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any tips for debugging RFI issue? #ubitx-help

Mike KK7ER
 

Our uBITX v3 behaves just fine when transmitting into a dummy load.  But when transmitting into an end fed wire with SWR 1:1 it misbehaves.  When transmitting CW, the uBITX gets stuck in transmit for a few seconds after I release the key.  When pushing the PTT switch, it also sticks in transmit but the menu also comes up on the display.  If the internal wiring was too long, I would expect the issue to show up with both dummy load and antenna.  I suspected the counterpoise (coax shield) was bringing RF back into the radio.  A number of sources seemed to suggest an "air choke" so I wound 20 feet of coax tightly around a plastic bottle and placed that between the antenna and feedline/coax but it made no difference.  Any tips from the antenna gurus here would be very much appreciated.

73 Mike KK7ER

Clark Martin
 

Is your uBitX in a shielded enclosure?  It’s possible RF (normal radiation from the antenna) is getting into the circuitry.


Clark Martin
KK6ISP

On Aug 1, 2018, at 7:25 PM, Mike KK7ER <groupio@...> wrote:

Our uBITX v3 behaves just fine when transmitting into a dummy load.  But when transmitting into an end fed wire with SWR 1:1 it misbehaves.  When transmitting CW, the uBITX gets stuck in transmit for a few seconds after I release the key.  When pushing the PTT switch, it also sticks in transmit but the menu also comes up on the display.  If the internal wiring was too long, I would expect the issue to show up with both dummy load and antenna.  I suspected the counterpoise (coax shield) was bringing RF back into the radio.  A number of sources seemed to suggest an "air choke" so I wound 20 feet of coax tightly around a plastic bottle and placed that between the antenna and feedline/coax but it made no difference.  Any tips from the antenna gurus here would be very much appreciated.

Mike KK7ER
 

It is in this aluminum bottom / steel top case from Jameco (shown here at first power on -- alligators clips working around bad encoder switch are now replaced with a toggle).  Transmit tests done with case fully closed.

 

Mike :

It certainly sounds like RF getting into where it doesn't belong.  End-fed wires are notorious for bringing RF into the shack.

I would suggest trying to put a ferrite choke on the power leads and keeping them as short as possible.
 
Any external wires (PTT, Mic, key, power, headphones) might be acting like an antenna and allowing
RF in where it isn't wanted. 

Is there any way that you can try a different coax fed antenna ? Maybe try the same configuration at a friends place where they 
have different antenna setup ?  That would at least allow you to rule out some internal issue in the radio.  If you can't reproduce the
issue elsewhere with a real antenna on it then it is likely something specific to your use of the end-fed wire and resulting RFI. 

Cheers

Michael VE3WMB 

Lawrence Galea
 

You said you are using the coax shield as the counterpoise?
RF pickup on all bands or all bands?
Try changing the length of the LW + or - and use a 1/4λ counterpoise connected to the case.



On Thu, Aug 2, 2018 at 4:23 PM, Michael Babineau <mbabineau.ve3wmb@...> wrote:
Mike :

It certainly sounds like RF getting into where it doesn't belong.  End-fed wires are notorious for bringing RF into the shack.

I would suggest trying to put a ferrite choke on the power leads and keeping them as short as possible.
 
Any external wires (PTT, Mic, key, power, headphones) might be acting like an antenna and allowing
RF in where it isn't wanted. 

Is there any way that you can try a different coax fed antenna ? Maybe try the same configuration at a friends place where they 
have different antenna setup ?  That would at least allow you to rule out some internal issue in the radio.  If you can't reproduce the
issue elsewhere with a real antenna on it then it is likely something specific to your use of the end-fed wire and resulting RFI. 

Cheers

Michael VE3WMB 


ajparent1/KB1GMX
 

Mike KK7ER,

What form of end fed 1:9 with random wire very bad without counterpoise.
Or resonant half wave (swr under 1.7:1).  FYI if not low at the desired frequency
read the manual and tune it.

FYI: a dipole without a balun or with one and a bad coax routing has RF on the coax
and off center fed is pretty bad too.

