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Author Topic: MFJ 1026 - How to make a recieve only aux antenna  (Read 6721 times)
K6MMS
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Posts: 33




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« on: September 06, 2012, 08:54:24 AM »

So I have a new MFJ-1026, and all of the reviews I've read suggest that the internal antenna that is supplied with the unit is not worth using. That said, what type of aux. antenna can I use?

I've heard that I can make a random wire, receive-only antenna for this by simply soldering a long wire to the center pin of a pl-259 connector. Is that really all there is to it?

Another option I have it to utilize an existing roof mounted cb antenna. Would that work as well for receive only?

Thanks,
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KE5PPH
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« Reply #1 on: September 06, 2012, 10:35:48 AM »

Yup.
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WB6BYU
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« Reply #2 on: September 06, 2012, 11:40:06 AM »

Even easier:  put a banana plug on the end of the wire (many of them don't require soldering)
and plug that into the center pin of the coax socket.

Now, one problem with such a simple approach is stability:  the noise canceling effect
requires that the amplitude and phase be carefully adjusted for the two antennas.  If
either antenna moves around, you might not be able to get a stable null.  Since the
ground system is part of the antenna, minor changes such as touching your radio
or coax with your hand can shift the received signal, and hence require readjustment
for maximum null.

Ideally you would set up your noise antenna so that it picks up maximum noise and a
minimum amount of the desired signal.  If you know what the source is - a
switching power supply, power line, or network cable perhaps - you can arrange your
noise pickup antenna to maximize that pickup.  But if your noise antenna picks up a lot
of noise that is NOT picked up by the receive antenna (as might be the case if one
was indoors near a lot of electronics and the other outdoors) then the cancellation
process won't work as well.


So, for example, you could use a CB antenna as a noise antenna if your regular antenna
is a dipole and you want to null out vertically polarized noise (which would be stronger
on the CB antenna).  But if your regular antenna was a vertical at the other end of the
roof, so both picked up about the same signal and noise, then it may not work very
well.
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K6MMS
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« Reply #3 on: September 06, 2012, 12:46:43 PM »

Thanks for the input.

Here's my situation:
My regular antenna is a Comet CHA250B 24' vertical. It is 32' up in the air on the peak of my roof (tip of antennas is 56' off of the ground) . The CB antenna is a 4' vertical on the lower edge of the roof, about 15' away from the Comet. Both antennas are smack dab in the middle of two 12kv power lines, both of which are about 25' away from the main antenna. It's the only location for the main antenna so I'm stuck with these conditions.

With that being the case, would you recommend that I use the CB antenna (most convenient) of create some sort random wire antenna as described?

Thanks,
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W8JI
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« Reply #4 on: September 06, 2012, 03:43:10 PM »

I assume you read the manual. I was heavily involved in the design.

I would never suggest a random wire for that unit, and I really didn't like the internal whip. The whip was only intended for in-house noise.

I understand you are in a difficult situation, and need to run what fits the room you have safely. You need to get a picture of what the system is.

The Comet antenna is pretty low efficiency, and also has terrible common mode response on the feedline. In other words, the shield of your Comet antenna feedline acts like an antenna. The feedline can act like more of an antenna than the actual thing we call the antenna.

This means any noise in the shack or house or coupled through grounds to the shack will get right into the antenna and the receiver. That can be a bad situation for noise. Keep in mind you might want to buy a couple good line isolators and install them at a few points in the antenna system. One near the antenna, and one near the house entrance. This will probably hurt lower-band transmitting performance, because the feedline won't radiate as much, but the noise should drop. 
 
You can use the CB antenna as a sense antenna, but you might have a little problem because the coax shields will want to be radiating. What I would do is try it, but watch the fuse lamp in the 1026 carefully to see if it ever starts to light. If the fuse lamp lights, or you cannot null noise, you will need to ask for more help. I would be prepared to try some line isolators.

73 Tom
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WB6BYU
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« Reply #5 on: September 06, 2012, 04:06:47 PM »

If the primary noise is coming from the power lines, then a horizontal wire on the ground
near one of them (whichever is causing the most trouble) might be a good choice - that
would maximize the noise pickup.

