Jim right of course -- the exception being the old Alphas with Pi-L
outputs.
At the the old K1AR M2 we ran from my house we used a lot of stubs because
we only had two towers.
Stubs made with hard line have less loss and are deeper, but narrower
nulls. Fixing 40 into 15 is a little trickier because it as a third
harmonic. I used an open 1/4 stub on 15m with a compensation stub to tune
out the imaginary part of the leftover conductance on 40.
Double and even triple stubs slightly offset across the band can be used to
make a broader null.
The most important thing to do is to measure the coupling from each
transmit antenna into each other antenna using 10w transmit and either a 50
ohm scope or a calibrated receiver.
I made a spreadsheet of all the coupling (transcontuce) between antennas to
plan my defense between bands. Some bands needed only a receive filer.
1500W on 160m used to couple over 50W into the 80m vertical array. You can
smoke receivers if you don't watch out!
- Ken K1EA
On Mon, Jul 18, 2016 at 12:04 PM, Jim Brown <k9yc@audiosystemsgroup.com>
wrote:
> Your logic is exactly backwards, Jukka, and it applies ONLY to optimizing
> the relationship between the stub and the power amp. When the output
> circuit element is a capacitor, the stub should be 1/4 wavelength from the
> amplifier at the harmonic it is intended to suppress. When the output
> circuit element is an inductor, the stub should be directly at the
> amplifier output, OR a halfwave up the line at the harmonic.
>
> But the relationship between the antenna and the stub also matters a LOT.
> A stub works by shorting the line at the frequency of the harmonic, and
> will be most effective when applied at a HIGH Z point on the line AT THE
> HARMONIC FREQUENCY. Most single-band antennas present a HIGH Z at the
> second harmonic, so the stub should be some multiple of half waves from the
> antenna.
>
> Note also the advantage of double stubs in portable setups where length of
> the line between antenna and stub cannot easily be controlled. Even if the
> first stub (nearest the antenna) is poorly placed, the second stub, a
> quarter-wave nearer the transmitter, provides its full attenuation.
>
> All of this is covered in the applications note for which I posted a link
> last night.
>
> http://k9yc.com/LocatingStubs.pdf
>
> The next link shows how we implement stubs in the trailer that W6GJB put
> together for or CQP and 7QP expeditions. Note how close the 80 and 40
> antennas were to each other in the last setup, and the 40M dipole and C3SS
> were to each other in the daytime setups. At 600W from an Elecraft KPA500,
> we had zero problems with harmonics with CW stations on both bands.
>
> http://k9yc.com/7QP.pdf
>
> 73, Jim K9YC
>
> On Sun,7/17/2016 10:37 AM, Jukka Klemola wrote:
>
>> Placing a stub at output connector helps a tad in case your output circuit
>> that shows a low impedance to ground at the harmonic. That is typical for
>> nearly all our equipment that have a low pass circuit with a capacitor
>> going to ground as the last component of the output circuitry; a Pi
>> circuit
>> output.
>>
>
>
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