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Old 15th Feb 2018, 1:44 am   #33
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Join Date: Jun 2006
Location: Chard, south Somerset, UK.
Posts: 6,188
Arrow Re: My first frame aerial

Type of feeder with a frame / loop aerial.
Earlier, I advocated my understanding that for the optimum results with that type of aerial, a balanced feeder was the best (but not the only) choice.

My personal thinking about that goes along these lines.

1. The loop / frame aerial with its tuning capacitor, but not connected to a feeder, is a balanced aerial: it has no connection to earth / ground, etc.*
2. In that sense, as an aerial, it is similar to the classic centre-fed half-wave dipole, without a feeder attached.
3. It is widely accepted - and with good reasons - that a dipole aerial should be fed with a balanced transmission line for optimum results.
4. A consideration of 1, 2 and 3 above strongly suggest therefore that a balanced feeder should likewise be used with a frame / loop aerial for same reasons.

*Strictly speaking, that is not exactly true, of course.
1. There will be some capacitive coupling from each lower part of sides of the frame aerial to ground. And that capacitive coupling will be less at the top of the side windings. However, if the frame is carefully constructed to be physically symmetrical, left and right sides, those capacitances will be equal, thus the balance to earth / ground will be maintained.
2. A typical ex-broadcast receiver tuning capacitor is not a balanced variable capacitor: it is not constructed to be physically symmetrical, although I would expect that lack would have only a very minor 'unbalancing' effect.
However, that does not hold when that capacitor is adjusted (for obvious reasons). I would expect 'Hand-effects' to disturb the optimum settings for elevation and azimuth. But how much? I wouldn't like to predict!

I have added an attachment which is of some relevance and interest. I believe I scanned it from an old PW magazine which I was given a long time ago. Note that this author specifies 300Ω balanced line.

Just a few thoughts: my attempts to develop this fascinating thread.

Attached Files
File Type: pdf Constructing an AM loop aerial file 1.pdf (1.39 MB, 26 views)
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