RX-only inverted vee

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nanZor

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What - why rx-only?

Testing my "loop on ground" inspired by Matt, KK5JY reveals that the directional pattern is somewhat similar to a typical low inverted-vee. But there's a twist.

We all know that bringing a dipole or inverted vee low to the ground makes it more or less omnidirectional and kind of "cloud burny" with no low angles. :)

One way to get those low angles is to squash the pattern by going to the extreme, such as putting it right on the ground. That's the subject of the "log" or "dog" antennas seen here:

https://forums.radioreference.com/receive-antennas-below-30mhz/370110-160-20m-log-loop-ground-2.html

BUT, for those who don't want to believe, and need something "in the air", we'll do just what you nornally don't want to do when your antenna is designed to transmit! Get it *really* LOW.

1) Build your inverted vee, but since this is designed for rx-only, bring the ends as close to the earth as possible! We're talking no more than an inch off ground! Bad for tx of course, but good for squashing the pattern for low(ish) rx angles. Your typical 20 meter inverted vee lengths will do.

2) CHOKE AND ISOLATE the feedline at the feedpoint. Otherwise, don't waste your time. Slapping an ineffective "ugly balun" doesn't cut it! We're talking about using a galvanically isolated (physical separation) transformer. And now your sleeve ferrite chokes. Do what you have to to truly isolate the feedline.

3) Don't get hung up over resonance. This is a non-resonant broadband antenna. I haven't yet determined the best impedance ratio transformer, it 4:1, 9:1 etc etc, BALun. Preferably not an autotransformer, but if that's all you got, immediately follow up with a 1:1 galvanically isolated transformer. You get the idea - don't make the feedline part of the antenna! Don't go half-way with a few random ugly loops of coax.

You can see the results in EZnec - even the demo version. Bring those ends down to 0.5 (zero point five) inches away from ground.

Right - I haven't built this yet. My "invisible" rx-only squashed inverted vee is in the shape of a loop on the ground. :) But for those who want something in the air, purposely built for rx-only, this might be a great project.
 

ka3jjz

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hertz, where would you get these transformers and/or plans on the net for building them? Mike
 

nanZor

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I just use off the shelf stuff rather than winding my own, which is actually pretty easy. However I fall victim to instant gratification more often than not. :)

Mostly I use transmit-capable amateur products for rx-only. It's overkill for rx-only, but it works.

Right now, I'm using a Radiowavz B19CI 9:1 balun, which is an isolated transformer balun. I tested with my ohm-meter to make sure that there was no connectivity across the input and output. That gives me the impedance change. along with the *physical* isolation, killing two birds with one stone.

I've also used standard 4:1 baluns, which are autotransformers, and followed that up immediately with a Radiowavz "ISOX" 1:1 transformer which is galvanically isolated. Then, I follow that up with your usual sleeve-choke, like an MFJ 915. All at the feedpoint naturally. There are quite a few other manufacturers with similar stuff, but in the case of isolation, one has to make sure they aren't prey to marketing lingo, and there is TRUE physical isolation. :)

There are many manufacturers, like Balun Designs LLC. Great stuff. DX-Engineering has some nice stuff too.

The one advantage to using amateur-radio products is that if you are an amateur, you may be able to pump some power into the rx-only antenna to see if your auto-tuner will tune the antenna - not that this is vitally important for rx-only - but I guess there's no harm in heating up worms doing that with the rx-only antenna. At least the balun / isolators won't burn out if you tried doing that with dinky little rx-only stuff.

And since one isn't really transmitting for long if they are only trying to do tuning on the rx-only antenna, lesser quality baluns/transformers might survive much longer than usual. :)

I think the usual online amateur radio stores have much of the needed materials, but always be *specific* about what you need - demand PHYSICAL transformer isolation. Marketing materials can sometimes be slick about this.

Winding your own: there is a lot of "End-Fed 9:1 transformer" material online as well which contains good info BUT BUT these are usually wound in an UN-UN configuration, and not a BAL-un configuration, which is what we want. Palomar Engineers comes to mind for some nice kits wound the way you want.

AIR-CORE DIY. Not popular these days, but might be just the ticket! May not be the absolute most efficient, but there is leeway when doing rx only:

For a 9:1
1) Get 1 inch to say 1.5 inch pvc pipe. Not critical.

2) Wrap about 12 single-spaced turns of household insulated wire around it. THHN #14 house wire is what I use, but heck even zipcord split off into single wires will do. This is the primary winding that connects to each end of the inverted vee. (or loop for that project)

3) Over the top of this, wrap 3 turns in the center over the top of the primary winding and attach this secondary winding to your coax.

Note: This 4 to 1 ratio of *windings* give you the 9:1 impedance ratio. And is also physically isolated.

This ratio changes if you are using 75 ohm tv-type coax cable or 50 ohm coax, but I'll leave that for online searches to determine impedance vs actual winding ratios.

Right - so not the most efficient thing on the planet, but if you need to get up and running *tonight*, yeah, this will do! ;)
 
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ka3jjz

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I seem to remember somewhere that if you are putting up an inverted V, a decent way to go is the standard 234/f(mhz) - 3% or so. Does that still hold or does it even matter in this case?

Mike
 

NC1

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Great information. My neighbor was asking me not long ago if there was something for SW receive only that he did not have to put on the house or attach to trees. I could not think of anything at the moment other than just a random wire along his outside railing.

This antenna looks like it will be exactly what he is looking for. Just a few simple parts and it's done, with the best part being it is flat on the ground! I'll have to make one just out of curiosity but of course he is paying for the parts since it will be his once I'm done.
 

nanZor

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Well, 234/f mhz = feet required is the ballpark required for each leg. But since this is rx only, and broadbanded to boot, it is not that critical since we aren't trying to resonate it. 160m to about 20m without getting any strange lobes is the goal. I suppose if one actually wanted it to be resonant and high-q so to speak, they could drop the impedance transformer from the equation and prune and tune with either a transmitter or antenna analyzer. The problem there is that when "pruning", you have to make sure to put the ends of the wires back close to the ground for our rx-only needs.

Personally I would just put up a support, and use the longest symetrical legs that I could fit safely on the property down to 1 inch above ground. And nothing smaller than about a 45 degree angle otherwise we're headed back to cloud-burner territory.

My experience with the very small loops and small dipoles has been that while workable, things get fiddly when they are *too* short.

My preference for the rx-only inverted vee would be about 25 feet at least on each leg. I'd have to model it in EZnec to see trends when going to very long extremes and see if I was defeating the purpose!

NC1: the loop on ground is perfect for swl and stealth. My preference is at least 15 feet per side of a square (60 feet) for a general purpose 160-20m, and even higher if the bands were actually open up there. :) When the band shuts down, it's gone fast with the on ground loop.
 
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nanZor

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STOP THE PRESS about this "rx-only" inverted vee! My information is wrong.

My apologies to all. I knew I should have actually built this before posting about it to verify if EZnec was guiding me properly.

There is NO magic to bringing the ends down super close to earth.

What I had done was mistakenly transpose an EZnec description filename with another antenna I was testing, made some plots, but didn't take a look at the antenna itself.

Well, maybe all that hot air might get some to choke and isolate their feedline, but otherwise my rx-only inverted vee was basically bunk.

Apologies to all. Won't make that mistake again without actually building what I talked about.
 
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