Date: Sun, 11 Jun 2017 16:33:19 -0600
From: Larry Benko <xxw0qe@comcast.net>
To: amps@contesting.com
Subject: Re: [Amps] Ten-Tec Titan 10 Meter Input SWR
<Gerald,
I have done this exact problem this many times for a high power ISM
application at 13.56MHz.
1.) For example put an "L" network between the amp and the exciter.
2.) Adjust the "L" network for a 1:1 match at the exciter at full amp power.
3.) Assume the amp impedance at full power is 40-j10 (but you don't know
this value)
4.) For a low pass "L" network with the shunt capacitor at the
transmitter side a match occurs with L=340nH and C=112.5pF (you do not
need to measure the component values)
5.) Now take the "L" network, reverse it, and measure the impedance with
any VNA with the "L" network terminated in 50+j0.
6.) You will measure 40+j10 with the analyzer.
7.) The amp input impedance is the complex conjugate of the measured
value or 40-j10.
The Smith Chart is not useless to solve this problem.
73,
Larry, W0QE
## I used a slightly less high tech approach.
I put a hb PI net with adjustable arco compression caps,or
broadcast variables for the C1 + C2..and used the GM3SEK PI spread sheet
to calculate the values of C1 + C2 + the coil for XXX Q.
Most tubes will list the typ input Z anyway, so you can get pretty close on the
1st pass.
Tweak tuned input PI Net for min swr when amp driven full bore.
Disconnected output of PI net from cathode, and substituted a 0-500 ohm pot.
Install MFJ on input of PI net. Tweak 0-500 pot for 1:1 swr on MFJ.
Disconnect pot, and measure its resistance.
Then plug in the dc resistance value just measured into the GM3SEK spreadsheet,
but this time input the desired Q.
Spreadsheet spits out C1, C2, L values.
Measure the tube C from grid to cathode with lcr meter.... it will be the same
as what Eimac
sez it is on their spec sheet for any given GG triode.
Since the grid to cathode C is directly in parallel with the C2 cap, the tube
C is then subtracted
from the spreadsheet C2 value.
Tweak both caps + coil with my B+K 875-B digital LCR meter to above values.
Apply drive, then measure input swr. This will typ result in being 75-90% in
the ballpark for swr.
Then tweak both caps for 1:1 swr.
You are now done. Well not quite. Process is repeated for each ham band.
If the tweaked up C1 + C2 values dont jive exactly with the spreadsheet, too
bad, so sad. Its now functional.
On a side note, I installed bird + CD watt meters on both sides of my hb
tuned input, that uses a pair of
4 x section, 18-540 pf per section broadcast caps...with all 4 x sections of
each cap strapped in parallel.
8 ga solid wire used for the 4 uh tapped coil. Both caps padded on 160M only.
Since the tube I used has a 50 ohm input Z...I fed the output of the wattmeter
into a 50 ohm dummy load.
With 200 watts into 1st wattmeter, and C1 + C2 tweaked for 1:1 swr into
dummy load, the output wattmeter
would also read 200 w. Except on 15M. On 15M only, with 200 watts in...
it only read 160 W out....and coil
would get a bit warm...and a 1:1 swr. Increased uh on the 15M tap by
just 1/4 of a turn.....and a slight reduction in the value of the C1
+ C2 caps. Now its 200w in....and 195 watts out... and 1:1 swr. .
I wonder how many folks have measured the power output of their tuned inputs ?
Point here is, you dont want the Q too high on the upper bands.
If I had not have measured the PO of the tuned input, I would have ended up
with less drive to the tube, cuz of tuned input losses. This tuned input
was for a 160-15m amp... no 10+12m. I used a small 6 position switch for the
coil taps. And tapped for 160-80-40-20-17-15
I should have used a 10 position switch and added extra taps, to fine tweak Q
on the low bands. No big deal. I like to keep the Q a bit on the higher
side if possible, even if that doesnt result in covering the entire 160-80-40
band. So one setting for CW..and one for SSB. Kw tune + load caps have to
be tweaked anyway for cw + ssb. So both the manually adjusted tuned
input....and manually adjusted kw tune + load caps..and both bandswitches
are all tweaked in one go. I do not gang the tuned input band switch with
the kw PI net band switch. I want them separate.
Jim VE7RF
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