Quote:
Originally Posted by G0HZU_JMR
Can you give details about the 5 turn loop antenna.
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Ok, I just measured it. I used a loop pickup coil connected directly into the input of a 2465B scope. I had the loop antenna on my pantry TX with a VFO. The center frequency, that the antenna is tuned to is 1.48MHz.
The transmitted carrier amplitude drops down to 70% when its driven at between 1.46 to 1.465MHz and also down to 70% at 1.495 to 1.50 MHz, you should be able to deduce the bandwidth from that.
I tried another rest. Using another transmitter, this version has a fixed crystal L/O at 1.48MHz, I looked at the modulated audio carrier with 50% modulation from an audio signal generator. By 13.95kHz the modulation had dropped to about 70% of what is was at 1kHz, the peak carrier dropped by only about 10% . Some of this test result relates to the audio filtering.
In any case I have not detected any audible high frequency attenuation at all via my hacker radio, mind you I can only hear up to about 10 or 11 kHz these days, but I think the IF bandwidth in my radio would be the limiting factor, not my pantry TX or its loop antenna.
I have attached pictures of the loop, it is actually 6 turns but one is the coupling turn.
The input transformer is made from two toroids in top of each other to create a bigger core, these are Jaycar parts with a 10mm ID, about 18.2 OD and each about 6mm tall and wound to be an auto-transformer. When the antenna is peaked at 1.48MHz, it represent very close to a 50R load to the pantry TX's output amplifier.
The turns of 60 strand litz were drawn around by pull wires inserted into the PVC pipe assembly before it was glued up. As you can see its electrician's conduit. I machined a PVC base for the bottom of the junction box where the auto-transformer is to carry a brass disc with the PL259 plug connector soldered into it. This whole loop is just under 1/2 meter x 1/2 meter (small really) and its performance is outstanding.
Although the input matching network looks elaborate to get the impedance of the loop antenna to 50R, its much more difficult to get the input impedance of a whip antenna this low. It required a pot core transformer, and by the time its done the voltage at the base of the 1.5m whip gets to over 140V, more than enough to light the neon in it as shown in the two extra photos.
But there is no comparison between the whip and the loop with the same applied power, the loop wins out for transmission range.