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Base 11m dipole symptoms

Limeybastard

Limeybastard

Active Member
May 29, 2017
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Shady Hills Massive, FL.
Was wondering if someone can be kind enough to explain the following situation to me.

With the 11m dipole connected to my Stryker 955 my SWR levels are 1.2 and 1.4 , ch1 and ch40 respectfully.
But with amp connected the SWR level increases to 2.2 and 2.5 and the 12v fans on my amp stand which are powered independently stop spinning when keying down.

Using the same antenna coax and connecting it to another antenna with amp on all is back to normal. Could it be a faulty dipole antenna? Or perhaps claims of it's wattage rating not being correct? It's rated at 1.5Kw.
 

I am no expert, I have worked an improvised antenna with SWR above 2.5:1 and nothing was damaged, although it was inefficient.

The fans stopping might be indicative of a power supply issue on the A.C. circuit feeding your bench, (Asking too much from one outlet or power strip) or it could be RFI issues.
 
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The swr meter is last in the chain.

Everything is same and runs normally, only when I change the antenna to the dipole (using same coax line ) does the 12v fans on bench cease to operate and swr increases. I believe I've isolated it to one single component in the chain being the dipole.
 
Limeybastard said:
The swr meter is last in the chain.

Everything is same and runs normally, only when I change the antenna to the dipole (using same coax line ) does the 12v fans on bench cease to operate and swr increases. I believe I've isolated it to one single component in the chain being the dipole.
Click to expand...

Commercially built dipole with a dipole centre containing a balun? Sounds very much like you're over-saturating the ferrite in it and indeed as you suspect it isn't rated at 1.5kW. Over-saturation would result in increased SWR which got worse the longer you transmit and you would get a lot of common mode RF coming back into the radio as well which is what could be killing the PSU running the fans as RF gets into the mains wiring.
 
M0GVZ said:
Commercially built dipole with a dipole centre containing a balun? Sounds very much like you're over-saturating the ferrite in it and indeed as you suspect it isn't rated at 1.5kW. Over-saturation would result in increased SWR which got worse the longer you transmit and you would get a lot of common mode RF coming back into the radio as well which is what could be killing the PSU running the fans as RF gets into the mains wiring.
Click to expand...

Cheers M0GVZ, appreciate your comments and guidances. I ceased to transmit after just two keydowns and used other antenna. My amp is doesnt put anything near that figure out. In its current state it modulates no more than 280W pep.

I best include my PSU setup also, which I forgot to mention in my original post.
One 30A PSU set at 13.8 V connected to the Stryker 955 and feeding the 8 hub 3 pin 12v fan hub which has the two 12v fans connected to it.

One 75A PSU set at 13.8V feeding the Texas Star 500v amp.
 
Limey,
I am 99% sure you have common mode on the coax messing with the fans, That's why i told you to put ferrite on the fan wires & .1uf cap across the wires, common mode issues when using dipoles is very common,

is the dipole horizontal or vertical with the coax coming away from the feed-point @90 degrees for some distance?

if you don't have a balun at the dipole feed-point try winding a coaxial choke as close as you can to the dipole,

are you re-calibrating the meter when you turn the amp on or just turning it on & seeing vswr go up ?
 
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bob85 said:
Limey,
I am 99% sure you have common mode on the coax messing with the fans, That's why i told you to put ferrite on the fan wires & .1uf cap across the wires, common mode issues when using dipoles is very common,

is the dipole horizontal or vertical with the coax coming away from the feed-point @90 degrees for some distance?

if you don't have a balun at the dipole feed-point try winding a coaxial choke as close as you can to the dipole,

are you re-calibrating the meter when you turn the amp on or just turning it on & seeing vswr go up ?
Click to expand...

The dipole is inverted V with coax almost 90 degrees being fed into it, I say almost because I had to route the coax away from power cables in the attic as best I could. But, the coax is near one of the lengths of the dipole, which I assume is could be causing common mode? I didnt get a chance to recalibrate the meter with amp on with this antenna but did with the antenna that works and runs fine. SWR was too high for me to keydown to calibrate.

Ill draw picture of the coax in relation to the dipole. the dotted line is the attic floor, the dipole ends are not quite that close to the attic floor. But you have brought me to the attention of perhaps the feedline in relation to the dipole end being close or causing common mode ? With amp off, the swr is fine, the other antenna with amp on is also fine.Untitled.jpg
 
make sure its not near the leg connected to coax center,

it does not have be coupling due to been near one of the dipole legs to have common mode issues,

imagine coax is not 2 wires but 3 wires connected to the dipole,
the outside of your coax braid is the 3rd wire and its connected to one of your dipole legs,

hey presto the coax length & resulting rf impedance of the coax braids path to ground effects how much current flows on the coax,
or anytime you have a voltage difference between one part of the coax braid and another,

try a 4.5 to 5 turn choke on about 4-1/4" none conductive former ( 2ltr pop bottles work for a quick bodge ) up near the dipole,

look chokes up on here you will find plenty of info on how to wind chokes effectively & how not to wind them.
 
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Limeybastard said:
I didnt get a chance to recalibrate the meter with amp on with this antenna but did with the antenna that works and runs fine. SWR was too high for me to keydown to calibrate.
Click to expand...

Mate you have to recalibrate when you increase power or the reading will be high.

Issue with the fans is most likely CMC, as Bob said .the choke should fix most if not all of it. rerouting the coax away from the leg wont hurt either .
 
vkrules said:
Mate you have to recalibrate when you increase power or the reading will be high.

Issue with the fans is most likely CMC, as Bob said .the choke should fix most if not all of it. rerouting the coax away from the leg wont hurt either .
Click to expand...

Of course, but the stryker 955 cuts out power due to high swr. Although its inbuilt meter isnt accurate it works fairly well with ballpark figure. It was shooting past 4 swr on its meter then shutting down.

Im assuming dipoles behave differently regarding CMC since fans weren't shutting down with other antenna and same feed line.
 
Radio measures SWR between radio and amp. So amp input does not seem to be 50 Ohm.
Can you try that on dummy load? That would cut our guesses.
Mike
 
sp5it said:
Radio measures SWR between radio and amp. So amp input does not seem to be 50 Ohm.
Can you try that on dummy load? That would cut our guesses.
Mike
Click to expand...

Indeed, since the radio is connected to amp then amp to meter. Antenna to meter. Using same feedline but different antenna works normal with low SWR.

I was using the oil filled ameritron dummy load while adjusting deadkey and such when i first got amp.

What shall I try? Cheers
 

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