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Author Topic: LOG Not Performing Well  (Read 490 times)

Offline ChrisSmolinski

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Re: LOG Not Performing Well
« Reply #15 on: September 10, 2021, 1823 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.

Yes, exactly ad NJQA noted, connecting in parallel negates this. You just end up with less resistance and inductance vs a single conductor of the same gauge, so it's a win win.
Chris Smolinski
Westminster, MD
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Offline RobRich

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Re: LOG Not Performing Well
« Reply #16 on: September 10, 2021, 1850 UTC »
Size was not too critical to my design, but I was shooting for a loop circumference somewhat under the 40m band. It ended up being about 148' after laid out and deployed.

Here is the basic design of my LoG:

https://www.hfunderground.com/board/index.php/topic,29940.msg114696.html#msg114696

About a transformer, try it without one. Just connect the loop straight to your coax and see what happens.

Another consideration is knowing a LoG is mostly a skywave antenna. If you model a LoG, its primary lobe is vertically upwards, thus it can be a good solution for receiving via NVIS and high-angle signals. That is part of the reason your LoG might fair poorly during the day but much better at night, especially at low HF and below. Typically I use my LoG more as a regional antenna these days.

Yet another consideration is your soil conductivity. The characteristics of an antenna on the ground can vary considerably depending upon the actual ground. I have sandy soil with poor conductivity, which explains why my LoG performs about the same as when it was deployed years ago despite it now being mostly covered in shifting sand, leaves, etc.
« Last Edit: September 10, 2021, 1916 UTC by RobRich »
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Offline ultravista

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Re: LOG Not Performing Well
« Reply #17 on: September 12, 2021, 1356 UTC »
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.

Chris, I have a large amount of CAT 5 cable. Please explain 'with all the conductors wired in parallel', meaning, all pairs connected together?
« Last Edit: September 12, 2021, 1402 UTC by ultravista »

Offline ultravista

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Re: LOG Not Performing Well
« Reply #18 on: September 12, 2021, 1416 UTC »
Yet another consideration is your soil conductivity

I am in North Georgia, the ground is red clay, very hard and compacted.

Offline ChrisSmolinski

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Re: LOG Not Performing Well
« Reply #19 on: September 12, 2021, 1630 UTC »
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.

Chris, I have a large amount of CAT 5 cable. Please explain 'with all the conductors wired in parallel', meaning, all pairs connected together?

Correct. I just stripped the insulation from all eight conductors one each end, and twisted them together.  So it appears, electrically (DC anyway), as one conductor.  Connected to a transformer and then coax to the shack.
Chris Smolinski
Westminster, MD
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Offline RobRich

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Re: LOG Not Performing Well
« Reply #20 on: September 12, 2021, 1720 UTC »
Ground conductivity extremes can influence a LoG by like 20dB+, but your southern mountain soil is likely average to maybe slightly below average, so you should be able to disregard that issue.

https://forums.qrz.com/index.php?threads/loop-on-ground-antenna.622669/page-15#post-5742886
http://www.w8ji.com/receiving.htm

Your current LoG is basically *way down* on gain. Like others have noted, go larger for both the conductor and the loop circumference, though even then you might still need a preamp as frequency increases. Just part of dealing with a typical LoG antenna.

Also I would not worry too much about ordering or building an impedance transformer for the loop feedpoint for now. Start with a basic RF choke if you have *actual* common-mode noise. Get the LoG far enough away from local RFI sources, and you might need nothing but the coax between the loop and receiver.

http://www.karinya.net/g3txq/chokes/
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Offline ultravista

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Re: LOG Not Performing Well
« Reply #21 on: September 14, 2021, 1531 UTC »
I swapped the DXE Beverage transformer with the Nooelec Balun One Nine, connecting both wires to the Nooelec using the same thing cross connect wire. The change in transformer didn't seem to do much but I am having good results with the 60 foot LOG now.

It is less noisy than the Pixel and PA0RDT mini whip (both amplified) and is working surprisingly well. Perhaps there was something else going on on day 1 testing, but it is working now and in some respects, on par with the Pixel and Mini Whip. Time of day or other conditions must have had an influence.

The 60 foot diameter does nothing for NDB hunting < BC AM, it is completely dead there. Last night for example, I could pickup WWVB @ 60 kHz on the Mini (strongest) and Pixel (weakest) but not on the LOG. 60 feet may be too small. I am by no means an antenna expert, I dabble with antennas and only understand the basics.

