Zip-Cord rx loop in 15 minutes

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May 28, 2009
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#1
A rainy day here prompted me to come up with a small hf rx-only loop that could be made and installed in between rain showers with a minimum of parts.

Zip-cord for loop AND transmission line. Loop is 16 feet circumference, or 4 feet per side for a square.
MFJ snap-on chokes for common mode control and choke balun.
Easy build for all ages 6 to 60 / 90.

Objective - get a small loop outdoors - even if it is just outside the window. Of course this could be indoors near a window too, but just being outside, even if on the other side of the window is like having a 10db preamp.

Build it:
I had a reel of common 18-gauge zip cord available. Soo....

1) Split the zip cord down 8 feet. Cut off an inch of insulation off each end and twist / solder together. Wrap with electrical tape.
2) At the end of the split, I wrapped 3 turns of the zip cord inside of an MFJ 700B4 ferrite snap-on choke.
3) Since the loop is just on the outside of the house, I needed about 12 feet of zip cord as the transmission line to reach the window where I transition to coax.
4) Opened up the loop and hung it from my 8 foot tall backyard awning in the shape of a square. That means the bottom wire of the loop is about 4 feet above ground.

5) Since the loop is not very high, when I mounted the loop, I put the feedpoint where the choke is at a corner of the loop which is closer to the house. This was way more convenient than trying to have the zip cord feedline coming down from the middle. At these low heights, and with such a small loop, placing the choke/feedpoint in a corner does not really affect the overall omni pattern. Very convenient.

Notes:
Everyone knows that zipcord is lossy, especially as you go higher in frequency, especially above 15 mhz. BUT, my run to the window transition is so short, and since I'm not using the loop for anything above 15 mhz, this loss is acceptable from an rx-only standpoint.

CHOKING - I had a 4-pack of the larger inside diameter MFJ 700 series of ferrite snap-ons. I had the 700B4's on hand, which allows for about 3-4 turns of the zip cord inside. Be sure that it closes properly and is not pinching a wire.

Convenient! The snap-on choke also serves to mechanically keep the loop wires from splitting further, and the turns wrapped inside keep the choke itself from sliding away. Nifty.

Near the window: Use another choke from the 4-pack to wrap 3 turns or so of the zipcord inside it.

Optional:
You *could* take a few inches of the zipcord past this choke on the end and directly connect to coax indoors. In my case, I used another 1:1 balun just to make the transition easy without soldering. Since I'm rx-only, I tested both and LDG 1:1 indoor balun, and even a 4:1 W2AU balun.

Get as critical here with your transition as you want with specialized baluns, but in the end remember that the very small loop is NOT trying to be impedance matched - there is more than enough signal without having to go nuts about it. Use what you got.

The biggest point is to utilize your ferrite chokes at the feedpoint, and also at the end of your zipcord run. The HF-rated snap-ons, with just a few turns inside them make it easy. Use as many turns as you can fit inside the choke. For me, 3 turns inside seems to be doing fine.

When I mounted the loop under the backyard awning, I did it in such a way to put the loop wires perpendicular to the house - just to get them a bit further from the house wiring, and not any sort of pattern management. But do what you gotta' do.

This wasn't an exercise in perfection for small-loop operations. It was just a way to get it outdoors, use what I had, and also make use of zipcord as the feedline, rather than rig it up with coax, although you can certainly do that. The choked zipcord feedline, in reasonably short lengths for a "just outside the house" kind of antenna seems to be doing REAL well from 160 to 20 meters. RX only of course.
 
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#2
Unexpectedly good testing for something just lashed up under the awning with some twine ...

NO problem hearing AM BCB and 160 meter stations *cleanly*.

The simple choking of the zip-cord transmission line at the feedpoint and at the window entrance was just fine. This kind of changes my mind about a galvanic physically-isolated transformer winding being absolutely necessary.

Unless I hunt them down, I can't hear most of the squirrely carrier interference coming my switching supply that I'm using as a test bed. And, unlike just hooking up directly to the LDG 1:1 voltage balun (which even with a balanced loop is susceptible to common-mode like most voltage baluns are), the ferrite chokes wrapped around the zipcord are doing their thing where BCB and 160m were a joke before. Now the only purpose the LDG 1:1 balun is just that - a simple transition to my indoor coax.

