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Author Topic: Ladder Line and Interference...  (Read 2634 times)
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W3MMR
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« on: September 09, 2019, 06:26:58 AM »

So, as some of you know, i've installed a 600 Ohm ladder line fed 125' doublet here at the QTH. Its been great so far, outperforming a 1/12 wave 75m inverted V that was oriented the same way, except the doublet is 20'+ higher, and it's outperforming my G5RV I used for 20m and 40m. The noise floor is the same on the doublet as it was on the other wires, but I get more switching power supply, or random noise source interference. For instance, ive never has an interference source on 3.875, now I do. Never had one on 7.163, now I have one. On my SDR, I can see the interference, and tell by what it looks like, I call it a buzzsaw or spike ball, on the panadapter. And on the NC-303, I can hear the same interference's. So, after some research, i've found that some folks say that if the ladder line is unbalanced it can introduce noise sources into the feedline. Most say that overall, there shouldnt be a difference in nosie between coax and ladder line. I live in the city/suburbs so there's noise all around me. Its always been a constant battle. My noise floor on 75m is s9+ all the time, s7 on 40m, s5 on 20m. On both coax fed wires and the doublet. Just more individual interferences on the doublet. Has anyone had a similar experience? And what is the best way to check the balance of the ladder line? Ive heaed of putting a lightbulb in series with each line and check the brightness of the two, which is kind of barbaric. 
I guess i could check the voltage lf each line but wouldn't i need to do that at the feed point?
Thanks again guys.

-Perry

W3MMR
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** Studio "A" - Anan 200D, Shure SM7B, AL80BX, Internal & External Processing <--> Studio "B" - Heath-Kit DX100, D-10/4, National NC-303 **
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« Reply #1 on: September 09, 2019, 09:18:55 AM »

The light bulb method, the MFJ balanced ammeter, or old-school thermocouple meters in series with each leg tell you about current amplitude -- but nothing about relative phase, which is quite important.

What you really want to know is: Is there any substantial common mode current on the line? You could place a large current sensing transformer around the whole feedline and feed it to a detector, which should work though I've never done it. You could compare it to identical sensors connected to each leg individually and come up with a useful ratio.

On my open-wire fed inverted L I have noticed much more local RFI with the voltage balun built into my Dentron tuner than with a Balun Designs current balun in its place. This effect is most noticeable on 40 meters and above, especially on 20 meters and higher. I believe that the balance of the built-in voltage balun is poor, and gets worse the higher in frequency you go.

A current balun makes more sense, anyway. What you want is low common mode current, and that's exactly what is choked off by a current balun. A voltage balun is just a transformer or (more commonly) autotransformer that tries to give you equal voltages and wishes you good luck getting equal and opposite currents in the real world.

73,

Kevin.
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W3MMR
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« Reply #2 on: September 09, 2019, 09:43:38 AM »

Thanks for the reply. Just an uodate on things. I hooked the G5RV back up and that interference i spoke about is still there. So it's gotta be a new noise source that just coincidentally reared its ugly head at the time i put the doublet up. So, it appears its no an unbalance issue causing this. Just living whwre i do is causing this. What i would do for a low noise floor. Some folks dont know how good they have it. Im going to try the wheat bulb method anyway to see what happens. And i see what youre saying about common mode currents. Im using an MFJ989C Versa Tuner V, the older model, and according to what ive read, it has a 1:1 current balun. My father suprised me by ordering the parts to build a 4:1 balun for me, so we'll see what that does. Hopefully ill have the new TenTec tuna by sunday and ill see how that works out.

W3MMR
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** Studio "A" - Anan 200D, Shure SM7B, AL80BX, Internal & External Processing <--> Studio "B" - Heath-Kit DX100, D-10/4, National NC-303 **
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« Reply #3 on: September 09, 2019, 10:16:00 AM »

Thanks for the reply. Just an uodate on things. I hooked the G5RV back up and that interference i spoke about is still there. So it's gotta be a new noise source that just coincidentally reared its ugly head at the time i put the doublet up. So, it appears its no an unbalance issue causing this. Just living whwre i do is causing this. What i would do for a low noise floor. Some folks dont know how good they have it. Im going to try the wheat bulb method anyway to see what happens. And i see what youre saying about common mode currents. Im using an MFJ989C Versa Tuner V, the older model, and according to what ive read, it has a 1:1 current balun. My father suprised me by ordering the parts to build a 4:1 balun for me, so we'll see what that does. Hopefully ill have the new TenTec tuna by sunday and ill see how that works out.

W3MMR


Noise! I live at the edge of hundreds of acres of woods in western Pennsylvania, but nearby power lines and the very few neighbors I have still put out all kinds of unwanted noises anyway.

One of my projects for the next few months is installing two orthogonal on-ground unterminated Beverage receive antennas to see if that nixes most of the local garbage.

I did a noise-suppression experiment with a portable shortwave radio the other day. I walked out into the woods with my portable, while listening to N2DTS. Brett was strong, but there was definite line noise crackling in the background as I held the radio at around chest level. But lowering the radio to several inches below knee level, I could still hear Brett perfectly clearly, but the line noise seemed to disappear completely. The desired signal went down maybe 10 dB or so -- but the noise went down much more, making a remarkable improvement in signal-to-noise ratio.

There seems to be a noise-free (okay, I'm exaggerating -- but still) zone close to the Earth. Maybe on-ground Beverages will take advantage of that.

Good luck with conquering your noise!

Kevin.
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Carl WA1KPD
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« Reply #4 on: September 09, 2019, 12:28:21 PM »

I went from a G5RV to 230 ft of centerfed by 600 ohm OWL. The antenna, although longer, is in the same place as the G5RV  and is fed by a Palstar BT1500A 1500 Watt Double L. I live in a small toiwn urban area with one acre. I noticed a substantial increase in signnal strength from other hams and a drop in the noise floor.
YMMV
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Carl

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
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« Reply #5 on: September 10, 2019, 07:44:49 AM »

I use a 989c also.  Had the same problem.   I now use an external high power current baluns by DX Engineering.   Both 4:1 and 1:1 depending on impedance. Good balance and not much loss as evidence by cool operation of the balun when running 1 kw
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