking up stations better than the radio in my car outside the garage!

Thanks for the help Tavvi, just got to sort out wing mounting it on the drivers side.