Re: OT: the dreaded FM halo

In message <slrnf8d417.3t.abuse@xxxxxxxxxxxxxxx>, Paul Ratcliffe <abuse@xxxxxxxxxxxxxxxxxxxxxxxx> writes
On Sat, 30 Jun 2007 17:24:11 +0100, charles <charles@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:

I thought you said baluns were unnecessary at VHF?

You misread one of my previous posts. I said that Antiference once stated
they were unnecessary. ;-(

OK, but I don't remember you disagreeing.
What is the signal loss in dB due to mismatch going from 300R aerial to 75R
tuner input compared to that from 75R aerial/balun to tuner?
What is the signal loss due to balun typically?

Because the folded dipole is bent into a halo, it probably won't be 300 ohms. It will be lower - say 200 ohms instead.

From open circuit, the pot-down into 75 ohms will be 75/275, or 0.27, appx -12dB.

With a 75 ohm aerial, from open circuit, the pot-down will be 75/150, or 1/2, ie -6dB.

The mismatch loss is therefore 6dB.

For a 300 ohm aerial, the mismatch loss will be 8dB.

I hope I've done the sums right!

Ian.
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