3 - 30 MHz Choke

[VK2CSW]

I decided to knock up a common mode choke as I think I am getting some RF in the shack, especially around the 15, 12 and 10m bands.

I got some bibs and bobs and put together a double stacked FT140-43 with 15 bifillar turns of 1.25mm2 enamel wire.

Sadly the SO235 connectors turned out to be the bodgey M16 versions, so final assembly is on hold.

The NanoVNA results look pretty good.

   

   

[VK4ADC]

Colin

Of course one might ask why you have a lot of RF around the shack in the first place..

Is it proximity to the radiator/s ?
Coupling from the radiator/s to the feedline (assuming coax) and whether there are BALUN/s in use at the far end ?
Lack of an effective RF earthing system in the shack ?

And the big one - are you planning to install one of these chokes at EACH end of the relevant transmission feeder ?

It's all very well stopping it coming down the feeder but sometimes making sure the feeder is perpendicular to the radiator to minimise coupling is enough if a BALUN is in use.

Have you tried one (or more) of the simple clip-on RF ferrite devices where the coax enters the shack AND at the back of the gear ?

I think I saw a mention somewhere in one of your postings about an end-fed half wave antenna (see, someone does read them !), well that needs to be fed against an effective virtual earth to not have feedline radiation. That virtual earth needs to be a REAL earth - and not a flimsy excuse for one - for the antenna to be efficient at its purpose, let alone not have the feeder radiate.

Of course, again, I might not be remembering antenna and transmission line theory correctly...

Doug

PS Your graph has no textual references as to what you were measuring. If it is common mode rejection then 45dB at 5.xxx MHz is ok, otherwise a 45dB signal loss input to output (S21) makes for a very efficient dummy load to dump power into...

[VK6ZFG]

Colin

Of course one might ask why you have a lot of RF around the shack in the first place..

Is it proximity to the radiator/s ?
Coupling from the radiator/s to the feedline (assuming coax) and whether there are BALUN/s in use at the far end ?
Lack of an effective RF earthing system in the shack ?

And the big one - are you planning to install one of these chokes at EACH end of the relevant transmission feeder ?

It's all very well stopping it coming down the feeder but sometimes making sure the feeder is perpendicular to the radiator to minimise coupling is enough if a BALUN is in use.

Have you tried one (or more) of the simple clip-on RF ferrite devices where the coax enters the shack AND at the back of the gear ?

I think I saw a mention somewhere in one of your postings about an end-fed half wave antenna (see, someone does read them !), well that needs to be fed against an effective virtual earth to not have feedline radiation. That virtual earth needs to be a REAL earth - and not a flimsy excuse for one - for the antenna to be efficient at its purpose, let alone not have the feeder radiate.

Of course, again, I might not be remembering antenna and transmission line theory correctly...

Doug

PS Your graph has no textual references as to what you were measuring. If it is common mode rejection then 45dB at 5.xxx MHz is ok, otherwise a 45dB signal loss input to output (S21) makes for a very efficient dummy load to dump power into...

Hi All
 
The end fed ground independent antennas are misleading as they do actually require something for them to work against.  This is because all electrical circuits require two conductors for them to work.  While the coax feeder is provided with two connection points the end fed dipole is only has one connection and needs a second one for it for the feed to it to work against.
 
If an earth, or alternative, is not provided for the end fed antenna to work against it will find one by resorting to using the coax for this function.  This provides the illusion of it being ground independent.
 
Igor
VK6ZFG

[VK2CSW]

The antenna is a FAMPARC multitap vertical mounted on the chook shed about 30m from the shack. This works surprisingly well.

(The EFHW is still under construction - the capacitors arrived today.)

Igor, the EFHW will have a counterpoise, either a 0.05 wavelength wire (at 40m) or I will attach it to a handily located wire fence. I simply don't want the coax to be the counterpoise.

The radio bench has a groundplane under the radio which earths to the shed structure, which in turn, earths to an earth stake. Every chassis earths to this too.

The shield of the feedline joins this earth on entry to the shed. I have tried with and without this.

Every cable in and out of the radio either has clip-on ferrites or is looped through a toroid.

I have built a couple of other chokes that have less common mode attenuation, something in the order of -19 to -25dB across the bands. Unknown ferrite mix bought years ago from **** Smith. I have experimented with these at one end or the other and both. To little avail.

The issue I occasionally see is that during transmit on digital modes on 10 and 15m the PC will drop the connection to the radio. Below this there is no issue. Curiously 12m seems to be less affected.

Now, when I first set up digital stuff at my old QTH in Sydney I did have similar random drop outs. I fixed it by simply moving the laptop further away from the radio. This fix is a little more awkward on the current bench, but not out of the question. In order to test it I need to get a couple of longer USB-C extension cables (this is a different laptop so, of course, new cables are required.

While I will press the new choke into service for the vertical set up as an experiment, I am actually building it and its sister for both ends of the feedline from a G5RV Jnr (and 20m dipole) that I am currently trying to engineer a mounting system for (between and over a couple of sheds - it turns out tin sheds are hard to securely mount stuff to. Not like the old days of a hockey stick pole screwed to a barge board with some coach bolts). Oh and waiting on some replacement SO235s as the cheap ones I got aren't 5/8x24 UNEF, probably M16x1 - that serves me right!

The weather and having to go to work every day is a big limiting factor, as is keeping the orchard presentable!

[VK6ZFG]

Colin,

As long as you use a counter poise you should be fine.

A straight through wire in a clip on ferrite does not do much.  Measurements I have made with them confirmed that when you have one or more turns in them then they greatly improve in the isolation they provide over the straight through configuration.

73

Igor

[VK2CSW]

Igor,

Cheers for that. Where the dimensions allow I try to loop two or three times. Actually, where possible I try to not use clip ons in any case.

[VK4ADC]

Yea, discussion !!!!!