Coupling Loop Tuning

I constructed a 12 ga wire feeder loop somewhat longer than 36″ connected to an S0239 socket for the 15′ loop and this time it tuned up easily to an 1:1 SWR.  The wire feeder is cheaper and easier to construct than coax.   Prior to that I tried an rg59 loop faraday design with no luck, which would do no better than 4:1  SWR.

I learned off the forums that the feeder loop to primary loop impedance is a function of the area / aperture of the loop and not the length. The rule concerning length of the feeder loop as 20% or 1/5 the primary creates a feeder loop with 4% of the aperture of the primary.

This could be why distorting the feeder is necessary to tune in many cases:  changing the shape changes the aperture.   The loops need to cut through flux lines so positioning is also important – exactly where the flux lines are to cut is another question.

The purpose of the faraday design  is to shield the feeder from nearby electrical noise and improve the S/N.  Now I think that will be secondary since the copper primary works well.

Day 1. 24 July. Using the National Cap, the bandwidth is very wide however, like around 150 khz for 40M (24july).  This is probably due to ohmic losses.  While the loop in transmit mode got reception reports on the mainland, the end fed made qsos the loop did not.  day 1 of operations tests.

Day 2 26 Jul.   Different story. Loop made 7 qso’s in JA, vk4,  n7 n5 k7 w2.  599 reports and in other areas lower signal like 559.  Standard for DX in the Pacific rim.  Comparable to the EF on this night.  Buzzing noise background present throughout which seems to be drowned out in white noise on the endfed.

Daytime 7/26 — noise endfed s1, noise loop s0.

7/27 0400Z evening loop is gonzo. resoldered the center terminal (broke up so was cold) and maybe that fixed it (solder job is not great either so need to work on it) – the socket is good though and allows cable swapout.  Loop tunes to swr 5 near the railing and 1.0 in the garden so loop must be off the ground a ways to tune 1.0.  But the cap is very unstable tonight, plus the rain does NOT help.  On another note, at swr 1.4 the radio power is already way down … max may be around 1.2:1 before radio cuts back (switch antennas and power comes back up to normal so it is the swr that is killing the power).  OK at 1.1:1 definitely … maybe 1.2 not sure.

~ by marksun on July 25, 2013.

2 Responses to “Coupling Loop Tuning”

  1. The copper wire feed loop is a good way to test and obtain proof that the primary is working. Regardless of its simplicity, it works. It highlights again that the primary and feed loops are two separate entities. The primary loop needs the best possible low loss connections to the capacitor. The capacitor has to have the most direct low loss connections possible. A 150pf variable cap is nearly impossible to tune without a gearbox – 16:1 is not enough. For psk at 14.070, a visible movement of the cap plate represents 10-50 khz change of the resonant freq.

  2. Rewired the copper loop with a BNC plug. Initially didn’t work at all – coax pulled out of the -F- connect or on the trxline. Then I could not get a good tune by moving the “coupling” loop. This is the current status.

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