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The Q Question




 
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September 20, 2018, 01:19:39 PM *
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KA3EKH
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« on: September 13, 2018, 08:54:57 AM »

So now that I have all the bias issues out of the way and the 20 watt amp running in the PRC-108 the next problem is getting the radio to work with a eight foot whip antenna.
Just playing around with simple meter and not thinking about it I have managed to destroy one set of finals and am down to my last set of transistors trying to get the whip to work.
The radio works great with a 50 Ohm antenna feed from the 50 Ohm port but I want to use it now as a true portable using its internal tuner. The tuner consist of a big coil of about seventy turns with ten preset taps that select a capacitor and tap for each channel. I only have two channels that I am interested in using. Channel 4- 5.357(USB) and channel 5-3.885 (AM) channel 6-5.000 and channel 2-7.850 with channel 1 keeping one of the original channels that came in the radio 2.182
I did not get the original antenna that came with the radio so I built a plate that mounts to the same side plate and adapts an old CB antenna thatís just under eight feet.
Because I tend to overdo everything I borrowed a network analyzer from work and have been using that set up for monitoring SWR from port one connected to the 50 Ohm input of the antenna tuner. By playing around with fixed capacitors and taps on the coil I can get SWR below 2.0 or so at the frequencies (3.885/5.357) that I want to transmit on but the tuning network exhibits somewhat sharp dips at resonance. Do I need to have a flat SWR for around 10 to 30 KHz per channel? How do you increase bandwidth or the Q of such a simple circuit? Whatís the relationship between the value of the input capacitor and the inductance and how do you go about finding the sweet spot?
Attached a copy of the schematic of the tuner.



* tuner.jpg (247.58 KB, 1638x2123 - viewed 28 times.)
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PA0NVD
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« Reply #1 on: September 13, 2018, 10:31:51 AM »

Fun tranceiver!
I am afraid that you can't broaden the bandwidth of the tuner, a whip of 8 feet is VERY short for those low frequencies so the antenna will be a capacitive load with only a few ohms as radiation / loss resistance. Approx 90% will be loss resistance in ground and coil and the antenna behavior is very sensitive to the ground, carrying around, putting the set at metal like a car etc The Q will be very high and the antenna voltage as well  in order to radiate some signal. You can expect hundreds of Volts at the antenna.
I don't know if you can lower the power for tuning purposes  (e.g lower the collector voltage).  An antenna analyser is nice, but due to the high variation induced by the ground in a portable system, not really usefull.
It may be better to tune at max antenna voltage using a field strength meter or so, a neon bulb close toe the antenna, meter with a diode etc. at low power and than increase to normal power when you found the maximum. Indeed expect a sharp tuning!
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