Disabling SWR protection on a J** box

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Elopid
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Disabling SWR protection on a J** box

Post by Elopid » Mon Mar 11, 2019 11:42 am

Hi all,

So we have the same problem as many are experiencing with the poorly built SWR protection turning the rig off every few days.
The SWR for the antenna is 1.07:1, lower than into the dummyload but rig keeps shutting down. Haven't got it here with me to hand to take pictures but can get the box and take some pictures if necessary. The TX is the J** 150w (200w maximum)

We have tried adjusting the 'pot' but it makes no difference so disabling the SWR protection completely seems our only option.

RF-Head
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Re: Disabling SWR protection on a J** box

Post by RF-Head » Mon Mar 11, 2019 12:34 pm

Most of the problems are static in the air
The better built rigs have some way to deal with it.
maybe cut the cable from the SWR bridge

nrgkits.nz
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no manz can test innit
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Re: Disabling SWR protection on a J** box

Post by nrgkits.nz » Mon Mar 11, 2019 9:30 pm

Where is the SWR coupler mounted inside the box? If it’s close to the PA and/or output filter there’s probably stray RF being coupled in which will produce a false reading - if this is the case it’s probably adding stray coupling to the reflected side of the coupler, hence increasing the reverse power presented to the SWR lockout circuit. The coupler should be shielded from the rest of the rig, and you should have around a 1/4 wave length of coax between the coupler and output of the PA’s low pass filter - the coax should also have a Teflon dielectric or similar that can handle high temperatures otherwise the coax impedance will change with the heating effect, RG400 or RG141 are ok they will handle high power and not go out of shape with the heat generated

Albert H
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Re: Disabling SWR protection on a J** box

Post by Albert H » Tue Mar 12, 2019 3:56 am

You really don't want to start messing with a rig you don't fully understand! If you can't identify the sections yourself, you're probably going to do more damage than good if you go inside the box.

The SWR of your antenna may be wonderful at home, but what is it on the actual site? Also, are you using the same feedline?

Is it a "DC open" antenna like a simple dipole? If it is, then static could be your problem. Some years ago, I was thrown across a lift room by a static belt from an aerial feeder. After that, I always used "DC-shorted" aerials like J-poles or other colinear types!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

Elopid
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Re: Disabling SWR protection on a J** box

Post by Elopid » Thu Mar 14, 2019 12:47 am

I can identify them myself and I will very easily be able to do this if anyone is going to be kind enough to help me simply find out how to do it.
The rig needs to be sorted and has already been back to the builder and was given back to us "all fixed" after an adjustment but still doesn't work properly, sometimes it lasts a few hours, sometimes a day or two or three. But every other box been fine no issues ever.

The aerial is a gamma matched dipole at 1.07 SWR on the roof, has been used with various other boxes and had abslolutely no problems with those because those boxes all work properly. Feedline is WestFlex103

No the problem isn't with the aerial what so ever and gives better SWR on the roof than my Dummyload on the bench, the problem is 100% with the transmitter because the SWR protection circuit is faulty. Please don't worry about if the 15 pounds transistor or any other parts need a few minutes to be removed and replaced, that's a very minor issue compared to this.

nrgkits.nz
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Re: Disabling SWR protection on a J** box

Post by nrgkits.nz » Fri Mar 15, 2019 9:32 am

Elopid wrote:
Thu Mar 14, 2019 12:47 am
I can identify them myself and I will very easily be able to do this if anyone is going to be kind enough to help me simply find out how to do it.
The rig needs to be sorted and has already been back to the builder and was given back to us "all fixed" after an adjustment but still doesn't work properly, sometimes it lasts a few hours, sometimes a day or two or three. But every other box been fine no issues ever.

The aerial is a gamma matched dipole at 1.07 SWR on the roof, has been used with various other boxes and had abslolutely no problems with those because those boxes all work properly. Feedline is WestFlex103

No the problem isn't with the aerial what so ever and gives better SWR on the roof than my Dummyload on the bench, the problem is 100% with the transmitter because the SWR protection circuit is faulty. Please don't worry about if the 15 pounds transistor or any other parts need a few minutes to be removed and replaced, that's a very minor issue compared to this.
If you can post some photos of the inside of the rig, one of us should be able to help you. How are you measuring the antenna SWR? If its only with an antenna analyzer, you may find the SWR changes when you plug the rig back in. Your antenna will also not be a perfect 50 ohms, realistically it will either be above or below slightly. You should try using a total coax length which is a multiple of a 1/4 wavelength of your frequency and also a multiple of a 1/2 wave length - one of the other will get you a slightly better SWR match and slightly more power out. This guy explains it really well:

I now tend to carry around a few spare coax leads of various short lengths when I set up gear - adding one of them on the end is either going to reduce the power slightly (increase in SWR) or do the opposite.

Albert H
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Re: Disabling SWR protection on a J** box

Post by Albert H » Sun Mar 17, 2019 3:48 am

NRGNZ - you really want to avoid resonant feedlines!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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