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Author Topic: "New" Tuner Causes TVI  (Read 561 times)

Posts: 68

« on: February 24, 2006, 11:13:28 AM »

I have an Icom 706 MkIIG and a LDG AT-11MP Autotuner. It feeds a short run of coax (about 6 feet) that leads to a 4:1 balun, then 450 ohm ladder line to the dipole which is suspended just above the roof top.

I had a LOT of extra ladder line bunched up (not coiled) on the roof, but for some strange reason, the XYL didn't like the looks of the excess ladder line. Go figure.

When I finally tired of hearing her complaints, I trimmed the excess ladder line, which left about 40-50 feet from the balun to the antenna. Due to my really tiny in-town lot (my house nearly fills the lot), I have little option for antennas. It's either a vertical, or my shortened (via loading coils) dipole.

After cutting the ladder line, my LDG tuner would no longer load 160, 80, 40, nor 17 meters. It never would tune 15.

A ham buddy at work sold me a silver faced MFJ-949E Versa Tuner. It literally will tune everything from 160 to 6 meters with this present antenna setup.

My joy would be complete, except where I had no to very minimal TVI with the LDG tuner, both with the full ladder line run and the now shortened ladder line run, I now have LOTS of TVI with the MFJ tuner. I'm running cable TV with a high pass filter where it feeds into the back of the TV.

I'm on the 2nd floor, so station grounding is non-existant. If grounding is my possible answer, the only thing to ground to is an steam radiator in the next room. Even though I'm running a dipole with a balanced line, I wouldn't want to make my heating system a part of my antenna system.

Any suggestions?

David, N1ZHE

Posts: 2193

« Reply #1 on: February 24, 2006, 11:23:32 AM »

I just wonder if now you are radiating more than you were before and getting TVI because of how close your antenna is to your TV?

Posts: 68

« Reply #2 on: February 24, 2006, 11:32:27 AM »

The same thought crossed my mind.....

I've considered a vertical on the roof that would be further away from the TV's in the house, but the radials would be a pain.

David, N1ZHE

Posts: 2201

« Reply #3 on: February 24, 2006, 11:36:17 AM »

Previously, the ladderline feeder was probably radiating also, and thus actually part of the antenna.  When you shortened the ladderline, you obviously upset that and the antenna now doesn't tune the same.

Posts: 68

« Reply #4 on: February 24, 2006, 12:04:51 PM »

No doubt the ladder line was radiating as a part of the antenna.

Is there another way to cure the TVI, or will I have to find some way to add the ladder line back on without displeasing the XYL?

David, N1ZHE

Posts: 20406

« Reply #5 on: February 24, 2006, 12:09:07 PM »

"Suspended just above the rooftop" probably tells the whole story here.

With the MFJ, you're now actually putting all your power into the doublet, whereas before, you weren't.  And the antenna's too close, so you're coupling into stuff.

However, you really shouldn't be bothering a TV set that's cable-connected to a compatible cable TV source, which provides adequate signal strength and shielding.

I can run a kilowatt into an antenna in the living room and not bother any of our TV sets in and near the living room, at all -- because they're all served by either digital cable or digital satellite (DirecTV), and the signal to the TV isn't even an RF connection -- it's S-link.

If you use S-video or at minimum RCA phono patch cables from cable converter to television receiver, a high pass filter won't do anything because your signals provided to the TV aren't at RF, they're at video and audio.  Almost impossible to interfere with this stuff, unless something's seriously wrong.

So, although your ham antenna is probably too low and too close to the house, and that can certainly intensify interference which simply getting the antennas higher can often eliminate, you really shouldn't have any TVI with a cable connection.

If you have a legit cable connection and service you're paying for, I'd get a service tech out to fix it.


Posts: 9

« Reply #6 on: February 24, 2006, 12:57:45 PM »

An interesting observation:

LDG AT-11MP tuner is an "L" network with shunt caacitors - reduces harmonics

MFJ 949E is a "T" network with series capacitors - very little harmonic reduction.