The directional pattern of this low down slightly large loop is similar to the small vertical loop, but being a tad larger does not have super sharp nulls, but it does have the oblong pattern similar to my on-ground logs and dogs.

Mounted perpendicularly to the house, that means the major oblong lobes are pointed into and out of the house. I thought this would be an issue. Amazingly I'm hearing BCB stations *through* the house coming from the other side with no problem.

I'm so blown away that I'm going to take the next step and purposely run zipcord indoors - with reasonable spacing away from other conductors - along with some chokes at either side and see what happens. Overall I'm not exceeding about 30 feet, which should show up with some attenuation on 20 meters - we'll see how happy I'll be by losing maybe ONE S-unit doing that. :)

While I don't seem to need it, I'll put a couple of twists in the feedline outside and indoors as well.
 
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#3
Credit where credit is due ...

Although I had built these years before, I didn't know how to properly evaluate them - mostly because I had problems with common-mode current, and never thought to model what I was hearing.

So again a big thanks to Matt KK5JY for getting me interested in the LOG and DOG, and now inspiring me with this:

Small Loop Antenna for HF Reception

My versions are basically fixed under the awnings, and venturing into the crazy world of zipcord for feedlines to it. More to come tomorrow.
 
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#4
Kill the NVIS response?

This small loop does *both* high and low angles. But what if you want to kill the high-angle response, and have it act more like a traditional ground mounted vertical?

*** Modeled, but not tested. Raining. Night time. Dxing now ***

A simple mod will do it: assuming you have it mounted as a square, with the feedpoint being in one of the lower corners, just open up the upper corner opposite the feedpoint so it is no longer a true loop, but an open dipole folded to look like one.

But there's a catch - it isn't perfect depending on frequency.

At 1 to about 2 mhz, this cut-corner loop looks very much like a traditional vertical with only lower angle lobes and a deep overhead null.

As you move up in frequency, say to about 5 to 14 mhz, that overhead nvis response is still reduced, but the low angle lobe starts to lean in the direction of the cut corner. We're talking more about a 6 to 10 db front-to-back, with a very reduced overhead response.

So perhaps if you are contesting, or have specific dx requirements, then arranging the loop with a cut corner to fit your needs might be interesting.

Modeling is one thing - but listening another. Too busy with the zipcord feedline test tomorrow to put this cut-corner version into practice.....
 
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#5
Pattern usable up to 30 mhz

15 meters opened up today and I was pleasantly surprised.

I figured that with a 16 foot circumference loop, this goes well beyond a quarter wave in circumference for anything higher than 14 mhz, and stops becoming a "small loop" and expected a really funky unusable pattern.

Well, listening and later modeling (top wire only up at 8 feet mind you) shows that the pattern is totally usable up to about 30 mhz. Sure, there is useless overhead response for these higher bands, but you aren't penalized - there are still low angles there. Same holds true for the "cut corner" version. A bit uglier at higher angles, but you aren't using those higher angles anyway.

Not much "cosmic noise" directly overhead while I was listening to 15 meters. I suppose if that's a *big* worry, then just a touch of dsp / low pass audio filtering might make that tolerable. Not a problem for me so far. Still waiting for 10m to open up. :)

Panadapter / Spectrum tip:
With the lower overall signal strength, if you are watching one of these waiting for big-daddy carrier spikes to prove that the band is open - you are missing a LOT. In between the "grass", are perfectly armchair copy of signals that SOUND S9, but aren't even tickling your S-meter.

I like watching spectrum like anyone else, but with these high s/n antennas, you gotta "spin and grin" or you'll miss out.
 
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#6
Look Ma, no coax indoors!

Or, how 3 turns of zipcord inside an MFJ ferrite choke is not good enough for BCB / 160 meters ....

Interestingly enough, zipcord - when in reasonable lengths - performed ok indoors. I could move it around, have some on the floor, put my hands all over it and it didn't detune the antenna. And sure, 15 and 10 meters has some additional loss, but for now I'm running with the 10db preamp on these bands anyway. Won't compete with the yagi at the club, but fun enough for now.

But, it was a common-mode circus, which was easily fixed.

Here's the test setup:

1) Only zipcord from antenna outside and a run indoors inside to the rig. (Icom 718 to keep me honest). 3 turns around MFJ snap-on at feedpoint, window interface, and near back of the rig. Use banana jack into so-239 and ground screw for zipcord attachment.

