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Author Topic: LOG Not Performing Well  (Read 3726 times)

Offline ultravista

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LOG Not Performing Well
« on: September 07, 2021, 1447 UTC »
I built a 60 foot LOG yesterday using a DX Engineering BFS-1 at the feed point. The wire is solid core thin telephone cross-connect wire.

The LOG is very quiet and barely picks up distant signals. Compared to my Pixel 1B mag loop, this thing is practically dead.

I have watched numerous videos and read a lot about the LOG and mine is not as effective as others. This LOG is deaf on160 and 80 meters.

Any suggestions on what to do?

Offline ultravista

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Re: LOG Not Performing Well
« Reply #1 on: September 07, 2021, 2353 UTC »
Interesting, at night, the LOG is different. I am picking up Israel 4XZ @ 4133 Khz with the LOG and Pixel. Also found conversations @ 160 and 80 meters. Not sure why day/night makes a difference.

Offline Ray Wraye

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Re: LOG Not Performing Well
« Reply #2 on: September 08, 2021, 0220 UTC »
 Funny you should ask  :)  I have spent the past month or so working on a LOG for my LF/HF RX. Mine ended up 300 feet long, 75' per side square.  I made my own matching transformer (?) , a simple 5:2 wound with magnet wire using a binocular core torrid as my feedlines are 75 ohm.  It is incredibly quiet, but....I need to run a lot a gain sometimes (MOST times), depending on conditions. All I have to compare it to is (was) a 120' end feed longwire with a 9:1 balun, until a recent wind took it down.

The Longwire seemed to pick up more signals, but there was a lot of noise as well. The LOG signal levels are much lower, but I can really crank the gain with the low background noise.  As with most antennas I ever bought, built and/or used...Conditions are everything. During the day on MW and HF it is pretty good, but after sunset it reallys seems to come alive. LF is all but dead during the day, but at night it's a totally different story.

My radios are a SDR and a Kenwood R1000, so nothing incredible there, but I do use a homemade pre-selector and preamp and it seems to help, again...depending on conditions

Ray Ray


Offline ~SIGINT~

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Re: LOG Not Performing Well
« Reply #3 on: September 08, 2021, 0440 UTC »
The use of small AWG telephone wire may not be the best choice in antenna wire. This wire has a typical DC resistance of approx 98 Ohms per km and a characteristic impedance of 100 Ohms from 1 to 20 MHz.

Attenuation is in the range of:
2.6 dB/100m at 1 MHz;
5.6 dB/100m at 4 MHz;
8.5 dB/100m at 8 MHz;
9.8 dB/100m at 10 MHz; and
13.1 dB/100m at 16 MHz.

Unfortunately, the result being that the signals captured by the antenna are substantially attenuated by the length of telephone wire even before they have the opportunity to hit the receiver input.

Offline ultravista

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Re: LOG Not Performing Well
« Reply #4 on: September 08, 2021, 1249 UTC »
Ray Ray

Nice to meet another Ray on the board.

Offline ultravista

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Re: LOG Not Performing Well
« Reply #5 on: September 08, 2021, 1255 UTC »
~SIGINT~, thanks for the feedback. I wasn't aware that the small gauge wire could be the culprit. Interesting.

This is 60 feet. Is it enough to materially affect the signal?

What surprised me was the day/night testing. During the AM testing, the bands were quiet on the LOG but active on the Pixel. I commonly listen between 7-9 AM ET and get a lot of APAC stations. It was as if the LOG wasn't connected, it was so quiet.

The evening however was a different story. The same stations received on 160/80/40m by the Pixel were also received by the LOG. Albeit not as loud, the same stations were present, including Israeli CW @ 4133 Khz.

Day or Night, the LOG at 60 feet is dead to NDBs below the Broadcast AM band. The Pixel does a so-so job < 500 Khz and I can tune a NDB @ 415 Khz. The log received nothing.

Offline ChrisSmolinski

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Re: LOG Not Performing Well
« Reply #6 on: September 08, 2021, 1510 UTC »
What receiver are you using?
Chris Smolinski
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Offline RobRich

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Re: LOG Not Performing Well
« Reply #7 on: September 08, 2021, 2233 UTC »
You are using a beverage transformer designed for 400-500 ohms terminating resistance. You LoG is a fractional wavelength antenna below the 20m band, or perhaps more around the 30m band due to resonance changing as approaching the lossy ground. Either way, your transformer is taking an already low-impedance antenna and lowering the ohms even further as frequency decreases, thus increasing mismatch and losses.