If the noise is propagated from the power lines to your receiver via the AC wiring,
or the problem is other noise from sources inside the house that is similarly coupled,
then the line isolators on the feedline might solve a lot of the receive noise by themselves,
otherwise a loop antenna coupled to the AC wiring may help.

The most important thing is to try to identify the noise source, because that allows
you to optimize the noise antenna to give best performance.
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K6MMS
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« Reply #6 on: September 06, 2012, 04:30:51 PM »


You can use the CB antenna as a sense antenna, but you might have a little problem because the coax shields will want to be radiating. What I would do is try it, but watch the fuse lamp in the 1026 carefully to see if it ever starts to light. If the fuse lamp lights, or you cannot null noise, you will need to ask for more help. I would be prepared to try some line isolators.


Thanks for the input. So what exactly does a lit fuse light indicate? Should I stop transmitting if it does / am I endangering my other equipment?

And again, thanks for all the help, everyone. Sorry for all the newbie questions...
« Last Edit: September 06, 2012, 04:40:48 PM by K6MMS » Logged
W8JI
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« Reply #7 on: September 06, 2012, 05:56:31 PM »


You can use the CB antenna as a sense antenna, but you might have a little problem because the coax shields will want to be radiating. What I would do is try it, but watch the fuse lamp in the 1026 carefully to see if it ever starts to light. If the fuse lamp lights, or you cannot null noise, you will need to ask for more help. I would be prepared to try some line isolators.


Thanks for the input. So what exactly does a lit fuse light indicate? Should I stop transmitting if it does / am I endangering my other equipment?

And again, thanks for all the help, everyone. Sorry for all the newbie questions...

The fuse lamp is on the external antenna port. It lights with excessive RF levels, and will burn out if things get too strong. It is cheap and easy to replace.

I did a lot of the design work on that and other similar units. The point I am trying to stress is you really need to sense the noise away from the house, and not have the feedline shields acting like part of the antenna. Just because it looks like a shield doesn't mean it won't be an antenna. :-)

Both the CB antenna and the Comet antenna will have significant shield currents, and so would a longwire fed with coax, unless certain steps are taken. It is premature to worry about that.

The very FIRST thing you should do is listen on the CB antenna, and see how loudly you hear the noise that bothers you on the Ham antenna. If you clearly and strongly hear the noise, then the next step is to just try it on your planned bands while watching that tiny fuse lamp.

By the way, it could be the feedline is picking up a lot of noise on the Comet. It might not be. As slow and painful as it is, it will go faster if you methodically look at things. Since you have the CB antenna, you should just check it first to see if it hears the same noise as the Comet on the bands you want to operate, or even if the CB antenna hears signals clearer.

You can quickly compare using the 1026 as a "fader" to pick between the antennas.

 
 
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K6MMS
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« Reply #8 on: September 07, 2012, 08:50:51 AM »

So I finished all the connections for this unit last night. I also adjusted the jumpers to set it for "Maximum sensitivity" of an external antenna. While inside, I saw the light bulb for the fuse lamp. Question: why was that designed to only be seen when the cover was taken off?

When I started playing around with it, I noticed that I was unable to hear ANY difference at all no matter what adjustments I made. It's as if the box does not even exist at all. I can still hear stations coming in etc, but no adjustment that I make changes the noise level or the sound in general at all?
« Last Edit: September 07, 2012, 09:06:29 AM by K6MMS » Logged
W8JI
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« Reply #9 on: September 07, 2012, 06:14:49 PM »

You need to read the manual, or do the test I described.


Before you do anything, you need to connect the CB antenna, not the connect the main antenna or turn the gain down to zero on main antenna, and see if you can clearly hear noise of good signals just on the CB antenna.
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KB4QAA
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« Reply #10 on: September 08, 2012, 07:37:22 AM »

Two points that I found helpful in improving my units' performance:

-Polarization same for both antennas
-Separation (distance) between the antennas helps

[edit]
I want to emphasize Tom's point about having good reception of the offending signal on the Aux side.   This is important as well when making first adjustments when operating.  Your ability to null the offender will be limited by both the Aux signal strength and how well it matches the waveform of the main Rx side.

When you find that some signals can be nulled and others can't it is because of a difference in reception between the two antennas (for example near vs. far source).  I guess that ideally we would like to have two identical antennas.
« Last Edit: September 08, 2012, 07:52:19 AM by KB4QAA » Logged
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