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Fluctuating SWR reading

B

basicallyimjoey

Newbie
Joined
May 1, 2023
Messages
1
I’m having the issue where my SWR will not stay rock solid. If I get it just perfect, it does. Any other time it’ll start to climb excessively, then hold steady. Or it keeps increasing. The PA cuts power & I’m forced to power off. Equipment is a Motorola MTR 2000 with the hated Notch Reject “flat pack” duplexer. Jensai I believe. 6 cavity duplexer. The SWR reading will behave different after tuning the notches with a VNA. I had it solid at 1.36SWR, so I know its possible. Moved the duplexer to a more permanent location, got paranoid & I’ve been chasing a good SWR ever since. Heres a screenshot of Motorolas software during transmit. It’ll start at 1.X, then gradually increase as the station transmits. Highest it goes now is 2.8 before bounce around between 2.6-2.8. A brief 30sec pause in transmit will put it back down around 1.8-2.0, then it starts climbing again. Brand new jumpers, Feed line is routed neatly, not corroded or pinched. Connection is waterproofed with thin coat of dielectric grease around outer threads only! Temflex top to bottom, electrical tape around that for UV protection. I can’t find the cause for a good, but rising SWR anywhere. Any input anyone has would be tremendous! Thank you!
 

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mmckenna

mmckenna

I ♥ Ø
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Jul 27, 2005
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24,836
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NMO's installed, while-u-wait.
Usually behavior like that suggests water/corrosion in the coax, connectors or duplexer.
You need to go through and isolate each component and test independently into a 50Ω load.

Might be the antenna is wet inside.
Dielectric grease/electrical tape might not be enough for your outdoor connectors.
Nick in the coax jacket.
Flakey duplexer.
Bad jumpers
 
F

freddaniel

Member
Joined
May 6, 2011
Messages
120
Location
Newport Beach, CA
As mmckenna suggests, try a 50 ohm load in place of the coax to the antenna. If the SWR is lower or stable, then your issue is with the feedline or antenna. If the SWR is still poor or varies into the load, then your duplexer or jumper cables are suspect. Chinese duplexers are always suspect.

I have found over many years that variable performance usually comes from a small amount of moisture in outdoor connectors. Changes in barometric pressure and hot/cold day/night causes connectors to "wick" moisture over time, even those with grease and tape. The only effective solution is to thermally insulate the connector, in addition to a good seal. I use one layer of Scotch 33 electrical tape, to protect the connector plating, covered by at least 3 layers of Scotch 2242 or 130C RUBBER high-voltage tape that is about 0.030 thick. The RUBBER tape will, over time, become solid and cannot be unwound. It can only be cut off.
 
jeepsandradios

jeepsandradios

Member
Feed Provider
Joined
Jul 29, 2012
Messages
2,215
Location
East of the Mississippi
Id start with the base. Put a dummy load on it and key it for some time. Ive seen the flat pack duplexers have issues espeicailly in arears with other gear. Not tuned right would cause all sorts of issues. If the base seems good add the jumper, then duplexer then feedline to antenna and so on. One step at a time. I'd also get a real watt meter on the base with a quality jumper.
 
L

lenk911

Member
Joined
Feb 24, 2007
Messages
117
Location
St Paul, MN
mmckenna said:
Usually behavior like that suggests water/corrosion in the coax
Click to expand...
I second that. Years ago I sold a small county sheriff's system. Found out from street gossip the sheriff would come in the office in the morning and put a dictionary on the microphone's PTT paddle and then go to coffee. When he returned problem solved until the next morning. The system was under warranty and I got the antenna cable replaced. Tough GE Mastr Pro base station to run continuous duty with a high VSWR until it drove the water out of the line.
 
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