Start with removing the transformer IMO. Wind a 1:1 or whatever if needing common mode current isolation from the feedline, though as for balancing the LoG itself, the antenna already should be rather balanced regarding differential currents.

I only have a few snap-on ferrites on the coaxial feedline at the the feedoint my 148' LoG. I do have a decent RF choke back at the receiver end, but it is not really needed for my particular deployment, either. YMMV.
« Last Edit: September 08, 2021, 2235 UTC by RobRich »
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Offline ~SIGINT~

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Re: LOG Not Performing Well
« Reply #8 on: September 09, 2021, 0054 UTC »
Quote
This is 60 feet. Is it enough to materially affect the signal?

Absolutely as you are dealing with signal amplitudes in the micro volts (uV) range which need to travel down the conductor to the feed point. The smaller the conductor the higher the resistance therefore you will get attenuation along the conductor. You also have less surface area to capture those waves.

Did you remove the insulation on the antenna wire? That as well has an attenuation factor. Then there is also the feedline loss. Every little bit makes a difference because all those little bits add up. Even the quality of the conductor can make a difference. Cheap mix alloy conductors with very little copper content make for poor antennae.

I also looked at the DXE-BFS-1 instructions and they specify:
Antenna wire: You will need a minimum of 3/8 wavelength of wire of any gauge between #8 and #20.

Your current wire is most likely 24 or 26 gage. 3/8 wavelength at 160 metres is 200+ feet of wire. Even at 1/4 wavelength you are looking at 137 feet of wire.
« Last Edit: September 09, 2021, 0112 UTC by ~SIGINT~ »

Offline NJQA

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Re: LOG Not Performing Well
« Reply #9 on: September 09, 2021, 1343 UTC »
I donít think you want to remove the insulation from the wire on a LoG antenna.

Although this isnít the case for you since you just installed your LoG, both LoG and BoG antennas suffer performance degradation if they get subsumed into the ground.  These antennas need to be sitting *on* on the ground, not under it, even a little bit.


Offline ultravista

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Re: LOG Not Performing Well
« Reply #10 on: September 09, 2021, 1417 UTC »
What receiver are you using?

Chris, I am using a Yeasu FT-950.

RobRich, I also have the Balun One Nine (1:9) and a Palomar Engineers Bullet 9LF (9:1) UNUN. Would either of these two be suitable? Your 148' LoG, how did you arrive at 148 feet, trial and error or planned?

~SIGINT~, the wire is jacketed. I will replace the wire. I had no idea that cross-connect wire wasn't suitable for antennas.
« Last Edit: September 09, 2021, 1423 UTC by ultravista »

Offline ChrisSmolinski

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Re: LOG Not Performing Well
« Reply #11 on: September 09, 2021, 2042 UTC »
I donít think you want to remove the insulation from the wire on a LoG antenna.

Although this isnít the case for you since you just installed your LoG, both LoG and BoG antennas suffer performance degradation if they get subsumed into the ground.  These antennas need to be sitting *on* on the ground, not under it, even a little bit.

Agreed, removing the insulation would not be helpful  :)

Regarding the type of wire, for a small antenna like this, 60 ft, the resistive loss from using the telephone wire may be trivial.   I used unshielded CAT 5 (or was it CAT 6?) for my 1000 ft LoG, with all the conductors wired in parallel, seems to work fine as well. Granted, it's a *large* antenna, so that helps.

Which brings up this... signal levels from a small LoG are going to be quite low. If your receiver has a pre-amp, by all means use it.