Result: S9 or more of TOTAL noise on BCB and 160 meters. Not surprising. But still happy that the feedline doesn't seem to be totally touchy around indoor materials. No changes in signal, um noise strength.

2) Placed an MFJ 915 inline ferrite choke off a 2 foot jumper at back of rig. Used banana jack and careful ground screw attachment at other side of choke.

Result: AM and BCB is back! Sounds like a normal band again. BUT, I can hear every single birdie that my testing switching power supply can throw out. Also moved the choke to the window interface with zipcord on both sides. Same result. An improvement, but still not good enough.

3) Swapped choke for galvanically isolated line breaker. Same result as #2 above. Not good enough. Nor is a combo of both choke and isolator.

4) Improved ferrite choke : a Myantennas rx-only CMC-0510-R. BANG, not only is BCB and 160 back, but all the birdies from the switcher are gone. If I *hunt* for them, crank the gain waaaay up, I can tell where they are. But from a normal operating perspective, gone. Second runner up is a tx-capable CMC-130S-3K from the same place.

Fascinating. It shows that zipcord in reasonable lengths is usable indoors, and from an rx-only perspective is not as critical in placement as I thought it would be. Hence a common-mode choke of GOOD quality is necessary. Nothing new to learn here I suppose, but the hands-on demo in my own shack was enlightening with this zipcord antenna.

How to be fooled: this was the biggest enlightenment. You, as an experienced listener, promptly install your *linear* power supply (or battery) in place. You hear no birdies, or anything obvious. BUT, just because you can't immediately hear any problems, does not mean you don't have common-mode issues which can affect the antenna. Keep that switcher around as a test instrument to improve your grounding and choking, and THEN go back to your linear power supply.

AWESOME way to have some fun on these rainy days!
 
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#7
Voltage baluns are out

When testing the original outdoor loop to indoor coax run, I wanted to see how bad the voltage baluns (LDG and W2AU) actually were when it came time for common-mode rejection. Yep, hardly any.

Swapped the W2AU for a current balun, in this case an MFJ 913 in the junkbox.

That produced results similar to the inline chokes - that is BCB and 160 are now usable, but still indoors I hear the switcher power supply carriers.

Only the rx-only CMC-0510-R truly took it all out to my satisfaction. Some may be satisfied with less.

So right now I'm just running straight from the zipcord loop, with zipcord feedline into the CMC-0510-R and a coax jumper to the rig for kicks.

But as a rainy-day project that rocks - don't use a voltage balun unless you have other common-mode choking measures in place for the indoor zipcord.
 
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#8
No longer afraid of open-wire transmission line !

Finally ended up with an LDG 1:1 *current* balun (black box) inline as the indoor transition to coax again. The 4:1 current baluns performed similarly, and basically wasn't about the transformation ratio, but just controlling the common mode. Don't be afraid to use that if you got one.

In the end, be it my coax run, or my zipcord feedline run indoors, both needed common-mode control beyond what some simple snap-on choke windings would do, and not just this antenna. This problem which shows up mostly on BCB and 160m, is not being "received" by either the coax or zipcord, but is being directly placed onto it by my poor rf grounding, and "whole house antenna". Those ground wire runs to a single point ground rod just aren't good enough. Good to know which direction that noise is coming from - from my side of the equation. :)

So in my case, even though I take care to have galvanically isolated transformers at the feedpoint for other antennas and all that, I still have issues starting first at the connection to the antenna jack itself. Hence the inline choke right there. Once that is in place, the necessity of having fancy isolation transformers at the feedpoint lessens. Nice, but not mandatory if you take reasonable care (chokes etc) at that end too. Coax or zipcord - results and fix was the same.

This opens up some new avenues for portable antenna using *reasonable* lengths of zipcord, or better yet, say some 300 ohm quality stuff (not decrepid tv stuff) for some runs where it is called for.

BUT, *just like coax*, one needs to pay attention to common-mode filtering. I can say now that most of my failed antenna projects using twinlead / open wire, were not due to it being a very sensitive transmission line, but more due to lack of attention to common-mode problems - which are easily fixed with the same care one would do with coax.

