MAKE A VHF / FM AERIAL THE BBC WAY
Here is an interesting little project for you to contemplate on a boring winter evening - or even on a boring summer evening - Make your own high specification VHF/FM aerial.
If you haven't got DAB and don't intend getting it, then make the most of your Hi-Fi FM tuner and provide it with the signal it deserves and construct a new antenna.
The measurements could even be scaled down to make an aerial for your new DAB radio, should you eventually decide to invest in that new-fangled technology.
Hi Mike, I am very keen to build the BBC VHF (FM) radio aerial, but I have a couple of questions if you don't mind.
From the instructions, is the dipole polythene mounting block the only piece of insulation needed? Is there no further (electrical) insulation required between the boom, reflector, dipole, directors and balun? Also, I may need to (somehow!) reinforce the arms as we have big birds here!!
Many thanks, Bill Dobbs.
Hi Bill, Thanks for your email. Yes, the polythene block is the only insulator used, to ensure that the driven elements cannot make any contact with the boom but are electrically connected via the balun pieces. The directors and reflector are attached directly to the boom metal to metal. Using metal saddles will strengthen the construction - I would consider them essential to strengthen the elements against birds! You may be able to salvage suitable saddles from old broken aerials, or obtain new ones from amateur radio antenna specialists such as Sandpiper Aerials perhaps.
The supporting pole will have an effect on performance and should not really be positioned as in the photograph! If the aerial needs to be mounted vertically then the mounting pole should really be non metallic, such as fibreglass. However in the UK most VHF / FM aerials need to be mounted horizontally, in which case using a metal pole will have no effect on the aerial.
Note: Most BBC VHF transmitters use mixed polarisation with an equal vertical and horizontal component, in which case use a horizontal receiving aerial. However some smaller relay stations may use vertical polarisation, in which case the receiving aerial should also be vertical. Many commercial radio stations also use mixed polarisation with equal power transmitted in the horizontal and vertical planes, in which case use a horizontally polarised reveiving aerial. Some commercial radio transmitters can use just vertical polarisation, or have most of the power in the vertical plane with only 25 % of the vertical power amount transmitted in the horizontal plane - in which case use a vertically polarised receiving aerial.
I hope that helps. Best wishes, Mike.
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