Before building the 1000 ft LoG, I played around with smaller ones. The first was 50 ft or so, and I found to be be quite deaf, to be honest. To the point of not being remotely useful. I then went with a 350 ft LoG, that worked much better. Which is what led me to expanding it to 1000 ft.
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Offline Josh

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Re: LOG Not Performing Well
« Reply #12 on: September 09, 2021, 2235 UTC »
A few gov and private party studies on buried hf antennas showed a dipole was efficient enough to be useful and certainly easier to deploy than a loop. They typically buried the antenna only a few inches below the surface, and used RG8 sans shield (meaning insulation and center conductor only) as the elements, waterproofing as needed. As you may have guessed these were EMP-related studies; can't be blown over if it's not above ground to begin with. Speaking of buried antennas, tests on hf within mine shafts showed some interesting properties, think it was a quartz mine so very dielectric, worked very well on 80 and 40, The ARRL antenna handbooks have a lot of these tests if you're interested

As for log types, special forces have long been searching for an antenna that was low profile with few showing any real promise. Those that do show some promise are fantastically expensive and are literally huge in area covered, at least for a wire antenna.
Here's one example;
http://www.hflink.com/antenna/elpa/

The Shirley and Jamaica Antenna has been suggested as workable;
http://arrl-ohio.org/SEC/nvis/nvis.pdf

As for hobbyist logs, I suspect ones with a single conductor, meaning no multiconductor phone wire, will be best, with as much wire on the ground as can be made to fit for reasons of gain. I suspect multiconductor wire will have odd capacitance effects and other undesireables as with using cat cable as an antenna - the pairs are specifically twisted so as not to interfere with each other signalwise.
« Last Edit: September 09, 2021, 2238 UTC by Josh »
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Offline NJQA

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Re: LOG Not Performing Well
« Reply #13 on: September 10, 2021, 1410 UTC »
I have posted this link before.  This gives the background on why buried Log or Bog antennas donít perform well.  Figure 39 is worth looking at:

https://rudys.typepad.com/files/qexjul-aug-2016-bog.pdf

This should be no surprise if you think about the difference in performance of buried vs elevated radials on a vertical antenna.  Verticals only need a few radials if the radials are elevated above ground, but they do need to be the right length.  Buried radials require many of them to achieve similar performance but the specific length of them tends to be less critical as ground losses become more dominant.

A few years ago I put up (down?) a 1000 ft BoG for the winter DX season.  I used a roll of cheap outside rated CAT5 UTP cable I bought off of Amazon.  This  cable was 24 ga copper clad aluminum (CCA) wire.  I twisted all the wires together at each end and fed it with a 9:1 (I think) balun.   The resistance of each of the individual wires was about 75 ohms, so it was probably about 9 ohms for the entire length of the antenna.  An external preamp was not used.

I was pleased with how well it worked for LW NDB DXing.  Part way through the season performance seemed to drop off a little, but I attributed that to conditions.  I was still hearing many new stations.

When I rolled it up in the Spring I found that I actually had 4 pieces of wire.  Some time during the Winter a rabbit or squirrel had chewed through the wire at multiple spots.  My LoG had become a large OCF-dipole-on-the-ground!

One of my original reasons for measuring the wire resistance had been for future troubleshooting, but during the course of the season I never rechecked it.  If I had it would have become obvious that the wire had been severed.

I will probably install another LoG this Fall.  I may use copper wire this time but I really donít expect a noticeable difference over CCA.  Receive antenna currents are extremely small and (I squared R) losses arenít very large.  People build Beverage antennas using aluminum or Copperweld fence wire all the time.  W8JI had some comments on wire types for Beverage antennas, but he said the biggest reasons to choose one over the other were largely focused on how well the wire survived environmental conditions and how difficult it was to work with.

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

There is probably a sweet spot regarding the minimum length of a LoG.  Too little wire wonít capture enough signal.  KK5JY suggests 15% of the target wavelength.

http://www.kk5jy.net/LoG/

My tests this Winter may use a roll of Home Depot THNN copper house wire (about 500 ft) instead of the 1000 ft CAT5 UTP as I want to see if this length will be useful at LF.





Offline NJQA

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Re: LOG Not Performing Well
« Reply #14 on: September 10, 2021, 1438 UTC »
I suspect multiconductor wire will have odd capacitance effects and other undesireables as with using cat cable as an antenna - the pairs are specifically twisted so as not to interfere with each other signalwise.

The twisted wires depend on current flowing in equal but opposite directions in the wire pairs to achieve cancellation of radiated energy.  For external induced EMI, the twisting ensures external fields induce equivalent voltages in both wires so that the induced energy can be negated by differential signaling.

Connecting all of the conductors together at each end negates these effects.  It just becomes a lot of wires in parallel.

 

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