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There is a loop antenna that was thrown around an attic interior several decades ago, of long forgotten dimensions. I found the feed line. Is there a way to measure what (U.S. amateur) bands on which it might radiate most efficiently (if at all) without physical access?

Possible resources: A vintage Philmore FS-45 field strength meter. An old Versa Tuner with an SWR meter. Two portable SW radios.

ADDED: Is there a way to test or measure the characteristics of this antenna without keying a transmitter?

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Since you apparently have access to the feed line, borrow an antenna analyzer and sweep the bands to find the resonant points and make note of them.

Some of us "OF's" still know how to use a "Grid Dip Meter" or "GDO" to loosely couple to the feed line and "DIP" the antenna to find it's resonant point(s).

But overall your best and easiest way would be with the "antenna analyzer" such as the MFJ-259 B or MFJ-269.

Check with your local club or other Hams in the area: chances are someone has one they will loan out and or help you make the test.

One other thought, since the antenna and feed line have been "in place" for a number of years. You might want to replace the feed line with something newer like coax. If you do test the antenna and find it useful, I would suggest a 1:1 balun at least at the feed point (depending on the frequencies involved and terminal impedance a 4:1 balun might be in order to give your transmitter a better "match").

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  • $\begingroup$ There is nothing I see in the question that suggests a 4:1 balun is in any way desirable. There's also nothing to indicate that the feedline isn't already coax. Or, perhaps coax isn't good for this application. $\endgroup$ – Phil Frost - W8II Feb 11 '14 at 15:34
  • $\begingroup$ Your Right Phil, "There's also nothing to indicate that the feedline isn't already coax." , What is indicated is that the antenna is several decades old in which case it would be a good idea to change the feed line to something "newer", assuming that the antenna it's self is useable. $\endgroup$ – W9WLS Feb 12 '14 at 12:45
  • $\begingroup$ It used to be pretty common practice back in the earlier part of the century to do this sort of installation for "Broadcast Reception ,, since a "LOOP" is essentially a "Balanced" antenna if he intends to use it he might need to make a transition to an "UN-Ballanced" (coax) feed line,,and depending on the resonate point's of the loop and it's other characteristics he might need or want to use a Balun to help keep the transmitter happy ! $\endgroup$ – W9WLS Feb 12 '14 at 12:57
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One way would be to connect a HF transceiver to the SWR meter, and that meter to the antenna. Assuming proper licensing to broadcast on bands - tune to the middle of each amateur band and check the SWR while transmitting a tone (or whilst whistling). The lower SWR reading would indicate closest resonance to that band. Checking at various points around that band could narrow the resonant frequency. Caveat (1) be licensed for the band/frequency you're going to test; (2) LISTEN on frequency before tuning and disrupting another QSO; (3) keep your test short and be sure to identify your call sign and the test as per regulations in your locale.

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  • $\begingroup$ An overall good approach, but I think worth noting that measuring the SWR doesn't necessarily mean the antenna is radiating effectively. The "antenna" could be a 50 ohm dummy load. $\endgroup$ – Phil Frost - W8II Feb 11 '14 at 15:37

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