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Old 15th Feb 2018, 11:33 pm   #38
Argus25's Avatar
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,520
Default Re: My first frame aerial

Originally Posted by Andrew2 View Post
Argus - surely any direct connection of a feeder to 'one side' of the loop would ruin the balance, spoil the null and bring about the 'antenna effect' in which the loop begins to act as just a random wire?
I still say a single-turn coupling loop (floating) fed with coax should be fine. A step-up ferrite transformer at the receiver may be necessary if the radio has a high input impedance.
It is a good question.

I have done much more work with loops as transmitting devices for pantry transmitters than receiving loops for vintage radios. In fact I do not have any receiving loops, only transmitting ones.

When the loop is in receiving mode though, it is sitting in the far field from the radio station's transmitter. At that point the ratio of electric to magnetic field has settled out at 377 ohms or 120pi Ohms. The loop responds primarily to magnetic part of the field. Therefore, in theory, if you grounded any part of the loop electrostatically, one end, its center or near one end, it wouldn't affect the function of the loop as a magnetic receiving antenna. As an electric field receiving antenna, the loop as pointed out, is very poor.

In the case of a loop for a transmitting antenna, I'm confident the magnetic radiation pattern emerging from the edges of the loop, in the plane of the loop, is unaffected by connecting one end of the loop to ground and the coaxial sheaths of cables and other equipments. This is because it it merely an electrostatic connection and doesn't alter the magnetic fields. In the near field, the magnetic component of the loop's output is high and the electric field very low.

When it comes to extracting energy from a loop, or injecting it for transmitting, I think alterations in the weak electric field by unbalancing part of it would probably not be noticeable, that is if you were receiving the signal with a transistor radio with a ferrite rod.

Simply the lower loop turn acts as a coupling coil into the main loop, and if the impedance is transformed correctly, the current in the coupling turn is maximized at the operating frequency. It is a case of "inductive link coupling" into the main resonant loop, so you don't really regard the coupling turn as part of the "main tank circuit" (you could isolate it if you wish as a separate coil one turn coupling coil and not a tap on the main resonant winding) and ideally in this coupling circuit, at resonance, the reactive elements cancel and maximum energy is transferred to the loop and when those conditions are correct it also matches the line impedance.

For my transmitting loops, I have arranged all of them with 50 ohm input impedances. That way I can use my 50 Ohm coax cables, use 50R RF power meters for forward & reflected power meters, SWR meters , 50R dummy loads to check my transmitters, etc and have the loop more remote from the transmitter if I want.
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