[Elecraft] Buryflex all the way to K3?

[Tom W8JI]

> I do have an HP 3590D spectrum analyzer sitting in the 
> shack, and put a
> simple loop on it to look at 2nd harmonics from my Ten Tec 
> 425 amp and 160M
> vertical. It was about -44 dBc when I started, and I got 
> it down to the rated
> -50 dBc by doing nothing more than changing coax jumpers.

Something else was going on Jim. Not shielding.

We cannot accurately measure harmonic suppression with a 
probe or loop unless the probe or loop is somehow frequency 
compensated for sensitivity as frequency changes. In short, 
the "antenna" used on the analyzer has increasing 
sensitivity for a given flux level with frequency. As I 
recall a very small loop open terminal voltage is about 
8*10-6th  times F times loop effective area. So your loop 
has 6dB more sensitivity as frequency doubles, although it 
could in practice be much more or even somewhat less because 
of other effects that might unintentionally compensate or 
enhance the change.

More likely you changed the impedance presented to the tank 
circuit on the second harmonic. Changing 80 meter impedance 
at the 160 meter tank can radically affect harmonic 
suppression. Harmonic suppression will be less with a low 
impedance terminating a pi network on the harmonic, and 
greater with a higher impedance on the second harmonic. Just 
changing the velocity factor (electrical length) of the 
cable a small amount can change the level of the harmonic 
significantly.

When we want to measure something, we have to be careful to 
actually measure what we think we are measuring. :-)

Case in point, I have a 2nd harmonic stub on my 160 
antennas. It is in a calculated sweet spot 1/4 wavelength on 
80 meters from my amplifier pi-network loading capacitor. 
This makes maximum possible 80 meter Z across the load cap 
on the 160 amplifier, so the loading cap looks like a more 
effective short on 80. By altering nothing but distance of 
the stub from the amp to 1/2 wave on 80 meters on the 160 
feeder, the harmonic suppression decreases 10-15 dB. Even 
the length of the cable to the matching system affects the 
harmonics, and each matching system is different! Changing 
cable electrical lengths will change harmonic levels 
significantly, even without a harmonic suppression stub in 
the system.

73, Tom