Most vertical with out a choke at the entrance are pretty bad as well. A Butternut
HF2V was the only antenna that gave me bites despite 8 radials.  The fix a 
length of LMR240 with about 50 FT50-43 toroids. before it cam into the house
and grounding the sheild.

Solutions, one is a choke balun (air or ferrite) at the shack entrance and also ground it there.
The other is load up all the wire you suggest has RF on it with ferrite.

Then again I ran into that with someone and moving the antenna (he was
literally less than 10ft under it). Didn't help that the coax used had broken off ground
in the connector too.

Insure the lead from the board to the coax connector really has the ground lead to
ground and not swapped.  You laugh...

More than like if the picture is correct... The wires hanging out of it could be as much the cause.

Power supply leads can be an path in.

Allison

ajparent1/KB1GMX
 

Forgot one...

Painted box, did you clear the pat at the coax connector and use the star washer?

Same for the board mounting standoffs, All of them.

Allison

 

Mike,

I agree with everybody else that your getting RF back down the feed line, it happens bad on my Par EFHW on 20 meters. Thanks for unwittingly giving me a tip on where to buy my case for my uBitx, I haden't thought of Jameco. I just placed the order.

Joel
N6ALT

ajparent1/KB1GMX
 

Joel,

Try changing the length of the coax on the PAR.  Something like 5FT can change that.

For cases Allied Electronics too.  They stock a lot of the Hammond products.

Allison

 

Hi Allison,

I need to redeploy that antenna, I have the feed point at the same level as the far end and the wrong coax on it, so I know it's my fault it doesn't perform any better than it does. I threw it up in the air about 5 years ago and am just too lazy to take it down and do it right, I'm too busy building radios:-). This fall when the forest fire smoke clears I'll do it.

Joel
N6ALT

ajparent1/KB1GMX
 

I was wondering how close those fires were.  The rule here if if you smell the smoke
its too close.  Take care.  Keep building!

Allison

Mike KK7ER
 

Allison,

Thanks for all the suggestions!  Regarding your points:
  • I do not have a star washer at the coax connector on the uBITX.  Nor do I have star washers under the mounting posts.  I'll give that a try.
  • The matching box is this one:  https://www.ebay.com/itm/191226640113
  • There was no manual but the seller pointed me to this page:  http://udel.edu/~mm/ham/randomWire/  I had 100ft of wire and wanted to use 40m-10m so I cut it in two to make a 39ft piece and a 61ft piece.  Both those lengths fit the 40m-10m range in the colorful table on that web page.  I started with the 39ft one at first then switched to the 61ft piece.
  • The SWR is 1:1 at the desired frequency of 14.074 MHz.
  • FYI, I'm in a townhouse.  The 25ft coax goes through the wall up into the attic and through the soffit to the matching box hidden under the eave (about 20 feet above ground).  The antenna wire runs under the eave south for about 13 feet, makes a 100 degree turn West and slopes downward with the end of the wire being about 10 feet above the ground.
  • I had a ferrite on the power cable (you can almost see it in the photo) and tried building and inserting an air choke where the cable came out of the wall.  But if the coax is radiating that badly then the part beyond the air choke inside the wall 2 feet from the uBITX likely renders that moot.
  • Would installing the air choke at the matching box be an option?  It would mean just a few inches of counterpoise/coax before the air choke.
  • Would installing the 31 feet of extra wire on the counterpoise side help?
Thanks again!

73 Mike KK7ER

ajparent1/KB1GMX
 

That is a 1:9 random wire box.  Expect RF on the coax if the ground lug is not used with a .25 wave wire.

The SWR may be good with the wires in use but you have an off center fed antenna and RF on the coax
is not much better.    

For both grounding the braid and using a Air or ferrite choke at the ground point to the radio is advised.

Using a choke at the match box will require a counterpoise to be used or the antenna may be ineffective.
I also expect the SWR will change.  You need a .25 wave counterpoise with that matchbos and the coax
is not a good choice to do that service for the problems you are already having.

Your likely have a lot of RF on the coax and that is a source of the problem.
You are up against the wall (townhouse and all) for antennas and I get that.  

Things to try...

