The AM Forum

I'm out here in Colorado helping set up for the W5CZ radio museum opening today.   There are six boatanchor operating positions each with four receiver/transmitters pairs including a Western Electric 1937 broadcast transmitter, a 30K1, A-lines, Globe King, S-lines, Johnson, etc., all live (except for that 32V-2 incident last night:  "That wasn't a fuse"), all connected to a switch panel of antennas.   Fun.

Well, as part of getting authentic we switched out MFJ high power antenna tuners for KW matchboxes at two of the operating positions and can't get them to match for beans.  The feedlines are all unbalanced so this is a bit of an unnatural act for a matchbox but it's supposed to work.   Yes, the strap on the back is connected from ground to the edgemost antenna connection.  Should I give up and put back in the MFJ's or is there a trick I don't know about?

Amidst all this splendor there is no antenna analyzer.   The antenna I'm trying to match is a 75M dipole resonant somewhere.   I was able to match it from the lab (I forgot to tell you about the lab) when I switched the same antenna to a matchbox albeit through a longer stretch of coax.  The antenna relays in the matchboxes are bypassed.   

Jon

Hi Jon:

My limited experience with the matchbox showed me that this tuner sometimes cannot match certian impedances at certian frequencies. As a chance try the following:

1) Add a few feet of coax to either the input or output.

2) Hide the MFJ, put it in between the transmitter and matchbox to partially correct the tuning so the matchbox can tune it.

3) Ya got a 4:1 balun there?

Other ideas are welcome.

73 and GL in the contest...

Dan
W1DAN

Are you feeding the relay on the matchbox w/110 volts during key down?

I'm not too familiar with the Johnson KW Matchbox myself, but it seems the relay has to be activated during tx.

The relay is bypassed.   The 4:1 balun or rather 1:4 unbal is an amusing idea.   I wonder if this would increase the matching range.   Hiding the MFJ works too.   If I can find a big length of coax I'll try that. 

That is correct. On key up you need 120 on the relay coil. Check for resistance on the relay coil to see if it is present. Maybe use an MFJ 259 to test tune. If you lack PTT switched 120 use full time 120 for transmit and receive. That works fb here and the relay can be keyed for hours without worry. Good luck

Mike

any chance of usin ladder line??   klc

If you already have a 75 meter dipole, why use the matchbox at all. The output network in the transmitters you mentioned will likely 'tune' just fine without a matchbox intervening.

Yes, I agree with this too. No need for a matchbox with coax. In fact, even if the coax fed dipole is a little off from resonance, the matchbox will add more insertion loss.  The matchbox is just to make the transmitter happy and has no effect on feedline loss. As Steve said, any pi-network should match the final tube to any reasonably cut coax fed dipole.

If you don't have any kind of antenna analyzer or swr meter to tell you where the dipole is resonant, try this:  First make  sure the flat top is near 121.5' to start with. Then load the rig up into a 50 ohm dummy load. Note the plate meter reading and the tune and load cap settings.  Then hook up the dipole to see if the readings are similar.  But if the flattop is near 121.5' and at least 35' high, you will be less 2:1 swr anyway. On 75M, this is great with coax, loss-wise.    Bear in mind that if you don't have a near 1/2 wavelength coax feedline  (84 ft including velocity factor) the impedance presented to the rig will vary from the true reading at the dipole's center input. That's why adding or subtracting lengths of coax can sometimes give you a 50 ohm match when there is swr greater than 1:1. 

73,
T

I drove over to the other side of Denver and picked up an MFJ antenna analyzer at HRO.   Here's the answer at 3875:  25 +/- 36j (the MFJ doesn't reveal sign of reactance).  It's a small wonder the matchbox wouldn't match it.  It gives up on low Z.

This position has the 30K-1, BC-610, 32V-2 and Apache.  The BC-610 does not have the matching antenna tuner and does have some unexplored modification to unbalanced out.   I suspect that one but not the others needs an external tuner.

The other position  where we were  trying to use the other matchbox has a KWM-1, R4B/T4XB/L4, HT32A, HT33.  I bet all of those would be fine sans tuner.

Since the guests will be arriving soon, the MFJ  989D is going back in.  I'll cross my fingers with the 30K-1 fires up.

Jon

PS:  Ladder line isn't a possibility.