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Low Band and Airband dog - wish me luck!

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#1
Since the HF loop-on-ground antenna is working well for me in the noise department, I thought I'd dip my toes into the pool of insanity with much smaller folded-dipole versions for VHF Low / 6M and Airband.

I haven't built them *yet*, but toyed around with EZnec enough to see that they might be viable for skywave reception, and possibly a solution to noise problems when no other measure is successful.

Very simple really up at these freqs. Folded twin-lead (or windowed ladder line etc). 4:1 (300/75 ohm) balun transformer. RG6 back to the shack. Maybe toss in some more ferrite common-mode choking to keep the pattern clean and guard against any noise ingress too, especially since you may not be using isolated transformers.

A 9:1 isolated transformer seems ideal, but I don't have any of those in my junkbox so maybe a cheapie 4:1 will have to do.

VHF LOW (3:1 swr or better) 40 - 55 mhz -- 8 feet long.
6M specific - (2:1 or better) 54mhz -- 7 feet long
Airband specific - (2:1 or better) 120 mhz - 3 feet long

CONS:
Right off the bat you are looking at least -12 to -15 dBi loss. That's db over Isotropic at the lower angles. Hot scanner front ends might do ok.
Useless overhead pattern. Well, maybe not for airband.

Directionality - not much - just a big hemisphere. I guess just put it down in whatever way you find convenient.

Line Loss: at these frequencies, wide banded-ness is not the goal. Line loss can rear it's ugly head, so resonance, such as is, (lower than normal) seems important. You also have a very heavy ground-loss reactance to deal with obviously.

For me, this will be more of a science experiment, because I don't really know what to look for on lowband, and I'm pretty satisfied with my airband reception with "normal" antennas.

I'm just thinking of it as a last ditch attempt to get a less noisy environment for those who are at wit's end and are willing to go to the ends of the earth (pun intended) to monitor without bleeding ears. :)

I'll need someone to join me in the padded cell later, so if you beat me to it, let me know.
 
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#2
NoElec 9:1 balun looks ideal for low-band

Whaddya know - the little NoElec 9:1 balun is supposed to work up to 90mhz.

AND, if you scratch through the only circuit board trace on the backside of it, that makes it isolated windings.

Awesome -- low band test here I come.

I was kind of hoping to just talk about it and not actually do it - but the NoElec is already on the desk. Maybe later today!
 
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YES - it works! Stoked!

Setup:
Laid down a 7 foot long folded dipole on the ground. Used the NoElec 9:1 with isolated windings. (scratch through the underside pcb trace). A 2 foot RG-174 jumper from the transformer output was wound about 4 times through mix 43 ferrites just for good measure.

Agrees with EZnec predictions!
Although I didn't use RG-6, I have about 50 feet of lmr-400 back to the shack. At first I didn't hear any activity, so I put a RigExperts AA-54 on it. Less than 3:1 swr from about 28 to 54 mhz. This is not the right way to do it, but at least it shows what the radio sees, and more or less agrees with EZnec trends.

Antenna was carefully laid as close to the earth as possible. Despite being horizontal on the ground, electrically the polarization is vertical.

And then bam - some activity:
No surprise - a 7 foot long folded dipole on the backyard dirt will smoke your rubber duck. :)

Local CHP blasting. Local 6 meter repeater blasting. Kinda expected that. So much so that attenuation on my Whistler 1040 handheld scanner made no difference. Probably need about 100db attenuation to make a difference there.

BUT, the important point is that it sounded CLEAN. No noise from my front led porch lights.

A weak CHP signal on 39.220 from South LA was heard cleanly up in the Santa Clarita Valley, with me on the other side of a mountain. An unknown on 39.120 was also heard. Compared to a "real" antenna, no big shakes, but I was glad to know I was hearing beyond the backyard. :)

So yep - kind of a niche antenna for the noise hampered, who can't get anything up and outside. Like a condo dweller, that kind of thing. Just getting the antenna *outside* and further away from noise sources, along with being down in the dirt - despite the additonal attenuation from that - may be a s/n ratio solution that works.

It may also be a nice antenna for amateurs who want to monitor 6M band conditions - who can't put anything more on the roof, or may have severe noise issues as well. With the better front-end of a dedicated 6M receiver, using a preamp might be a possibility.

That was FUN! Nope - it won't be part of my permanent installation, but it is definitely going into the antenna notebook for me as being a viable solution under extreme circumstances.
 

Ubbe

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#4
I beleive the isolated balun could be the secret. It removes common mode interference, restores loob directionality, increase frequency bandwidth.

Have you tried hanging it vertically to get those 12-15dBs back in the antenna or are you swamped with interference?

/Ubbe
 
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Have you tried hanging it vertically to get those 12-15dBs back in the antenna or are you swamped with interference?
Not really swamped, but low band can be a tad noisy for my "normal" quarter wave up on the roof.

This was more of a prove-it-to-yourself kind of thing. For some, it might be the only answer. For sure, it is a "niche" antenna, but for those who have a niche to scratch, it could be a hobby-saver. :)
 
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#7
Here's a nice little tutorial and diagram rom Arcticpeak :

Folded Dipole

Of course his are in the air. This project is on the ground, so the math may not be exactly right for length and don't pay attention to the directional pattern pic. Ours on the ground are different. You can use twinlead if you have it, but I just wrapped single *insulated* wire back around a ground stake in the folded pattern. Spaced maybe a couple of wobbly inches or so. Didn't need to get ruler-perfect. No major spacers or anything like that. Just a simple fold with insulated wire attached to the transformer in the middle.

I used the NooElec 9:1 for isolation (scratching through the only backside copper trace). 8 feet for VHF low public safety etc, and 7 feet for amateur 6M 52mhz ops. Seems a good way to get an isolated transformer without having to guess with most of the common TV variety transformers, which may or may not be isolated. If it works for you, I'd suggest a more permanent housing, not using the slip-in type wire connection but a soldered little jumper instead etc.

For testing though, the NooElec couldn't be easier to try - took about all of 5 minutes to build, since I had a matching adapter for the SMA connector that comes with it.

The quality of the ground is going to change things, but it didn't turn out to be super critical since I'm not transmitting, and there's plenty of lossy bandwidth to go around!

BUT, this is a great point - AnalogLowband just might find this a perfect solution that isn't just a total dummy load, and can be conveniently hidden!
 
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Airband pre-test thoughts

Desense - overload even with a 3-foot long folded dipole on ground?

For some scanners and situations, yes. According to EZnec, on airband we could expect anywhere from -8 dBi at zenith, to about -20 dBi (isotropic reference) attenuation at single-digit low angles. Of course the noise floor is lower, but we've been through that. And of course the directional pattern isn't going to win any awards. But we're going to forge ahead anyway.

So if you are experiencing FM broadcast or VHF hi pager interference on your existing antenna, you can probably be sure you'll need filters for this as well.

I can't use the NooElec 9:1 transformer, since that is only spec-ed up to 90mhz, so I may have to rewind a junker for isolation - or more likely try to find some commercial TV 4:1 300/75 transformer solution that actually specifies if the windings are isolated or not. In the past, I used the higher-end Radio Shack versions which actually were isolated for other things. Or, just bury the feedline near the feedpoint in #43 mix ferrites.

When I do actually put one down, I'll let you know how it goes.
 
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