This following antennas were inspired by Harry Lythall (SM0VPO) 'flat-dwellers' frame antenna which he uses for Rx (and low power TX) on 80m and 160m. I have only tried it on the 80m band. Below are some close up pictures of my version. I found I had to remove one of the turns. The first few photos are of my copy of his design followed by my modifications and experiments.
According to John Kraus (the antenna pioneer) the impedence of a small n-turn loop (small compared to the wavelength) is dependent on n2. The 'DC' resistance (effective DC resistance after the skin-effect has been considered) will be dependant on n. So I was hoping that a n-turn (n=4 in this case) might have some advantages over a single turn loop. The first advantage is that because the 4 turn loop has a higher inductance than a single turn, the capacitor can be much smaller for the same resonant frequency. It's much easier to make a small capacitance wide-spaced capacitor which will (I hope) cope with the high voltages on Tx. I hope the later versions e.g. the 8mm copper tube (see below), will radiate something! All versions are great Rx antennas.
This loop might make a nice antenna for CW or for PSK31 experiments on 80m for example, especialy for those like me who dont have much space for antennas.
The completed copy of the frame antenna. It sits on a weighted tripod and can easily be rotated for null.
Showing a corner of the loop and also part of the coupling coil positioned using nylon cord. The wires are feed through holes drilled in the four spreader arms.
The wood structure was varnished after assembly which also helped to secure the windings.
Showing the high voltage capacitor with reduction drive and waterproof enclosure.
The loop connections go in at the bottom of the box through long sleeve rubber gromets.
Connection of the 50 ohm coax to the triangular coupling coil.
The central support showing the butterfly nut fixing to the tripod mast.
4sqrp News Letter article August 2012
Some thoughts about the 80m band
home-made tripod details
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