[DSP-10] Brickette QRO QRV at n5bf/6

[Courtney Duncan]

Yes, I have a copy of the QST article, in fact, a color one that I 
printed from arrl.org.  Higher resolution pictures!


kd7ts wrote:

> Confirm the winding of L4 and L5.

This is a little confuing.  Figure 1 says 5 turns on a T-25-17, but in 
Figure 2 it looks like four turns to me.  I have tried both what looks 
to me like four turns and five turns.

Originally I built it with what looks to me like 5 turns on l2, l3, l4, 
and L5.  To get it working with the SWR light out, I removed one turn 
from each.  At that point I had inductors that matched Figure 2 and 5.1 
watts out forward.

> 
> If these are wrong, spreading or compressing their turns has very
> little affect on tuning.
> 

In that configuration, spreading or compressing L4 or L5 had little 
effect.  Today I took them out and rewound them with what looks to me 
like 5 turns again.  Now I can get some roll off by compressing the 
turns.  If I push them all up together, it drops under three watts.  I 
think this is the right behavior.  Increasing the inductance like that 
should lower the cutoff frequency of the low pass filter.  I spread them 
out to where I'm getting 4.9 watts forward again.  On the schematic it 
looks like L2 should be involved in low pass filtering also, so I took 
it out and rewound it too to what looks like to me is five turns.

With this I can get reduced power by compressing L2 also and so spread 
it out to where it does 4.9 watts forward again.

Since this is a low pass filter and since I possibly had inductors too 
small, it is possible that the difference between 4.9 and 5.1 watts 
forward on the Bird wattmeter to a 50 ohm resistive dummy load is power 
in a harmonic that is now adequately suppressed.  I don't have any way 
here to measure for this hypothesis but, thinking this, I think I'll 
leave it this way.

I cannot see how L3 is involved with anything but turning the RF and SWR 
lights on so I left it as it was, at what looks to me like 4 turns.  It 
had to be retuned to re-extinguish the SWR light and now I'm getting 
expected RF and SWR light behavior again.  Is there some reason why L2 
and L3 have to be the same value?  I simply couldn't get reflected power 
to null with L3 at what looks to me like 5 turns.

> If the peak power (forward) is well defined by changing the spacing,
> they are correct.
> 

The peak should be broad, right?  I'm getting a peak at about the widest 
spacings on L4 and L5 (and L2) and can certainly reduce power by 
compressing turns now.

> check the pins of the T/R relay for connection and solder bridges.

Done.  Nothing anomalous found.

> 
> confirm the input and output coax shields are connected at both ends.
> 

I originally used a good piece of some scrap RG-58 for this which was 
big, ugly, hard to bend, and could have been a problem.  I've replaced 
all three RF connector jumpers (Drive, Rx, Ant) with nice miniature 
coax.  The ends are all pretty and well made.  There is no change in max 
output.

> Do you have a copy of the original QST article ?
> 
> 73 KD7TS
> 
> Mike
> 
> 
> 

So, with all this, I still think I just have an underperforming module. 
  The only other thing I can think of is inadequate DC but I don't time 
right now to try to get above the 12.4 V (after connector drops and 
things from a 13.8 V regulated supply) that I currently have, when at 
max out (13.6 Rx, and I think I could rewire the power to get like 13.0 
at max out).

The low pass filter is supposed to be a matched low pass filter, not 
particularly a matching network, right?  Everything should be 50 ohms 
end to end and the low pass filter is just designed to be that way but 
not to deal with any mismatch.  Yes?

Oh, on the matching network L1 and C4, I do see a few percent change 
when I bend L1 around.  I have it peaked right now too.

Thanks again Mike for the consultation.  This isn't where I can't live 
with it operationally, but the mystery is part of the fun and if there 
is something grossly wrong I'd like to understand and fix it.

Courtney