Using a Small Transmitting Loop

Having constructed, and positioned the magnetic loop antenna- and run the co-axial feeder to the transmitter and control line into the up/down control box for remotely tuning the loop, what then?

First step is to tune the loop coarsely on receive. The loop acts as a very narrow band pass filter, and although loops are relatively insensitive to electrical noise, when connected to a receiver, the noise floor significantly comes up when the loop passes through resonance.

Look at the plot below of SWR against frequency. The noise from the receiver varies inversely with SWR shown here and you can see that there is relatively little change in frequency for change in match. The 80m loop is narrower still - and it is quite possible to build a loop that is too narrow for ssb transmission!

An SWR bridge is the main tool used when tuning an STL.  A cross needle or other direct reading SWR display is especially handy.

The received noise level peaks significantly when on tune, and the SWR dips very sharply when fine tuning with a half a watt or so on transmit.

Having tuned the loop broadly on receive, switch to transmit and using a watt or so of energy, fine tune the loop on transmit for lowest SWR. The lowest SWR I can manage is unmeasurable on a good day, but when the leaves are thick, or its very damp, then 2:1 can be good. This is probably a sign that the housing of the antenna is leaky - or that the tress that grow through the 80m loop need pruning again!

Turning the band is not as simple as with wire antennas - but with practice it isnt difficult to track the antenna with the receiver. Of course some of us have valve P.A.s that take just a mite of adjustment before hitting the TX button.

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Carl GW0TQM's Magnetic loop page
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