For Stormy Days when you don't want to connect an outdoor antenna to your radio.
A dual loop coaxial Magetic Loop antenna, designed specifically for 40m but will also work on 80, 60, 30, and 20m.
> Coax: (Loop) M&P Airborne-5
> Coax: (Feedline) RG-174
> Cap: 44 - 163 pF (ARC-5 (surplus) capacitor)
> Spacers: SP-50 Spreader (from DX-Wire)
> Mast: Spiderbeam 35mm (dia.) fiberglass . . . tube, 1.15m long.
The base of the mast slides over a 22mm bolt, enabling the antenna to be rotated by hand.
The 22mm bolt is attached to (through) the table-top of the operating desk, with the head under the table and the thread side above the table.
Snowy days drove my antenna work indoors.
I have been planning to build another magnetic loop antenna for many years.
Finally I took the leap and began building.
This loop is primarily receive-only.
The 70 year old variable capacitor out of an Army ARC-5 radio is perfect for the job. The problem with magetic loop antennas is the need to re-tune every time you QSY just a few kHz.
With most variable capacitors, tuning is very tricky. You must v e r y c a r e f u l l y turn the knob. This ARC-5 cap is turned through a worm-drive with a reduction ratio of 98:1. This means, to rotate the cap 180 degrees (from min. to max.) takes 49 rotations of the knob.
IMPORTANT: This capacitor is not very good for transmitting because its contact on one side (the rotor) is a sliding contact. This will cause a problem on TX due to contact resistance, but it is not a problem on RX.
The butter-soft tuning when using this capacitor is a huge advantage. However, if you wish to transmit with this antenna, you must use a butterfly capacitor, which has no moving contacts.
Typical SWR Curve:
NOTE: The 3:1 SWR bandwidth is 30 kHz. Though this level of SWR sounds high, it is only high when transmitting. As long as we are only receiving, there is not much difference in signal strength within this 30 kHz range. If you QSY farther, you will need to re-tune.
This antenna is primarily for Amateur Radio use. It was designed to cover the 80m, 40m and 20m ham bands.
With the jumper OUT and the optional Capacitor OUT, the antenna tunes across the 20m band.
For 40 and 80m, the jumper must be inserted.
For 80m, the optional capacitor may also be inserted.
Othere frequeces may be covered by inserting a different value of capacitance instead of the 138pf Cap.
Next Question. (hi)
Initial comparison tests on 40m, comparing to my 80m 807-HDL OCFD antenna show about 25 dB loss compared to the outdoor OCFD that is mounted high and in the clear of all objects - about 60m away from the house.
Compared to my YouLoop antenna, my home-brew tunable loop is about 3 S-Units (18 dB) better.
Effectively, if the station being received is S5 or better on a very good outdoor antenna, it can be copied on this tunable indoor magnetic loop antenna.
This is not earth shattering performance, but keep in mind that the antenna is small and located indoors. It may be used safely for receive during thunderstorms. In addition, its performance is adequate for copying 41m and 49m shortwave broadcast stations at night.
More performance info to follow, after the new coupler-loop is built . . .