Modern rigs with bandpass filters seem much better in regard to harmonics and TVI than the old tube gear. With your Icom-706 either tuner should be fine with regards to TVI.

But your specific problem with cable reception is most likely due to your strong signal being coupled to cable-tv wiring, AC house wiring, TV interconnecting wiring. Try ferrite clamp-on chokes on the aforementioned wiring and experiment with better grounding, both at the TV and at your station. As a last resort, change to coax feed with choke balun and add extra dipole elements as in a fan dipole configuration to get multiband coverage.

Posts: 12777

« Reply #7 on: February 24, 2006, 02:23:40 PM »

Certainly the reason that the LDG a won't match the
antenna after you shortened the feedline is that now it
is presented with a different impedance on each band.
(Though I presume you checked to make sure that you don't
have an open connection in one conductor - that can make
a mess of things.)  Changing to a 1 : 1 balun rather than
4 : 1 might make a difference... or not.

Posts: 1368

« Reply #8 on: February 24, 2006, 05:06:43 PM »

>It literally will tune everything from 160 to 6 meters

Before or after it catches fire?

Posts: 1714

« Reply #9 on: February 24, 2006, 05:54:09 PM »

An antenna driven by a mismatched transmission line of arbitrary length can present a variety of impedances at the feed point at different frequencies.  Some of these may be extremely high, others may be extremely low. What you get is pretty much random, unless you calculated it or made some measurements. When you changed the feedline length, you changed the impedances seen by the balun on all bands. The balun may not operate very well with large impedance mismatches.

Here is some speculation. There are a couple of possibilities I can think of.

1. You may have a mismatch large enough to produce a lot of heat in the balun (even with 100 watts).  Excessive heat can destroy a balun. Heating will not generate TVI. Balun breakdown is a possibility.  Arching between primary and secondary is a possibility.  This will generate TVI.

2. If you saturate the core of a balun, this will generate TVI. This is a possibility for a low power balun with a particularily bad load.

Solution? Try adding more feedline between your balun and the antenna.
You may also need a new balun. If you find a band that won't tune, a change in the length of the ladder line will probably fix it.  This is kind of a trial and error approach, because any length change effects all bands, so you have to go back and check all the bands after making a change.

Jerry, K4SAV

Posts: 9787

« Reply #10 on: February 24, 2006, 06:19:15 PM »

on most ldg you have 3 sets, just touch the button and it goes to the last memory, , press for more than 1/2 second but less than 3 seconds and it tries last memory and the tweaks it, and hold for more than 3 seconds and it does a reset from scratch, try holding it for more than 3 seconds with the tune power from the radio on..

Posts: 9302


« Reply #11 on: February 25, 2006, 03:10:18 AM »

David, you may need to get rid of that 4:1 balun. The correct balun for your application would abe a very high common mode impedance 1:1 balun, NOT a 4:1.

Most likely what has happened is altering your feedline length has greatly increased impedance presented to the balun. 4:1 baluns of any type ALWAYS have considerable magnetization of the core. They is always some winding that is connected directly across the feedline. This means flux density in the balun core is related to feedline impedance for a given power, and even with very low power you could have considerable magnetization of the balun core.

A very good 4:1 current balun might handle high Z loads, but most cheap 4:1 baluns will not. They will unbalance and perhaps even overheat, and you can even saturate the core creating harmonics.

I'd toss the 4:1 balun out with the trash and buy a very good 1:1 current balun with high common mode impedance. A string of beads will NOT work well, and neither will an air core coil of coax. You'd need something with very high common mode impedance to be effective. Try looking at the DX Engineering site, or you can build your own by winding multiple turns of teflon transmission line on a medium to high mu ferrite toroidal core.

73 Tom

Posts: 543

« Reply #12 on: February 25, 2006, 08:24:20 AM »

>>It literally will tune everything from 160 to 6 meters

>Before or after it catches fire?

I worked France on 20 meters with 100 watts, tuning up a rain gutter with an MFJ tuner.

Try grounding your tuner really well if you haven't already.  That could make a difference.
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