The Curse of SWR..

[Ed/KB1HYS]

I had cooked my MFJ Tuna with built in SWR meter some time ago.  Since then I built a K1JJ style Big Arsed tuna (copper tubing and breadslicer) to feed the 160m dipole here. Since I didn't have a SWR meter to indicate anything, i just tuned up into a dummy load, and adjusted the tuna until my plate current was the same as it was into the dummy load. Simple, straightforward. RF left the antenna and was received. Noted the positions and they stay pretty constant. All happy, UNTIL...

... I went and bought an SWR meter,(it's a reflectometer type greifkit AM-1) so's I could use my other rigs on the same antenna. I figured I'd start at the same points that I had been using for 80 and 160.  Well. The lil' ole SWR is just not right. When it indicates a good match, I get no forward power indicated. When I tune off, I see some forward power, but usually not where I should be, and of course the reflected power goes up quick. For example, when I tune into the Dummy Load, I get a very low foward power indcation, but also a very low reflected inidcation. (so some good some bad?)   The meter checks out OK FINE. (I looked into it before I used it on the station).

I started reading about RF circulating currents and such Figureing that I had just messed up something.  Now my brain is circulating...

I would through the dang meter into a box, and go back to the way I was doing it, but I would like to know that I can use my Kenwood with out letting all the smoke out of the finals... 

SO can anyone explain What is happening here???  I'm just confused...

[WA1GFZ]

Reverse the coax in and coax out into the dummy load and see what happens. Sounds like you have a bad diode. Sometimes lightning will take out a diode and mess up both directions. My reflected diode was taken out by lightning in my HB wattmeter. Also Todds KWM380 had a bad reverse diode that made both directions look bad. Change them both if you suspect a problem. 

[flintstone mop]

Hi Ed
You explanation of tuning the tuna has circulating currents in my head. I don't think your gonna get reliable results tuning for the same readings as a dummy. The antenna is a totally different beast with a lot of variables that cannot be tacked down with your procedure. The MFJ259 is the secret tool for finding the sweet spot in the tuner to get the dipole to "look like" 50 ohms to your radio. I know there's more dollars spent, but it's the only way I know to get your tuner to cooperate. Can you borrow this instrument from someone? You can mark the coil taps and cap positions to get you in the ballpark, then it's a matter of a little tweek for lowest SWR on the Kenwood. Minimal time with the radio seeing high SWR.

Fred

[Ed/KB1HYS]

Yep, I was thinking Diode too. but I checked em prior to first hook up.  Maybe i do have the leads reversed. Not everybody is right handed    and that WOULd be something I would do...

[WA1GFZ]

A quick way to get your tuner close is to put a 6 or 10 dB 50 ohm pad in series with your RX to normalize it to 50 ohms then tune the tuner to maximum background noise or a signal on the S meter.
ED if the secondary of the current transformer was wired backwards the thing would read backwards. I assume you are using the cube sized watt meter not the old reflectometer. If it is the older one make sure the right termination resistors are used on the pickups. I had that kit and you could set it up for 70 ohm coax. It will read screwie on 50 ohms. I like 1N5711s rather than the crappy old geraniumum diodes.

[Ralph W3GL]

He stated it is a "Greefkit AM-1"...

That is the old reflectometer unit and they can be set up for either 50 or 75 ohm  line use.

My guess is both diodes are bad...  too much leakage...

Also check the resistors to confirm it is set for 50 ohms.


I do not remember but, I think the set up for 50 ohm line was 150 ohms.

[Ed/KB1HYS]

The greifkit is set up with 150 ohm resistors, which means it's set up for a 75 ohm line (the resistors act in parallel). It's actually an AM-2 not that there is much difference. A reflectometer type meter. Plus it has a Cool Name.  a partial manual is on BAMA, has the schematic and component values, so it's good.

Anyway, I only checked the diodes for conduction in one direction, and open in another. I really didn't look for leakage current or other stuff.  I figured if they had gone, the would either be shorts or opens.   

I do use the tune for max noise in the Reciever trick, to get close to my optimal settings for the tuna.  Gets close and then I would tweak for best plate current.

I guess I'll set the meter up with a matched pair of 100 ohm resistors and see what happens.  I had thought that if I was set up with a 75 ohm bridge the best VSWR i would see was 1.5:1 or so. maybe I'm screwed up though.

Welp, back to the bench...

[WA1GFZ]

To check the meter for proper function reverse the coax connections and the reverse now becomes FWD and FWD power becomes reverse. Check the values of the resistors and dump those crappy germanium diodes.