First and foremost use a counterpoise wire at the matchbox they provide a lug for that
the hanger hook is that in the picture.   The wire should be around 16.5ft for 20M.

Try attaching to the radio chassis itself a wire maybe 16-17ft (quarter wave) to force
the case to a current node (sorta ground).  Just lay the wire out straight on the floor
or ceiling (if ground floor) as a test.  See if that helps 20M (likely makes other bands worse)

One thing you might try is replace the matchbox with a vanilla 20M center fed dipole (two 16.5ft wires)
even if temporary.  If your high enough one of the wires can just dangle (forms a center fed inverted L).

Or replace the matchbox with a 1:49 type and use a 66-68ft wire (half wave end fed and working on harmonics)
You may still have RF on the coax but a lower magnitude and maybe less troublesome.

Also make sure the board and the connector are electrically connected to the metal case, try putting
the cover on as well but external wires will need to be choked with ferrite.

For 1:9 baluns there are a few magic lengths that work better for some bands or all.
table attached has that info.

Also that arrangement in the all band form requires a tuner at the radio as not all bands will match well.
A tuner does not help the RF in the shack issue, its there ot keep the final intact.

Let us know her if and of it helps.  I;m sure there are others having similar issues.

Allison

Mike KK7ER
 

Allison,

Thanks so much!  This is very, very helpful!  You have given me a lot of homework.  I will try the following and report back.
  1. Install air choke at matchbox (20ft RG8X wrapped tightly around a plastic jar).
  2. Attach counterpoise wire to ground lug of matchbox (I wonder why you specified 16.5ft.  My math gives 17.5ft for 1/4 wave at 14074 KHz.  I'll start with 17.5 and trim back).  I'll leave the counterpoise dangling to start and then find a way to hide it.
  3. Disassemble the uBITX and grind off the paint around the standoff and antenna holes and reassemble (no star washers in my junk box).
I'll leave the counterpoise-on-radio and antenna mods (shorten wire length to match table or use new matchbox  with longer wire) for phase two.

73 Mike KK7ER
 

ajparent1/KB1GMX
 

Its 17.4 in free space for a perfect thin wire.   Multiply that by .95 for reality.
If you used 234/f  (234 .25W is half of 468 .5W) you get 16.62.  
Even antenna wires have velocity factor.

That's how.

Allison

Mike KK7ER
 

Again, thanks for the explanation.  After adding the air choke and counterpoise (17.5ft -- I'll trim it back tomorrow), the uBITX now behaves properly when keying the mic and transmitting CW!!!

The SWR is a bit higher, though.

Also, it has moved a bit toward the Frankenstein's monster end of the aesthetic spectrum.


73 Mike KK7ER

ajparent1/KB1GMX
 

Mike,

You solved the source issue....

with the counterpoise as suggested I did say the SWR will likely change.
Generally the 1:9 scheme is problematic for RF in the shack unless you have
luck or resort to choking...

You might try a End fed half wave as when the wire is tuned you will have less RF.
Also if the bands of interest are the 40/20 there are a few that do them well.
Though a town home is likely a noisy are a for 80/75M.  

However that picture scares me...  ;)

Allison

Mike KK7ER
 

I shortened the counterpoise and things got worse.  But after hiding the counterpoise in the rain gutter, things got better.  :-)  Go figure.


73 Mike KK7ER

Timothy Fidler
 

Mike , which all goes to prove you are working at the cutting edge of RF technology and should be so grateful to be there :-) 

Timothy E. Fidler : Engineer BE Mech(1) Auckland , NDT specialist AINDT UT /RT3 , MT2 CB #2885, 
Telephone Whangarei   022  691 8405
e: Engstr@...



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To:
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Sent:
Sat, 04 Aug 2018 22:09:18 -0700
Subject:
Re: [BITX20] any tips for debugging RFI issue? #ubitx-help


I shortened the counterpoise and things got worse.  But after hiding the counterpoise in the rain gutter, things got better.  :-)  Go figure.


73 Mike KK7ER

ajparent1/KB1GMX
 

Mike,

Putting the wire in what is likely an aluminum gutter made it longer and likely not a bad thing.
Any metal you have available can be helpful!

Allison