High VSWR at Low power then comes good at High Power...

[VK3SMC]

Hello all.

I have a Cushcraft A3S with the 40m add on kit, what i see only on 10m is that i have a VSWR of 3.5:1 when i am running low power and to clear the fault i just need to run a carrier (usually on CW) at 100 watts, the VSWR then comes down to 1.25:1. I can then return back to 5 watts and its all good for a short period of time.

This only happens on 10m. I am about to lower the tower to the ground to investigate and am after some direction on where to start looking. Has anyone had this problem before? Could it be a faulty trap? Poor connection between joins in elements?

Thanks in advance.

Simon
VK3SMC

[VK3DXE]

G'day Simon,

You've obviously tried another SWR meter?

[VK3SMC]

Hello Alan.

Yes i have. I have the LP-100 Vector Meter connected all the time, and i have also tried a Daiwa CN-520. The TS-2000 has a built in SWR Meter and it also shows the same.

Simon....VK3SMC

[VK2DDS]

I'm not familiar with that antenna but my initial thought is that there's a matching unit component which is inefficient, getting warm and changing value. The only time I've seen power dependent SWR was when using small leaded 50V ceramic caps in a matching unit that got hot.

How long has it been installed? Could there be a connection that's corroded and it getting warm and therefore effecting a nearby capacitor/inductor?

[VK3SMC]

Antenna has been up 2 years. The Balun i have is a Hy-Gain BN-4000B. Its weird that it only does it on 10m. 15M, 20M, 40M are all OK.
But it may be worth looking at the connectors.

Simon...

[VK5RC]

Hi
I have a Cushcraft A4 which I think is very similar, the traps if I recall, have a bleed hole, are they pointing up? If so they could allow water inside and this may give variable SWR issues.
Regards Rob
VK5RC

[VK3SMC]

Hi
I have a Cushcraft A4 which I think is very similar, the traps if I recall, have a bleed hole, are they pointing up? If so they could allow water inside and this may give variable SWR issues.
Regards Rob
VK5RC

Hello Rob,

The A4s is the same antenna except that it has 4 elements.

As far as i can see all drain holes are pointing down, but this will be a good thing to check when it is lowered for service. Who knows spiders etc may have blocked one of the holes.

[VK2FAK]

Hi all.....


You mentioned you can switch back to 5w and read 1.25 swr......do you also read 5 watts out...

I was thinking maybe the radio is folding back because of the high SWR.....and does not reset itself for a while..

John

[VK3SMC]

Hi all.....


You mentioned you can switch back to 5w and read 1.25 swr......do you also read 5 watts out...

I was thinking maybe the radio is folding back because of the high SWR.....and does not reset itself for a while..

John

Hello John.

Yes it does TX 5 watts. I might take a quick video of what it does, its very strange. Also it does not normally need 100watts to clear the high VSWR, it is probably much lower.

[VK3AIF]

Sods law says that it probably won't play up while you have a camera pointed at it

[YU7EF]

I would first look at some loose connection at 10m element taper .... At high power (and much higher current flow)
loose connection start conducting and after a while at low power it looses it

Might also be aluminium surface corrosion at taper joint since antenna is in the air for 2 years exposed to weather,
probably to salty sea wet air

Pop, YU7EF

[VK3SMC]

Hello Pop.

Thanks for the information, i think it might be time for a full service. Time to check all connections.

Cheers
Simon.

[VK4TI]

Hello all.

I have a Cushcraft A3S with the 40m add on kit, what i see only on 10m is that i have a VSWR of 3.5:1 when i am running low power and to clear the fault i just need to run a carrier (usually on CW) at 100 watts, the VSWR then comes down to 1.25:1. I can then return back to 5 watts and its all good for a short period of time.

This only happens on 10m. I am about to lower the tower to the ground to investigate and am after some direction on where to start looking. Has anyone had this problem before? Could it be a faulty trap? Poor connection between joins in elements?

Thanks in advance.

Simon
VK3SMC

Generally those antennas are pretty solid , mine was and offer little trouble however the boom join and a couple of other joints tend to work a little if not torqued perfectly . Perhaps a quick rebuild with some decent aluminox and checking end caps and screw claps might solve it , assuming the trap holes ARE facing down with good drain and good luck

[VK3SMC]

Managed to get a video done.

See it here --> https://www.youtube.com/watch?v=TTOZriZ ... ata_player

Had to use the ATU to get the power up there but it fixed the issue.

Cheers

Simon... VK3SMC