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Author Topic: TX reflecting back into rig...can't figure it out!  (Read 879 times)
VE3ECMW2
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Posts: 1




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« on: February 26, 2006, 05:39:09 PM »

Okay, guys, I built my 20m hamstick dipole... looks very good, it's relatively high up (equivalent to about 4 stories from the ground), oriented E-W.

I never had this problem with the G5RV I used, but as soon as I hooked this dipole up to my rig and did some tests, anything I TX'd would echo into the speaker on the rig (TS-450SAT - internal speaker)... if I had headphones plugged in, it would reflect back into the headphones instead of the speaker.

I've tried everything I can think of:

6 turns of the premade RG-8 at the feedpoint;
Ferrite cores on the RG-8, the headphone speaker wire and the microphone...

Someone I spoke to at a 'fest today told me that the rig is too close to the radio; but I'm a little skeptical to that idea... the radio is at least 30 feet away.

Someone else told me that it's a good sign that it's doing this, as it means the antenna is radiating really well.

Either way, the reflection occurs regardless of power setting... TXing under 5W gives the same reflection as if I TX'd at 100W.

Anyone have any ideas what's causing this and what I can do?

(I'd resolder my PL259's, but they're premade, and I have 100% faith in them - not to mention I never had any problems at all with my G5RV)

Aside: The hamsticks tune to less than 1.4:1.
Radio: TS-450SAT
2 x 20m Hamsticks
Dipole mount
50' of RG-8 between radio and antenna
Antenna is 2 stories above the rig and about 20 feet to the left of the rig... is the antenna actually too close?
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VE3IOS
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Posts: 79




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« Reply #1 on: February 26, 2006, 06:00:11 PM »

You mean your audio is coming back into your speaker or headset as RF?
Jeff
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VE3ECMW2
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Posts: 1




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« Reply #2 on: February 26, 2006, 06:03:01 PM »

Um, I guess I should be clearer:

When I TX through the microphone, regardless of whether I have headphones or the speaker enabled, I hear everything I'm TXing through whatever speaker is active.

Is that any clearer... I'm not sure how to explain it any better.
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K8MHZ
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Posts: 115




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« Reply #3 on: February 26, 2006, 06:04:49 PM »

Is your radio grounded at all?

Sometimes (not always) a poor ground can be the culprit of RF being in places you don't want it.

73,

Mark K8MHZ
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VE3ECMW2
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Posts: 1




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« Reply #4 on: February 26, 2006, 06:09:49 PM »

I never even thought of that.

It's not grounded at all... My apartment is on the second floor of the home I live in, and grounding is difficult.

I could run some wire to the utility conduit running up the side of the house... would that help?

What should I use? Braided grounding strap is not okay; it'll have to be black-sheathed wire to minimize visual impact. How big gauge wire should I use?
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K8MHZ
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Posts: 115




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« Reply #5 on: February 26, 2006, 06:26:18 PM »

Being on the second floor can create some problems.

If you try to run a conductor to ground from where you are it may have too much impedance and act like a radiator.  What may be happening is the grounded conductor in your AC line, being so long, is acting like a radiator.

You may want to try to choke the AC mains grounded conductor.  This can be dangerous for someone not versed in electrical work.

Removing the grounded conductor may eliminate your RF problem, but presents you with a safety issue.

(Following for information purposes)

A GFCI can be used as shock protection without a grounded conductor.

If you live in an older building there is a chance that the center terminal is not actually grounded.  

Sorry to sound so confusing, but the grounding system is certainly something to take a look at.  

The other antenna was fed with balanced feedline.  Are you feeding the dipole with coax?  That may be your problem.  You may have to feed the dipole with balanced feedline or put a true 1:1 balun at the feedpoint.

If you have a coax fed dipole and the outer part of the coax is not grounded it will radiate, sometimes severly.  By balancing the feed that should be less of a problem.

Good luck,

Mark K8MHZ
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W5ONV
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« Reply #6 on: February 26, 2006, 06:36:09 PM »

Do you have the transmitt monitor turned on in the menu system ??
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AA4PB
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« Reply #7 on: February 26, 2006, 06:38:09 PM »

Is the "reflected" audio clear sounding or very raspy and distorted? If it is clear perhaps your radio has a monitor function and it is turned on. The monitor function on some radios is designed to let you listen to your transmit audio in the speaker or headphones.

A Hamstick dipole is a balanced antenna and you really should not have a lot of RF coming back down the feed line provided your feedlne does not run parallel to the dipole elements. Grounding should not be a requirement. I've used many dipoles, including Hamstick dipoles and never experienced this problem.

Have you tried a dummy load to make sure it really is an antenna issue?
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VE3ECMW2
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Posts: 1




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« Reply #8 on: February 26, 2006, 06:47:29 PM »

I checked the TX M and RX M settings, and that's not it... I get it regardless of whatever it's set to.

The reflected audio is very raspy.

I'm going to try few things:

1. It's not difficult to run a ground line: there's a grounded conduit line literally right outside my window, so I'll give that a shot and see if that helps any.

2. I'm going to place a 90 degree adapter on the dipole mount to make sure I'm not getting RF into the shield (which might be part of the problem).

