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Author Topic: 80M home brew Mag Loop VS. inverted L comparison  (Read 156 times)

Offline Antennae

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80M home brew Mag Loop VS. inverted L comparison
« on: September 22, 2019, 1645 UTC »
I've had this big 9' magnetic loop in 2 pieces made out of plumbing copper pipe and I never had a good way to test it.  With my newly acquired ham ticket I was able to send out a carrier wave and listen to remote SDRs across the Pacific.  It was 630am for me and about 1130pm for Australia's East coast.  The propagation conditions were not changing for me yet.

I wanted to see how far this big magnetic loop would transmit compared to an inverted L . 

The inverted L is about 30' high and is terminated into a tuner and a ground rod. Its length is cut for the upper 80m band. 

The magnetic loop has an adjustable air capacitor and it sits on a brick above my lawn.  It has a length of coax used as a capacitor in parallel to bring my air capacitor into the range of 80m.  The small loop hooks into some coax which goes into the same  tuner on the same ground rod. 

1st test: 50w carrier.  I had them both up one morning and the inverted L was about 1 S-unit stronger than the magnetic loop into Freeman's Reach, AU and Marahau, NZ.  They were fadey and near the noise floor.  AKKY, in Fukushima, Japan did not pick up the mag loop, but it picked up the inverted L at the S-2 noise floor.  The signal was so low that I could hear it and hardly see it on the online remote SDR. 

I wondered if the inverted L was resonating with the loop. I didn't expect much from my magnetic loop.  It has clean connections, but I figure it has loss compared to those fancy vacuum variable loops on the internet.  And it doesn't have a ground wire system.  It goes to the tuner, then gets grounded at the rod there.  Anyway I had to do another test. 


2nd test the next morning.  30w carrier.  The mag loop was tuned a few kHz away from the test on the inverted L to prevent resonation.  (when testing the mag loop it might charge up the inverted L and that would transmit as well. That would throw off a nice scientific test because the antennas wouldn't be isolated.)  I tested the inverted L, then I took it down, and then tested the loop.  The signal was about the same into Freeman's Reach, AU. Near the noise floor, fadey carrier.  There was interference in NZ, and i didn't test Japan (I was still sleepy. maybe I did test it and didn't write it down? we will never know, unless I do another test tomorrow). 

I don't know which one is uglier for the neighbors. The tall pole? Or is it the large loop peaking over the fence with a big air variable capacitor on it?  I spray painted the loop camouflage and its now less noticeable until you do notice it, then your mind messes with you because you know its a circle but you can't place it easily because its broken up with camo colors.  If you don't know WTF it is, its more scary because I'm trying to hide it from you, neighbor.  And I'm up at 630am on a Sunday messing with it.  And talking sweet things to it. "Thaaaaats a gooooood girl." 

So, the loop is about 1 S-unit weaker than the inverted L and they both make it all the way across the Pacific.  But its hard to tell, they're pretty close.   Pretty dang cool, mang.  Its the not so little loop that could.

California Coast
RCVR: Radio Shack DX-398 portable, remote SDRs 
Antenna: usually a random wire with tuner

Offline Josh

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Re: 80M home brew Mag Loop VS. inverted L comparison
« Reply #1 on: September 23, 2019, 2128 UTC »
Nice work. Loops like these are always going to be lossy and bad for dx on the low bands, but once you get above 40m they can be fairly decent but still lossy due to the huge amount of current that zings around inside em. For dx, you'll be making it easier on yourself by going to a vertical.

On the interaction, you can turn the loop, if possible, to a right angle with the other antennas to place the antenna in the null of the loop. Also to make for fair testing you want both antennas to put a lobe onto the rx area, so don't have the loop crossways (null towards the rx site) to the target area. Have fun!
Conveniently located near Vincennes Indiana.

Offline Antennae

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Re: 80M home brew Mag Loop VS. inverted L comparison
« Reply #2 on: September 24, 2019, 0002 UTC »
Oh, cool, that makes sense to point the null toward the other one. 

I tested again this morning and the inverted L went into AU and NZ again while the loop wasn't even there.  BUT...there was a layer of dew on the loop. Perhaps its the culprit. 
California Coast
RCVR: Radio Shack DX-398 portable, remote SDRs 
Antenna: usually a random wire with tuner

Offline ThElectriCat

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Re: 80M home brew Mag Loop VS. inverted L comparison
« Reply #3 on: September 24, 2019, 0310 UTC »
Nice work. Loops like these are always going to be lossy and bad for dx on the low bands,
this is true, but if you are interested in NVIS communications (Near Vertical Incidence Skywave), which is really good for shorter range HF communications, loops can be excellent performers.
there are lots of online resources on NVIS on ham websites for anyone who is unfamiliar.
In another life, I could have been a telephone engineer.

Offline Josh

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Re: 80M home brew Mag Loop VS. inverted L comparison
« Reply #4 on: September 24, 2019, 1847 UTC »
Yep, like cat said, loops can be great for nvis, where losses aren't really going to matter much. Just make sure the loop is aligned with the target area or you'll be shooting yourself in the foot with rf.

Might help with aiming the loop;
https://ns6t.net/azimuth/azimuth.html
Conveniently located near Vincennes Indiana.

Offline ThElectriCat

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Re: 80M home brew Mag Loop VS. inverted L comparison
« Reply #5 on: September 25, 2019, 0352 UTC »
If you have both antennas, you may want to use the loop for RX, where its tuned, narrowband inherent characteristics and deep nulls will help to reduce interference from strong local stations.
Of course, for RX only, a much smaller, multiturn loop can be used if you are so inclined
In another life, I could have been a telephone engineer.