- December 4, 2020 at 2:40 pm #116372Don HallParticipant
Total posts : 8
I’m just getting underway with my Part 15 project after 25 years in commercial radio programming and production (ending with a relocation to an area where no one knew me). I just retired and thought it would be fun to find an unused frequency and invent something.
I got one of the Spitfire transmitters from 6v6.co.uk and it’s been an adventure. Nice little unit, but I had a lot of trouble with hum. I fixed that (mostly) by relocating the tx outside, to the base of the antenna mast. Now, however, I’ve lost most of my coverage area. I was able to get 300’ or so down the street, but now can barely copy the signal in the car in the driveway. Tx is grounded to an 8’ copper clad rod about 3 feet away, audio is from the PC soundcard, frequency is 1650khz. Antenna is the supplied 10’ wire, vertical on a PVC mast, about 10’ from the next-door neighbor and 4’ from my wall.
My idea is that when the installation was indoors, the RF was somehow coupling to the house wiring and then re-radiating (to some advantage) into my receivers and the neighborhood. Now, less re-radiating but more shielding and signal absorption between stucco houses.
Thoughts? Suggestions?December 4, 2020 at 3:47 pm #116374MarkModerator
Total posts : 658
A few thoughts. As I understand the Spitfire has a red light on the front that indicates when you are peaked for max signal to the antenna…either it goes off or on? Maybe with your outside set up without a meter or voltmeter to actually see if you are peaked to max you don’t know and you are getting less of a peak with your set up than you did indoors. If you can put an analog multitester in place of the LED to see actually where you are peaking it may answer your reduced range outside.
Secondly the Spitfire wasn’t made to be outside with extra ground lead to your copper stake. It was made for indoors. To get better range with your set up with 3 feet of ground lead lessen the length of the antenna by say 1 foot and see what happens. Or put the transmitter closer to the grounded stake to have 1 foot of ground lead….see what happens.
Also and most important the Spitfire is a broadband final amp with no loading coil on board which means to make it work better you have to make your own coil at the frequency you transmit at and tapping the coil at the right spot for the 3 meter antenna. Here’s a video of the idea but you’d have to make a coil for your medium wave frequency….https://www.youtube.com/watch?v=tDT8Aq5tmGI
The stucco has little effect as it’s not metal. As for the hum this is partly the supplied power supply that not that good and other things plugged in in the room.
Here’s a picture of a transmitter with the loading coil on board….
December 4, 2020 at 5:14 pm #116375Don HallParticipant
Total posts : 8
Thanks for the prompt response. I have not tried to re-peak the ATU since I relocated the tx, and that should be my next move. On the Spitfire this isn’t a fun thing to do, as the trimmer cap is small and looks fragile, but takes a bunch of torque to adjust. It destroyed two of my nonconductive alignment tools in the initial installation! Also, rather than adjusting for one, obvious peak, I find myself with several smaller peaks and final adjustment is a little inconclusive. The LED seems to be an OK indicator, as it varies in brightness with the collector current of the final transistor. Maybe measuring the voltage at that point would give me a better indication. I could also tweak for max on the s-meter of my communications receiver.
I will also try changing the ground arrangement, but the lead I’m using now is no longer than the one I used in the house.
Construction of a loading coil is described in the Spitfire instructions. I intend to try something like that, but I want to get back to my baseline performance before I introduce too many variables.
Finally, I can assure you there is PLENTY of metal in the stucco on the houses in California. A stucco job starts with an uninterrupted layer of chicken wire onto which is troweled the stucco plaster. Maybe they do it differently where you are.December 4, 2020 at 8:55 pm #116376MarkModerator
Total posts : 658
The chicken wire
Shouldn’t affect you outside on a house next to you that much. May affect you from indoors getting out a little. Should be no different than a wire fence.
Tuning the capacitor to peak has to be done anytime something changes even a bit. Also the selector jumpers have to be the right selection for your frequency range.
If I remember right on that transmitter there’s a selection for external or wire antenna?
The trimmer cap shouldn’t be that hard to turn and if it broke it’s easily replaceable with one that works better with the same value at electronic parts places. Using a volt meter would work by just connecting the leads to the LED wires. An analog one with meter and pointer on the x1 resolution.
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