I would like a LOG that is capable of pulling in signals < BC AM and would like know a suitable size to get there.

Offline ultravista

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Re: LOG Not Performing Well
« Reply #22 on: September 14, 2021, 1534 UTC »
Start with a basic RF choke if you have *actual* common-mode noise.

Dumb question ... how do I determine if noise is *actual* common-mode noise?

Offline RobRich

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Re: LOG Not Performing Well
« Reply #23 on: September 14, 2021, 1810 UTC »
My 148' LoG can receive longwave BCB stations with no preamp when propagation cooperates, but those stations are using extremely high-wattage transmitters. I am not really into NDB monitoring, but I have noted some when spinning the VFO down there.

An "ideal" start for a fractional loop is probably around 15% of the the desired wavelength.

Let us look at the bottom of the AM BCB band for an idea:

530KHz = ~565m * 0.15 = 277.5m

So around a 900ft loop there. Proximity to ground will lower resonance by a few percent, but we are dealing with such large loop sizes now, it is a "whatever" IMO.

If we disregard noise, ~1000ft+ would be a good start for an "ideal" low-frequency BoG. However, low frequencies are typically swamped by atmospheric noise, so you simply need an antenna with enough capture area for the receiver to detect the desired signal. Above that is basically just added gain, and it is about SNR instead of absolute gain.

TLDR? For longwave and lower, deploy the largest LoG you can. ;)

----------------

About common-mode noise, let us start with a very basic test assuming we are talking about a typical residential deployment. Tune your desired frequency with an antenna in the house, even if it is just a few feet of wire. Now hookup your outside antenna. Regardless of the signal level, do you hear the *same* noise? If so, you might have common-mode noise. If common mode is suspected, to further evaluate if the noise is being picked up by the antenna or the feedline, you would need to evaluate if the same noise is present at your antenna location. For example, by using a portable receiver.

----------------

Your Nooelec balun still is acting as a impedance transformer, though it is so small I am not sure what effective impedance transformation it is doing at such low frequencies. Hmmm. Considering a 60' loop is going to resemble almost an electrical short at longwave and lower, an 9:1 might actually work better if you turned it around at the antenna feedpoint. YMMV.
« Last Edit: September 14, 2021, 1824 UTC by RobRich »
Tampa, FL USA | US Map Grid EL88
Airspy HF+ Discovery | Kenwood R-600 + R-2000 | 2x RTL-SDR V3 + NE602 | Si-Tex 200 | Soft66LC4 | Yaesu FRG-7
148' "Shielded" Loop-on-Ground | 18' End-Fed Vertical | 9' + 31' Verticals | 2x PA0NHC MiniWhips

Offline ultravista

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Re: LOG Not Performing Well
« Reply #24 on: September 15, 2021, 1956 UTC »
... turned it around at the antenna feedpoint. Not sure what you mean.

Offline RobRich

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Re: LOG Not Performing Well
« Reply #25 on: September 16, 2021, 1212 UTC »
Your 60' loop has extremely low impedance at longwave and lower. Complex impedance aside, simple impedance likely is a few ohms at best.

You have a low impedance antenna, around 50-75-ohn coax, and probably around 50-ohm receiver. Your antenna is a significant mismatch, even to the point of almost appearing to your receiver as being *short* at low frequencies, thus incurring even greater losses for an already negative again LoG.

A 9:1 lowers the impedance even further by a ratio of.... well, 9 to 1. ;) Turning it around to become a 1:9, you can use the transformer to up the impedance match between the antenna and feedline/receiver, thus improving the match and incurring less losses at low frequencies. Your Nooelec balun will not have the right connectors if turned around, but you can overcome that with a little soldering.

Alternatively, the mismatch is part of the reason I said to try connecting the loop straight to the coax. Having no impedance transformation should be better than having a 9:1 transformation, at least at very low frequencies for your small LoG.
« Last Edit: September 16, 2021, 1214 UTC by RobRich »
Tampa, FL USA | US Map Grid EL88
Airspy HF+ Discovery | Kenwood R-600 + R-2000 | 2x RTL-SDR V3 + NE602 | Si-Tex 200 | Soft66LC4 | Yaesu FRG-7
148' "Shielded" Loop-on-Ground | 18' End-Fed Vertical | 9' + 31' Verticals | 2x PA0NHC MiniWhips

Offline ultravista

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Re: LOG Not Performing Well
« Reply #26 on: Today at 12:33 »
Got it, thank you.