Using a BE FX-30 with a Harris FM5K1

[TVJunkie12]

I was wondering if anyone else has seen this. I've been trying to use an FX-30 with the FM5K1 transmitter but there is far too much reflected power and the FX-30 just won't work. My co-contractor was told by someone at Harris that the high reflected power is normal and was a characteristic of the solid state IPA when you are at a frequency at either end of the band. We happen to be at 106.1 and there is quite a bit of reflected power. The MX-15 exciter is able to handle the 5 watts forward with nearly two watts reflected but it's much worse with the FX-30 and we can't even get it to drive the transmitter. I've tried different length cables and nothing worked. The only solution that Harris offered was to obtain a circulator but those aren't easy to find for a reasonable price. Anyone know where we can get a circulator or know of another workaround to this problem?

 

[rfry]

You might try putting a 3 dB, ~15 W, 50 ohm r-f attenuator between the output of the exciter and the IPA input. That will add 6 dB of return loss to whatever load the exciter sees -- could make it happy.

 

[TomT]

I have a Bird 25 watt 3 db attenuator around here someplace. Bought it for project that never went past the arguing stage. Has "N" conectors in and out. Will sell it for 75 bucks. Should cure your problem, at least warm up the transmitter shack.

Interesting problem, almost sounds like something else is happening here. I use an FX-50 to drive the PA on a 2.5 H directly, and while I see a fair amount of vswr when the transmitter is off it doesn't seem to bother the FX. Of course, powered up, I have maybe a watt or so reflected. Use a shorting stub "T'ed" into the output as a safety measure since I'm looking at the grid of the 1500B directly through a tuning cap.

(The IPA in this beast shorted out one holiday weekend--and I connected an MX-15 directly to the PA to get the station back on. Station sounded better, final more stable so I left it that way until we raised power from 1.8 to 3.6 kw ERP. The MX had enough drive for the original 1100 watts TPO, but not the 2450 needed with the upgrade. Hence the FX-50.)

Have you tried using the FX-30 into a different, but tuned load? Such as a small dipole? I have a Continental 802A that won't work into a translator antenna, but is happy into a dummy load (or connected through the Bird attenuator to the translator antenna). Direct into the antenna (SWR CP single bay), it shuts down immediately with VSWR protect. An MX-15 hooked to the same antenna is quite happy, and is translating in the back room as I type. Consensus is that the PA in the 802 is oscillating---you might have a similar problem with the FX-30.

 

[TVJunkie12]

Have you tried using the FX-30 into a different, but tuned load?

I ran the FX into two different service monitors-one an IFR and one Motorola and all was normal. I'll try the attenuator route first-I think I've got one somewhere. I also came across an article dealing with a similar problem that I need to read through and see what it says. Thanks for all the suggestions.

 

[TomT]

You might need a spectrum analyzer to actually see what is going on.

 

[RadioMan203]

Ray Topp's "Radio Guide" had a very interesting article this month about interfacing a solid state exciter into an older transmitter. The author was a gent from a tube rebuilder that tests their tubes in an older Harris rig before shipping them to customers. I don't have the article in front of me since I took my copy home, but I recall something about putting in a attentuator and a x/x wave stub between the exciter and transmitter.

Perhaps someone has a copy of the article and can cite the particulars for TVJunkie12.

 

[rrsounds]

I was wondering if anyone else has seen this. I've been trying to use an FX-30 with the FM5K1 transmitter but there is far too much reflected power and the FX-30 just won't work. My co-contractor was told by someone at Harris that the high reflected power is normal and was a characteristic of the solid state IPA when you are at a frequency at either end of the band. We happen to be at 106.1 and there is quite a bit of reflected power. The MX-15 exciter is able to handle the 5 watts forward with nearly two watts reflected but it's much worse with the FX-30 and we can't even get it to drive the transmitter. I've tried different length cables and nothing worked. The only solution that Harris offered was to obtain a circulator but those aren't easy to find for a reasonable price. Anyone know where we can get a circulator or know of another workaround to this problem?

I had a similar problem with an as-delivered BE 10 kW. i had removed the FX-30 exciter and racked it external to the transmitter, and replaced the cable with one of a different length. Silly me. The exciter turned out to have a VERY high sensitivity to cable length.

Kind Regards,
David

 

[Info-warrior]

Cable lenght seems to be a funny issue. One school of thought says using a various lenghts of 50 ohm coax is an acceptable way to match a transmitter to a load. Others say that it doesn't actually match the device, it just fools the SWR meter/protection circuit by causing the incoming wave to be at a deminishing point on the voltage slope. The contention is that actual reflected power remains the same since as the voltage goes down, the current is going up.

I suppose the results you get from the former make everyone happy and you can't argue there. Yet, the theory on the latter side seems true. This doesn't apply to using 75 ohm coax as a matching stub in a 50 ohm system, i.e. when making a simple co-phase harness for two antennas.

 

[TVJunkie12]

Perhaps someone has a copy of the article and can cite the particulars for TVJunkie12.

I did find a copy of that article. Thanks.