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Author Topic: RFI problems  (Read 349 times)

Posts: 8

« on: August 21, 2006, 07:36:07 AM »

Hey all,
Ok here is my problem.  I'm using an Icom 7000, the AH4 Autotunner with a 100+ foot random wire, and I just put up a Cushcraft AV5 verticle that I picked up at a local ham fest.  
I put the rig on the air last December with a G5RV, adn MFJ tuner, and then this spring added the wire and AH4 Tuner.  I was using some 10 guage wire to 2 ground rods just outside the house.  No problems at all, everything worked great.  Then I cganged the ground wire to soem heavy duty audio speaker wire, 8 guage, and added the verticle.  I installed the verticle on the roof, with 3 radials for each band.  The antenna tuned up great.  (also made a 8 turn choke 4" diameter on a PVC pipe)  New RG8X coax.  Now the rig does not like to put out more than 50% power, when I increase the drive beyond 50% on CW or rtty as soon as i touch the paddle, or the keyboard the rig shuts down and then restarts.
On SSB I can operate for a min or sometimes several minutes then the rig will power down, then restart.
I relocated the rig to the ground, I have a ground rod (6') next to the antenna, right now the radials are above ground, the move flattened my swr curves on all bands, however it seems to have made the problem worse, on some frequiencies I can only go to 30% power.  But such as on 40m I can apply full power in the part of the band that is a perfect 1:1 once I start qsying from a 20khz swath the problem returns.  and my 2:1 bandwith is from the bottom of the band, to about 7.250. in fact at the bottom of 40m, I'm only showing about 1:6 or so.  
10m one time I can go to full power, then the next time (minutes later) I'm limited to 50% or so.
I now have the same problem with the wire tuned with the AH4, and up til now I have not had any problems with that antenna RFI wise.
So is it an RFI problem, or is it a radio problem?  Why did switchign from roof mount to ground mount appear to make the problem worse?  Why one specific frequiences in each band does it appear that I have no RFI yet a quick qsy up or down bring the problem back?
Any advice will be greatly appreciated.
Scott, AL7II

Posts: 556

« Reply #1 on: August 21, 2006, 10:02:28 AM »

The problem MAY be in your 8 turn choke.  You said it was 8X.  Most 8X is foam and foam has a habit of letting the center conductor drift towards (if not TO) the shield in a tight bend.  Try replacing the coax without the choke, live with the feedline radiating for a while and see if your problems change or disappear.  I recently set up a 2M radio in the shack and stuck a 1/4 wave whip with a magnetic base to a square of steel and put the antenna on the very top of a shelf unit that holds some of my equipment including one of my power supplies.  When I transmitted on the radio two of my radios (including the 2M) got stupid.  That is they lost all their memories.  I put enough RF into the PS that it fed to the radios.  Similiar to your rig shutting down and restarting.

My opinion and worth what you paid for it.

Clint -  W5CPT

Posts: 671

« Reply #2 on: August 21, 2006, 10:53:36 AM »

You can try changing the lenth of coax going to the radio. This may "move" the RF away from the radio. Ferrite snap on beads may help also or a line isolater. It sounds like RF is getting into your radio. You might also try ungrounding everything and put one ground at the radio. Don WD8PTB

Posts: 8

« Reply #3 on: August 21, 2006, 12:39:51 PM »

the coax is brand new...I was using some older RG-8, and i had sealed in the package the RG8X so I decided to change out the coax...oh and I had the same choke on the RG8.  

Posts: 2269

« Reply #4 on: August 21, 2006, 12:50:03 PM »

I think your problem is not Intermod, nor Harmonic, rather pure Front End Overload.

With 3 HF antennas 25 feet apart, the sheer amount of rf energy is overwhelming the receivers.  

Using the bandpass filters is excellent, but separating the antennas as much as possible is paramount.   Orienting the harmonic antennas perpendicular to one another will help as well, e.g. 40M and 15M.  Using a common ground point for all the radios has been suggested to me, but I have not tried it.

The other point made above is using the best quality radios with narrow filters installed.   For example at last year's Field Day I used a 718 on 40M and it caused and received interference from most of the other radios.   Multi-op environment requires quality equipment and setup.

Good luck!  bill

Posts: 13026

« Reply #5 on: August 21, 2006, 01:36:43 PM »

Sounds like a classic case of RF getting into the rig,
power supply, etc.  The amount of RF coming back down the
coax will depend on the coax length and what is connected
at the other end - I know of one case where connecting
another antenna to the unused port on an antenna switch
was enough to throw off the SWR on an antenna, as it
changed the impedance seen looking down the coax shield.

