[Kenwood] Signal loss through outboard AT

[Dr. Gerald N. Johnson, electrical engineer]

That a particular antenna tuner removes AM broadcast reception is not
surprising, nor is it surprising that any different antenna tuner
affects AM broadcast reception differently.

The function of an antenna tuner is to correct impedances and reactances
to present a matched (generally 50 ohm resistive) load to the
transmitter at a narrow band of frequencies (minimum 2 kHz) at the radio
frequency desired. There are many circuits used to accomplish this, some
do it better than others, all have differing efficiencies (well
documented in QST a couple years ago) for different amounts of antenna
mismatch. And ALL have different bandwidths, depending on the antenna
mismatch. Far from the tuned band (and the original post says nothing
about WHERE the tuner is tuned, if its tuned) some act as high pass
filters, some as low pass filters, and some as band pass filters. The
most popular of T tuners with a shunt coil act as high pass filters,
which is not great for TVI prevention but deadly on LF reception. Figure
a T filter with 100 pf series capacitor, quarter microhenry shunt
inductor and another 100 pf series capacitor. At 1 MHz that's series X
of 1600 ohms, shunt of 1.6 ohms, series of 1600 ohms feeding a 50 ohm
receiver input. Great gobs of attenuation at 1 MHz, as expected for a
high pass filter tuned to 10 meters.

If you want strong signals on AM broadcast WITH a tuner, you need a
tuner made for the BC band, with LARGE components. No mere 25 microhenry
coils, no 250 pf capacitors, think 1500 pf variable capacitors, and a 1
millihenry variable coil. And then you have to tune that tuner to the BC
band at the FREQUENCY you are interested in. Expect a narrow bandwidth
unless your antenna is self resonant at your favorite BC frequency.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.