Since the off the shelf inline chokes and 1:1 baluns are doing the heavy lifting, my first thought was to just pull all the snap-on ferrites since they don't have enough choking impedance to a major job especially at BCB / 160. But, I think I'll leave them in just for good measure to help. Especially since I found that after wrapping only 3 turns inside, I can get a 4th turn in by threading the last one in (rather than wrapping) with a little spit lubrication. Yuck. Zip cord not tasty.

This opens up a lot of new avenues for me. And surely saves on a lot of coax jumpers, as proven by running the zipcord loop antenna and feedline all the way in to my choke right at the receiver. Although the zipcord will have to be replaced often when the sun burns it up, I have no moisture issues for the entire system enduring a weeks worth of solid rain either. The good 300 ohm stuff might be substituted shortly.

Pretty jazzed about this experiment. And the loop itself is doing just fine - about as clean as my log or dog antennas.
 
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#9
Put this in the attic!

Looking at this on the patio, it became obvious that I might be able to fit a 4-foot square loop vertically in the attic.

I used coax, and will detail more of the tests later. But the big hint here is that what I thought was "attic noise", was really due to common-mode problems of the coax. Once I gave the attic loop a fighting chance, I was surprised.

More detail in a day or so...
 
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#10
Going a bit larger and changing shape. Project still meant to be safe and sane for children of all ages.

I think this will be my last iteration for testing. I'm changing shape to an inverted delta loop. Basically an 8 foot flat-top, up at only 8 feet, with wires on each end coming down near to the ground for the feedpoint. Each of those sides of the triangle are about 11 feet long. In my case, the inverted delta is really 8 feet straight up, 8 feet over, and the last side run diagonally back down to meet at the feedpoint.

With the inverted delta loop, I'll still have some effective-mean-height so that electrically the loop won't appear so close to the ground. And, I won't have any vertical runs of coax feedline - it will all be on ground to help provide some measure of common-mode attenuation.

But to be sure, I'm still using chokes at the feedpoint and at the receiver itself.

Not expecting anything unusual. Sure will be quick and convenient.
 
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#11
A bit of dx gloating time ..

Just copied a VP2M station in Montserrat on 160m cw. NO preamp. From LA 2am pt. With zipcord. Getting a taste for 160m I'll tell ya. Wouldn't be possible without the Myantennas CRC-0510-R ferrite choke at the Icom 718 side...

Stations copied are not the best way to judge antennas, but it's a new one for me!

So far:
*SMALL* inverted delta loop with feedpoint at ground level, and one side only up 8 feet.

30 feet of RG-8X with aforementioned choke at rig, and another Myantenna CMC 130S-3K at feedpoint adapted to banana plugs.

18/2 *LAMP* cord (not speaker wire) for loop. Paralleled together to emulate single #12 wire for a 3db enhancement. Additional stray capacitance for this rx only antenna doesn't seem to be an issue by twisting ends of cord together. Since stranded #10 is too unwieldy, may just pick up some #14 or #16 and double them together just for 160m for a tiny additional boost. Feel no need to go nuts with tubing with this "medium" sized loop.

Mostly no rig preamp when bands truly open up. Plenty of signal in most cases to put a smile on my face - Just listening to 160m is a whole new thing to me anyway now that I'm choked nicely.

Once common-mode issues are worked out, this rx-only game is just too cool, even when the s-meter is pegged in the wrong direction. :)
 
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#13
Ah, but even a balanced feed is no guarantee. I needed ferrite choking with the zip-cord feedline too, as imperfect as that is from a loss standpoint.

I had done loops like this in the past, and got close to fully understanding them, but was still affected by common-mode skewing my enjoyment.

I think I have reached a state where I'm satisfied with a "Medium-size" loop project and tied much of the variables together. The temptation is to over-engineer it, or become an EZnec bench-racer and shoot it down comparing it to every other antenna out there. Step away from the mouse, and copy a VP2M on Montserrat instead on 160!

Objective:
1.8 to 30 mhz general purpose omni-directional coverage, accepting a lower-efficiency sacrifice here and there. More or less emulates a small vertical, without the ground wires, and the *addition * of high-angle reception when available.

