The AM Forum

Having acquired a really nice DX 100B about six months ago, I consider myself a newbie to the realm of vacuum tube tech. I need some advice on the very high (15:1) SWR readings I'm getting. Here is my setup: DX100B RF output into a home-brewed relay-controlled xmit/rec switch; from there to an Alpha-Delta coax switch (switching between antenna and dummy load); then coax to MFJ 828 SWR/Frequency counter; from there coax to LDG AT-600 PRO antenna tuner; from there coax to Alpha-Delta DXCC trapped dipole (not my favorite antenna, but I have limited space). I have replaced all the coax jumpers. The antenna analyzer (MFJ 259) shows the antenna by itself is under 4:1 at its highest on 75 meters, 3.3:1 at 3.885 mHz. Tuning the DX 100B, my meter readings are all good and tuned within manual specs. Loading control is at 4 and Amplifier is at 36. Yet the LDG antenna tuner cannot get the SWR below 15:1. Thoughts? Suggestions? Advice? What am I doing wrong?

I would first:
a. connect transmitter to SWR bridge to dummy load.
tune up to specs; read SWR (high/low?)
If everything appears normal then
b. Without touching the transmitter settings, connect transmitter to LDG tuner, to SWR bridge(Optional) to dummy load
Diddle the LDG tuner for matching to dummy load.
If the LDG can't match the dummy load, LDG probably has a problem.

Further with no tuner:
c. transmitter to SWR bridge to antenna.
tune up to specs; read SWR (high/low?)
For a tube transmitter with a Pi network output and a trap antenna, a 3 to 1 SWR  at some frequency is not that bad. If it's high at all frequencies, you might have to diddle with antenna length and/or feed line length depending on the design of the antenna system.

   I agree with Pete's suggestions. Only other thing
that comes to mind is tuning up on a harmonic or NOT.
On 3885 the VFO is running 1942.5.. That would do it!

   I have seen those inexpensive ASIC freq counters
pick a second harmonic over the fundamental.

   Also make sure you knobs are set correctly, the
"DRIVER" knob should point at 100 when the cap is Full
Mesh, Same with the "AMPLIFIER" knob. Loading cap should
be full mesh with the knob at "0". Always start tuning
at 100 with the "DRIVER" and "AMPLIFIER" knobs. Loading
always at 0..

GL with that...

73

/Dan

That MFJ-259 should tell you the story.

You have a lot of stuff in series with the coax leads.  Disconnect the coax connector from the DX-100 and plug it into the MFJ-259.  Use either a 50 ohm carbon resistor and/or the dummy load on the other end to verify with the 259 that all is well at 1:1 just before the antenna.  This will prove out the 50 ohm cabling and connections after the DX-100. (Do not include the ant tuner for this test)


As already discussed, the DX-100 needs its own test. You could start with a simple light bulb on the output of the DX-100 and use a freq counter to verify it is really on 3.885 MHz.  

A VOM to check continuity through the whole 50 ohm coax chain may be helpful too.  Check your individual coax jumpers to be sure they are not intermittent.  This is a common problem with the cheap crimped-connector stuff.

T

Emphasising the basics:

-A DXCC antenna should not require a tuner on any of it's designed bands. It is as resonant antenna!

-A tube type transmitter with a PI tuning section should be able to work into most reasonable antennas. They don't need a 50 Ohm load.  If it loads up within spec's it's fine.

-Line up:

TX=>PWR/SWR Meter=>Tuner=>Ant

Get rid of all that stuff.

1. Connect the DX100 directly to the dummy load.
2. Tune up per the manual.
3. Then connect directly to the antenna.
4. Tweek the tuning to obtain the exact same final plate current you had when connected to the dummy load.
5. Now add in the TR relay. You should see no change in plate current. If you do, the relay is the problem.
6. Get on the air.

I would not add the tuner or the counter back in. They are not needed. You can monitor your transmit frequency with your receiver.

I also use an LDG AT-600 tuner with a DX-100B.  You must use low power when tuning up or the tuner is overloaded.
Mike/WA8RVD

I have my dummy load connected to one of the antenna jacks on the tuner so I can push 1 button on the tuner to switch to my dummy and tune the transmitter. Then I reduce the loading to lower the power output. Switch to the antenna and switch on the B+. The tuner should then tune. Then increase the loading to full power. You should only actually have to be on the air for a few seconds to tune up! The tuning circuit will shut down to protect the relays under certain conditions of high power and high SWR. My antenna is a random wire thru a 4:1 balun.
Hope this helps.
Mike/WA8RVD

Me again!   Works on all bands 160 thru 10

As always, this group never fails to provide solid advice. You have prompted me to get more familiar with the theory of the pi coupled tank circuit. I know that's Vacuum Tube Theory 101 stuff. Today I spent some more quality time with the Heathkit manual. I will be getting rid of the superfluous gizmos that my solid state addled brain thought would be good to have for hollow state equipment. Mike, I very much appreciate the tips on your set up and want to compare notes sometime on our transmitters. But for now I think I need to go back to the basics: transmitter to antenna with maybe a T/R relay switch in between. Thank you, everyone. And if there are more thoughts and advice out there, I would like to hear them. BTW, I do want to insert a Heathkit SB 610 station monitor. I...think...I want to put it directly out of the XMTR and before the T/R switch. OR right before the DL. Opinions on where in the line to place it?

Doesn't really make much difference where in the coax line you put it. The SB-610 provides pictures on how and where to hook it up for transmitter use.