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Author Topic: Question regarding my first loop antenna  (Read 2697 times)

Offline MDK2

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Question regarding my first loop antenna
« on: January 15, 2017, 0645 UTC »
Most regulars here are probably aware that I have two homebrewed loop antennas that I've been using. Both were constructed with identical 365 pF variable capacitors for tuning, and as it turns out, both loops overlap coverage with each other quite a bit despite other important differences (namely one loop has a circumference of 15' using 1/2" diameter copper tubing as the conductor, while the other has a 10' circumference using 1/4" diameter copper tubing).

The bigger loop, the first one I built, is very top heavy and despite a decent tripod, has toppled over a few times. The variable capacitor got some of the plates bent (as far as I can tell) and isn't very usable at the moment. It can probably be salvaged, but I thought I would take advantage of the situation and replace it with something that can lower the frequency coverage I receive, if possible. I figure that a variable capacitor with a higher pF number is the way to go. With the old 365 pF capacitor, coverage leveled out at about 3.6 MHz. I'd like to go all the way down to 1.8 MHz if possible. Am I on the right path?

I know of this calculator form (below), but it's geared toward transmitting loops. It seems to say that I would need a capacitor with at least 2,000 pF. I don't know if I'm reading it correctly. It also says I would need a much bigger loop, but again it's in regard to a Tx loop, and I'm just interested in Rx. Thanks for helping out someone who knows little about electronics.
http://www.66pacific.com/calculators/small_tx_loop_calc.aspx
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Offline ChrisSmolinski

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Re: Question regarding my first loop antenna
« Reply #1 on: January 15, 2017, 1627 UTC »
Yes, increasing the capacitance will lower the resonant frequency, which is equal to 1 / (2 * PI * square root (inductance * capacitance))

Four times the capacitance will cut the resonant frequency in half. Insert lots of hand waving here for not taking into account stray capacitance and inductance, how the loop is wound, diameter of the wire, phase of the Moon, and the square root of the distance to Chuck Norris.

There's practical limits to how much you can lower the resonant frequency by adding more capacitance, and I defer to any advice from jFarley when loop antennas are concerned.

Another thing you can do of course is to go with more than one turn on your loop, if you want to lower the resonance frequency but keep the overall dimensions reasonable.
Chris Smolinski
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Offline MDK2

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Re: Question regarding my first loop antenna
« Reply #2 on: January 15, 2017, 1715 UTC »
Thanks Chris!  ;D
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Offline redhat

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Re: Question regarding my first loop antenna
« Reply #3 on: January 15, 2017, 1857 UTC »
The other thing to note is that as you add capacitance, your tuning range will shrink, both due to your tuning capacitor being a smaller fraction of total C, but also the lower frequency.

+-RH
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Offline jFarley

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Re: Question regarding my first loop antenna
« Reply #4 on: January 15, 2017, 1911 UTC »
The preferred approach would be to increase the number of turns in the loop. 
Joe Farley, Near Chicago
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Offline MDK2

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Re: Question regarding my first loop antenna
« Reply #5 on: January 15, 2017, 2111 UTC »
The preferred approach would be to increase the number of turns in the loop. 

Would this be done using the same 15' piece of pipe, turned to smaller diameter loops, or a longer piece of pipe to make loops of the same diameter?
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Offline jFarley

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Re: Question regarding my first loop antenna
« Reply #6 on: January 17, 2017, 1338 UTC »
To drop the the frequency range by a factor of 2 without compromising sensitivity, 1 or 2 additional turns of the original diameter should be added.  The fact that the loop is made from copper tubing is going to complicate the issue.

Joe Farley, Near Chicago
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Offline MDK2

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Re: Question regarding my first loop antenna
« Reply #7 on: January 17, 2017, 1526 UTC »
Just adding any turns is going to complicate things, just because I have a frame that supports only one turn. But I can figure something out. Thank you very much, jFarley!
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Offline Josh

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Re: Question regarding my first loop antenna
« Reply #8 on: January 17, 2017, 1608 UTC »
Do you transmit with the antenna? Reason I ask is that for rx only, you don't need the low loss copper pipe, it's really only needed for tx where there's a zillion amps running around the coil and the losses are huge.
Conveniently located near Vincennes Indiana.

Offline MDK2

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Re: Question regarding my first loop antenna
« Reply #9 on: January 17, 2017, 1931 UTC »
Do you transmit with the antenna? Reason I ask is that for rx only, you don't need the low loss copper pipe, it's really only needed for tx where there's a zillion amps running around the coil and the losses are huge.

It's Rx only. I went with the copper pipe because the original design suggested it, and even at that length and diameter it was affordable.
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Offline BoomboxDX

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Re: Question regarding my first loop antenna
« Reply #10 on: January 25, 2017, 1424 UTC »
Remember you can add capacitance with a disc ceramic capacitor.

I lowered the tuning of a MW loop with a disc ceramic, I soldered one side to ground and clipped it to the positive terminal with an alligator clip when I wanted the tuning range to drop.
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Offline MDK2

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Re: Question regarding my first loop antenna
« Reply #11 on: January 25, 2017, 1535 UTC »
Remember you can add capacitance with a disc ceramic capacitor.

I lowered the tuning of a MW loop with a disc ceramic, I soldered one side to ground and clipped it to the positive terminal with an alligator clip when I wanted the tuning range to drop.

Something to consider. Thanks!
Denver, CO.
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W6LVP active loop, homebrewed mag loops.
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Offline Josh

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Re: Question regarding my first loop antenna
« Reply #12 on: January 25, 2017, 1906 UTC »
I used to make and sell a tabletop frame loop I sold to some 160m and ambc  dx guys that used a switch to add in fixed caps as desired. If I could find cheap 365pf variables I might go back in biz.
Conveniently located near Vincennes Indiana.

Offline MDK2

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Re: Question regarding my first loop antenna
« Reply #13 on: January 25, 2017, 2317 UTC »
I used to make and sell a tabletop frame loop I sold to some 160m and ambc  dx guys that used a switch to add in fixed caps as desired. If I could find cheap 365pf variables I might go back in biz.

This is what I got for both my loops. I don't know if that's cheap by the standards of someone making a go of selling antennas, but I thought it was reasonable enough for my purposes.

https://www.amazon.com/Capacitor-Variable-Single-365-Rotation/dp/B00EQI9K9O
Denver, CO.
SDRPlay RSP2pro, Icom IC-7100, Grundig Satellit 750, Tecsun PL-600.
W6LVP active loop, homebrewed mag loops.
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Offline redhat

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Re: Question regarding my first loop antenna
« Reply #14 on: January 26, 2017, 0127 UTC »
I see the same thing on ebay.  It might be the antique electronic supply guys, as they used to sell a cap that looked just like that.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com