Medium-sized Vertical Loop antenna:
20 to 30 feet total wire. In any shape, square, diamond, triangle (normal or inverted). Inverted preferred when using ground-level feedpoint. Rotated the inverted triangle so that one side is vertical, top side horizontal, and other side diagonal because at high frequencies, the overall omni pattern is a bit better. Fixed in a convenient position, but if I had to rotate it, I'd have to use a "swing arm" method. For now it stays put.

Impedance matching:
Forget about it. At low frequencies, this loop is still very small electrically. At higher frequencies, you even pass quarter-wavelength loops, and get close to being a funky half-wave loop at 30 mhz - another bear to match. So we'll stay just inside that by not exceeding 30 feet total. Go direct with a 1:1 if you like. Current baluns preferred. Isolated transformer if you got one. Use a 4:1 current. Use a 9:1 current. My preference is to dump ferrite on feedline. Not much difference, unless one is trying to match a narrow-band half-wave loop with a 50:1 transformer for a 45K impedance. :) Use a relatively short quality feedline. Open-wire ok, but still a ridiculous match. Whatever floats your boat. Par EF-SWL transformer works fine, especially down to .5 mhz. Radiowavz 9:1 B19TX *galvanically isolated* does fine too. Banana plugs adapted to Myantenna CMC inline chokes do great. Run what you got. Improve if needed.

Pattern from 1.8 to 30 mhz:
Pattern goes from inline with the loop plane from 1.8 to about 10 mhz. From 10mhz to 21 mhz it is an omni-blob favoring no direction. As we go higher in frequency, it wants to become a half-wave loop, favoring a broadside to the plane. All frequencies have high-angle reception along with low, sometimes useful, sometimes not depending on freq. Will you notice in the real world? Maybe not.

Wire / Tubing:
Use as large a gauge as you feel comfortable working with or have laying around. At some point, cost and practicality may be the determining factor.

Tuner ?
Not necessary. Because the antenna is so ridiculous a match, you will just twiddle knobs and get a peak to nowhere. Having a "bypass" switch on the tuner helps prove that it is redundant after tuning. If it *IS* very helpful, then you don't have a lossy loop, but perhaps a very funky common-mode connection making a big random wire instead. Look out for that, hence the ferrite choking or other measures.

Overall, the simplicity and freedom of this rx-only general purpose antenna are alien concepts when donning my amateur hat, wanting to pin down every single variable. Instead, just put one up and see how it goes - then go back to the mouse. :)
 
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#14
Notes about the use of a preamp in the rig:

A little longer evaluation with some finer details ..

Normally, the use of a preamp for me is about the s/n ratio, depending on what I'm listening to. But using it for a huge coverage of spectrum from 1.8 to 30mhz, with a 20 to 30 foot loop, the preamp is toggled on and off depending ....

160 meters - Here the loop is so small compared to wavelength, and the feedline match so poor, that *when the band is open*, I run with the rig's 10db preamp on all night more or less. I could use just a tad more signal level, and I might just increase the wire gauge up a bit. Not enough to want to use tubing, but yeah, that would a small improvement. Liveable for now with just 12 gauge up there.

Now that I have choked off the common mode of the feedline, during the day I hear many of the neighborhood squeal-y little carriers and whatnot. At least that's not coming from my feedline. But once the band opens, and propagation changes a bit, those little carriers are buried. But still, I run with the preamp on all the time.

80 / 40 meters - Preamp on or off depending on my desired s/n ratio. Loop is still small compared to wavelength, and I feel the need for large tubing to be unnecessary. Wire gauge of about #12 sufficient.

30 meters - here the 25 - 30 foot loop is quickly approaching a half-wavelength, where the impedance is skyrocketed to about 45k. Of course the feedline match is nuts and a 1:1 to 9:1 doesn't make much difference. When jumping around the bands, the volume is unnaturally low in comparison to others. To help make up for that, the rig's 10db preamp is on all the time - day or night. Since the preamp works fine here, I'm not going to try and "slide" this around by changing the size of the loop. Preamp does just fine.

Doing the math for a half wavelength should put this near 20 meters, but electrically, it seems to be around 30 meters where this is happening in the real world environment for me.

20 - 17 meters - preamp usually OFF. Plenty of signal strength. Amazed because this is where the loop approaches a half-wavelength when doing the math and should be poor. But it works here. Real world interaction probably at play here.