As a quick check, take a quarter wave wire for 40m (about
32 feet long or so) and connect one end to the ground lug
on the back of the rig, then lay it on the floor (running
down the hallway and into other rooms as needed - this is
just temporary.)  Does this change the symptoms?  Try it
connected to the chassis of your power supply as well.
(You can also try disconnecting the coax cables to the
antennas you aren't using at the moment to see how much
difference that makes.)

And if you have any external connections to the rig, for
example to a computer or modem, see if disconnecting
them makes a difference.

If these change the symptoms, it is a good guess that
you have common mode currents on the coax.  The two
approaches are (1) to reduce the currents using baluns,
feedline chokes, counterpoise wires (like the one running
down your hall, but installed in a more sustainable
manner), etc., and (2) to reduce the susceptability of
your equipment to RF using ferrite chokes on wires,
ferrite beads, bypassing, etc.  If you can show that
the rig is shutting down because the power supply output
is dropping out, then bypass capacitors in the regulator
circuit might be a good start.

Posts: 9304


« Reply #6 on: August 21, 2006, 03:17:18 PM »


You have a classic case of the worse antenna system layout possible for RF in the shack. An 8 turn shoke is almost worthless in a case like that.

First, you should handle the RF problem BEFORE it gets into the shack wiring.

Bypass and/or ground any control, power, or coax cable to a common connection point at the shack entrance. This should also be the ground point for the power line electrical safety ground and any telco cables. I do this by using a single distribution point for everything in my shack. All the coax shields, equipment grounds, control cable shields, and the power line safety ground all ground at that single point.

Second, that vertical (or any antenna with a lot of common mode) needs more than a few turns of coax. That's a terribly ineffective choke. To have a reasonable impedance across middle HF you'd have to have 15 turns 4" diameter. Also the problem with air wound chokes is bandwidth is narrow. There isn't any way possible to have an effective choke from 1.8 to 30 MHz. Peak impedance is when the choke has somewhere around 1/8th or so wavelength of coax and goes through a MINIMUM when the cable is 3/8th wave or so long. So you can see the bandwidth is limited with an air coil of coax.

A string of ferrites would be much better.

Second, the long wire is a real issue. Every single ampere of current flowing into the longwire is macthed by an equal current exciting the case of the antenna tuner. That means you need a really good ground on the tuner at the tuner, or you can get by with a mediocre ground and common mode decoupling of ALL leads leaving the tuner. That means the feedline and the control cable.

If the tuner is in the shack it will probably have to be moved outside.

If the antenna is right there is no need at all for a RF ground on the station equipment.

Both of your antenna systems are an RF nightmare waiting to happen. The best thing to do is fix the system outside the shack. You will not only fix the RFI, you will have better reception and transmission.

73 Tom

Posts: 9890

« Reply #7 on: August 21, 2006, 05:20:31 PM »

you need to start with a dummy load in place of the one antenna, see how thta works, then come back behind the ugly balun, then to the back of the radio, and do this for each antenna setup. it will give yiou and idea if it is coax related, overload, or antenna proximity problems.

start with the dummy load, and go from there.

Posts: 8

« Reply #8 on: August 21, 2006, 06:44:28 PM »

Problem solved!!
Thanks all, it turns out I didn't have a problem, it was in my power supply, broke out the spare supply and the rig works great.  But I am now making some changes in my system based on the advice everyone gave me.  I see some changes that I need to do to improve my system, and to make the station better.
Thanks to all
Scott AL7II

Posts: 1292


« Reply #9 on: August 22, 2006, 05:12:04 AM »


Glad you solved your problem before you headed off on the wild goose chase.  

73  W9OY

Posts: 523


« Reply #10 on: August 22, 2006, 07:51:07 AM »

Bet It was a switching style supply. You do get less gross weight/AMP. However the old style regulator/pass transistor supplys do not emit tons of EMI.

  I have a 75 AMP switcher supply. Weighs 4 kilograms/less than 10 lbs. I had to use many ferrite
rings to get all the squaure wave harmonics and HF noise that switcher supplies can sometimes emit.

  Even conventional regulated supplies can benifit from a few turns around a snap on ferrit ring. I use the rad shack ferrits. 6 loops on both positive and negative leads can help and will certainly do no harm.

  Of course as JI pointed out..try to use one single electrical strip. Pulling power from other sides of a room will create ground loops.
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