PGXL SWR Observations and Responses

Ray - K6LJ

Hi - Two days ago we had a bit of rain here in Austin and I decided to operate while it was raining.

I have 2 antenna's here the first is a Titan GAP vertical  Mounted near the ground and the second is a G3TXQ Hex with a 40 meter Dipole add on.

The Hex antenna goes thru Couplers for my LP-700 signal Monitor. It goes thru an MFJ Versa Tuner III with the ability to select the HEX 40 meter dipole add on or the second position directly to the Hex for other bands. This antenna is about 125 feet from the shack.

I have seen several posts relating to the PGXL and Hi SWR so I was somewhat attentive.
While operating FT8 on the Titan on 40 Meters I decided to use the PGXL for more power while working a South African station.

Suddenly as I increased power I started getting PGXL Hi SWR alarms. Of course the PGXL reacted. The issue I saw was that the SWR alarm came in and out very fast and power fluctuated until the PGXL shutdown. Other bands on the Titan did not seem to be affected as much.

The hex was working fine so I switched to the Titan until the next morning. At least I could see then. What I found was interesting and shows we should check the antenna every so often especially when mounted on the ground. The Titan has a counterpoise for 40 meters with 4 spreaders and the wire is held on the ends by neoprene insulators. One of these insulators was almost melted. Upon inspection I found that a small bug (critter) had crawled into the open end got fried. The GAP people have seen this before saying bugs are carbon based so it shorted out the counterpoise. Thus the hi SWR. After cleaning the end cap the SWR returned to normal. I have also ordered a new end cap and wire as these were black and partially melted.

A lesson learned here when running FT8 is to be more observant to watch the SWR while transmitting. The Flex 6700 does not shutdown on Hi SWR I think so if the PGXL is giving intermittent Hi SWR readings and it doesn't shutoff then maybe the 6700 can get the amps damaged. I know the PGXL response is fast but intermittent readings seemed to take a bit longer to react to.

The second issue I found was while trying to troubleshoot the first titan issue. While switching to the HEX 40 meter antenna I switched the MFJ versa tuner incorrectly. A long story short is that I caused an arc in one of the coax connectors to the antenna. Suddenly the PGXL shutdown on Hi SWR (SWR was just about 3.5) and it also caused the power to fluctuate a lot until the amp shutdown.

I suppose a situation like this could result in 6700 output amp problems. I was lucky! Hoping I didn't fry something up on the Hex I started at the antenna and disconnected it then connected my antenna analyzer and it was OK. I worked my way back along the coax thru 2 coax couplers and I found one connection that had arced. It was black. I cleaned and wire brushed the connector and cleaned it well. The issue has been resolved.

I will be more observant in the future when running hi power for FT8.

A question to be asked of FRS is how will the PGXL respond to a hi frequency SWR  fault. I know it is fast but I watched the PGXL alarm then clear then alarm then clear all very fast. The power in the meantime was all over the place.

Anyhow my observations for what it's worth.

Ray 

WX7Y

The PGXL has a great SWR shutdown circuit and is super fast, the 6700 also has a good SWR shutdown circuit.

The problem is the VERY HIGH RF Voltage being generated by the AMP compared to the barefoot RF voltage from the radio. 
This can have voltages that can be maybe 800 to 1000 volts or more running barefoot and may be 5000  to 10,000 RF volts with the AMP in line depending on power output, Feed line / Antenna impedance, capacitance, Inductance and SWR can make for VERY VERY High RF Voltage and thus a large Spark gap much like a tesla coil is created at ANY week point in the Transmission line and Antenna,  This makes it easy for the ARC to start and once the OZONE and carbon trace gets started it is almost impossible to stop it once the RF voltage reaches the critical point.

If your not running top quality and Higher voltage Coax with silver plating PTFE (teflon) insulated connectors including the barrel connectors your asking for trouble running a AMP like the PGXL or any other full legal output AMP,  each Connector increases the chance for a fault to occur.  ONCE you have a ARC on a COAX connector it is best to change it out because it is almost impossible to get the carbon burnt into the connector insulation fully removed.

When running barefoot the RF voltage is much less and so the ARC won't start and thus won't have a High SWR trip on the radio running  barefoot if the voltage is below the ARC trip voltage point.

I have had this happen here across the end insulator that was 6" long on a ALPHA DELTA Dipole, the RF was dancing up and down the insulator and this was a commercial Antenna, the insulator was tied to a 12' rope over to the tower, the rope was wet from a storm earlier in the day but the insulator was dry, running Voice it was crazy to watch the blue arc dancing up and down the insulator, it left divots about 1/16" deep  every 1/4" all the way across it . 

Hope this helps a little
73's
Bret
WX7Y

Ray - K6LJ

Appreciate the comment

bahillen

Ray Bret’s input is right on. I have had a number of insulators arc over on my wire antennas. They are multi band antennas so that resonant elements can be isolated by insulators. Long before my PGXL arrived there was a thread here that suggested getting ready by changing old coax and connectors. We have a tendency to feel coax last forever. In reality, it is an inexpensive component to a high power station. Changing to a double shielded coax is worth the extra cost. I just changed much of my coax and my RFI was greatly reduced and noise pick up was reduced. That includes even short jumpers. Much of my insulator problem surfaces when there is rain or snow. Antenna resonate frequencies shift and SWR increases. That increases the voltages across the insulators. Moisture in the connectors increase an moisture creates a path that only gets worse with time. Big stations use large air line and Heliax, great ground systems ant other things they learned over time. Specifically, when it rains and snows or wind **** the wires around that is the time that rare station comes on I want to work. I throw caution to the wind. The good news is that the PGXL has not failed. It shuts down. When I see a problem if I back down to 500 watts the arcing stops, if not I go to the 6600. Eventually even 100 watts is too much. The insulator is burned and an obvious black track is melted and burned. I operate FT8 a lot so heat build up is an issue with non ceramic insulators. The better you antenna and the better your grounds grounds the higher your Q in the antenna. Thus SWR goes up quickly as you move frequency. The voltages and currents go up quickly too. This week my SWR was high. The snow shifted the antenna resonance 50 khz on 80 m. Over 3:1 SWR. PGXL shut down. This is like working with a puzzle, we know the pieces but the picture improves as you get the pieces put together. This Text may be too long but I have been intrigued with solving these problems during the journey. 73 Bill W9JJB

Ray - K6LJ

Thank you. Very good comments.

N2WQ

SS amps are rather sensitive to poor antennas, SWR close or above 2, and fluctuating SWR. I love my SS Acom 1200S for DXpeditions, but never run anything but tube amps at home, even though all antennas are resonant with SWR of 1.5 or less. IMHO, it will be quite a while before SS amps reach the reliability and ruggedness of tubes.