15 - 10 meters - this gets a little weird and will need more testing when there is more activity. Strangely, the preamp is OFF. But, I may be running into the cosmic noise from the overhead coverage of the loop, and of course that coverage up here is not doing anything useful. A *small* amount of DSP filtering works wonders to make it sounds like a normal band. So the problem of the useless overhead coverage is not a deal-breaker, but I need more activity and time to make any sort of conclusion. C'mon solar cycle!

I'm pretty jazzed at how well this simple loop is covering the bands - but the preamp is needed at two major extremes - 160 meters when it is actually open, and where the loop is nearing the half-wave in circumference around 30 meters. (20 meters if you do the math, but it seems to hit me at 30m) At least that's what the blob under the foil hat is telling me so far. :) Having things not be so critical like it would be with a transmit-capable antenna is blowing my mind.
 
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#15
No bias against small loops

Just a note to say that this antenna implies no "superiority" against smaller amplified loops like a W6LVP, Wellbrook, MFJ or similar.

What you get with these compared to a fixed-position 20-30 foot vertical loop is obvious portability, a much tighter pattern you can use to null local noise, and the ability to remotely run them with long feedlines.

So no salesmanship of the zipcord loop, it's just another option in the world of untuned loops.
 

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#16
But if you don't want the expense of one of the above loops hertz mentions. this is a world better than just throwing a wire on the floor in a condo or apartment. Probably a better performer if you can get it into an attic or hide it outside...Mike
 

K9DAK

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#17
Great write-up Hertzian! Gives me an idea for a BCB antenna in our family room where we have a window with a 26' circumference... I could hide the wire behind the moulding and have only a 10' feedline to the A/V receiver that I could hide behind the baseboard moulding. I like to listen to Da Bears games' audio on WBBM-AM and synch it with the TV video.
 
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#18
Thanks guys - pushing these antennas made me learn something about indoor or close-to-the-house situations ...

Much of what I thought impossible due to noise is mostly coming from the house wiring itself. Common-mode problems have to be hit **hard** . A few snap-ons or a handful of windings around a ferrite doesn't cut it. The simple MFJ 915's - at both ends - are a good start, but they will leave you wanting for the heavyweights like a Myantennas, Balun Designs LLC, or equivalent good common mode choke(s). Ie, it would not be unthinkable for an SWL'er to use a CMC designed to handle 3K of power. :)

Finding and curing the offender is always a good thing from direct radiation is great, but man - they surely couple into the house wiring itself real well which for me seems like most of the problem.

I've got a couple of Tripp-Lite Isobars coming for those mission critical devices that I just don't want to tear apart. I'm on a roll now. Snap-ons for the power cords were just not cutting it.

Indoor version of the inverted delta?

You got me thinking 'DAK! It would not be unthinkable to run a slightly smaller loop here - that is, a right-triangle version (which Moxon loved) slightly askew like mine outdoors. That is, 7 feet vertical, 7 feet horizontal, and a diagonal run back down to the feedpoint near the floor. Supposedly the right-angle (as opposed to an equilateral) improves the radiation resistance by 3 - 4 times. Not sure it applies much at these small loops, but hey, every little bit helps.

Again - CHOKE that thing first! :) If your interests include AM BCB, then use a choke that will actually do something down that low - the rx-only Myantennas CMC-0510-R does a great job there - much more so than most amateur chokes which struggle a little at even 160m. Highly recommended - just don't hit your tune button!
 
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#19
Tuner Revisited:

I'm going to take back what I said about tuners being redundant just a little bit ...

Yes, for the most part, the two "problematic bands" of 160m and 30m more or less mandated using the 10db preamp all the time for my tastes. And I'm not one who likes to bang the s-meter. Problem solved.

BUT, I prefer the full-bodied flavor of having less amplifiers inline if I can. On those two bands, 160 and 30 meters, the tuner DID make an appreciable difference - where it is now a judgement call on whether to use a preamp or not, whereas before it was mandatory. I suppose the high reactance of the system, along with the 30 feet of RG-8X being a transformer in it's own right needed to be tamed a tad.

Other bands didn't make any appreciable difference - just making me go through the tuning motions once I compared with the bypass switch.

Prior to getting my common-mode problems solved, tuning on 160 meters made no difference. It does now!

But at the end of the day, if you don't have a tuner, the 10db preamp will do just fine for most on problematic bands.
 
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