Honestly, I'm glad I'm having problems. I'd never learn any of this stuff if it worked out of the box.
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VE3ECMW2
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« Reply #9 on: February 26, 2006, 06:49:01 PM »

Further:

I don't have a dummy load to test with at the moment, so I can't be sure: suggestions?

If I can't make it work with the things I'm going to try, I'll have to try out a 1:1 balun and see if that helps too.

Ugh.
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K7UNZ
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Posts: 691




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« Reply #10 on: February 26, 2006, 07:48:08 PM »

If you are only on the one band, you might try a counterpoise or two at the rig, since grounding is difficult at your location,

Just cut a couple of quarter wave lengths of wire, attach 'em to the rigs ground, and spread 'em alond the baseboard in the shack.

Even if it doesn't quite eliminate the problem, if it reduces it you'll know you have a grounding problem.

73, Jim/k7unz
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N4CR
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Posts: 1650




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« Reply #11 on: February 26, 2006, 10:26:24 PM »

VE3ECMW2 said:

"I never had this problem with the G5RV I used, but as soon as I hooked this dipole up to my rig and did some tests, anything I TX'd would echo into the speaker on the rig (TS-450SAT - internal speaker)... if I had headphones plugged in, it would reflect back into the headphones instead of the speaker.

I've tried everything I can think of:"

Did you try hooking back up the G5RV to make sure it's the new antenna that is creating the problem?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K8MHZ
Member

Posts: 115




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« Reply #12 on: February 27, 2006, 03:31:46 AM »

A dirty dummy load can be made from a  light bulb.  Be advised, that this is not the same as a 50 ohm dummy load.  In fact, it would take a 288 watt bulb to be 50 ohms resistive.  But for your purpose it should serve well.

I just seems to me that you are getting radiation from the shield of the feedline into your shack.  Since you are on the second floor, grounding may not help.

Have you seen the plans to make a true current balun out of a piece of coax?  All you need is a 1/4 wave (this is from memory, it may be 1/2?) piece of coax.  If you can't find the plans I will dig up the page I found them on and post it here.

Just for the heck of it, check your resistances.  A dipole should show open at DC.  Then check from one end of the coax to the other, both conductors.

Try adding 10 feet of coax.

If that works, cut 10 feet off.  I have had such bad luck with connections that I would cut 5 off each end and re-do both connections.

Remember, a 1.4 to 1 simply states the ratio of waves going one way vs. waves going another.  It does not tell us how much of that energy is radiated as RF, and how much is radiated as heat.  It also doesn't tell us where the RF is being radiated from, nor which direction it is going.  Truly, the only value of an SWR meter is to assure the radio is not damaged.  Of course, that is a very important consideration.  Now, if an antenna builder is certain that all other aspects of the build are correct, an SWR meter can be used to properly tune an antenna.

Try the easy stuff first, then, if it were me, I would start dissecting the antenna.

Have fun,

Mark K8MHZ

73,

Mark
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W5DXP
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« Reply #13 on: February 27, 2006, 04:21:21 AM »

Are you using an amplified speaker? The speakers on my computer do the same thing when I transmit on 2m. 73, Cecil, W5DXP
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W8JI
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Posts: 9304


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« Reply #14 on: February 27, 2006, 04:43:00 AM »

Let me interject a few things. It doesn't suprise me in the least you have RF feedback with an antenna like that. Even a full size perfectly symmetrical dipole can have problems, and short loaded antennas can be a nightmare.

If you have a very short dipole you will have considerable electric field strength near the feedline, and yopu can alos have resoance with some pretty bad unbalance at the feedpoint. Worse yet with a very small antenna the feedline shield makes a better antenna than the actual antenna does.

The CFA antenna and EH antennas, as well as several other compact magical antennas like the Morgain Dipole all work basically because the feedline shield radiates! Your Hamstick dipole is almost like one of those antennas.

Your 8 turn choke near the feedpoint is useless. You need hundreds or thousands of ohms choking impedance, not a few dozen ohms.

Grounding the rig is a band-aid patch. If you have to ground the rig and do other things, it means the feedline is acting like the antenna. The best thing to do is fix the antenna, not patch the problem.

The problem could be aggrivated if the Hamstick connected to the shield is not tuned exactly to resonance.

 Here is what I would do.....

I would build or buy a very good balun. A string of beads is NOT a good balun, an 8 or ten turn coil is NOT a good balun for 15 meters, let alone 40 meters.

You'd need to wind more than 1/8 wavelength of coax on a 4" form  to have a high impedance with an air core choke balun. That would be 17-25 feet of coax for 40 meters, so with a 4" plastic form it would be somewhere around 15 to 20 turns. You might actually have to do that twice, once at the antenna and once more 30-40 feet away from the antenna.

Decoupling small antennas properly is a real pain in the keester, because they are balance critical. The antenna is so small and fields so intense around the antenna that the coax often becomes a better antenna than the actual antenna.

What you see isn't rare, it is pretty common.

Try reading this:

http://www.w8ji.com/verticals_and_baluns.htm

and the links. It explains this in detail.

73 Tom
     
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