HFU HF Underground
Technical Topics => Equipment => Topic started by: MDK2 on January 15, 2017, 0645 UTC
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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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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.
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Thanks Chris! ;D
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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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The preferred approach would be to increase the number of turns in the loop.
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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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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.
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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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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.
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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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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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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!
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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.
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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
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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
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RS used to carry a tiny and cheap 365pf vari that was more or less the same as used in cheap am transistor portables, the kind with plastic dielectric between the plates. As most of the construction was plastic ie cheap I figured someone would be making them today, but it looks like they're long gone, being replaced in the market by varactors and the 13 dollar variables as mentioned.