VHF indoor "half sloper"

[nanZor]

Here's something I built (with a picture included) that is mostly of interest to indoor / attic users.

I adapted an HF quarter-wave sloper, or "half sloper" for VHF use, and instead of using a grounded tower, I faked the vertical section with chokes.

Half-slopers are vertically polarized antennas, and have some gain in the direction of the slope. Mine definitely does.

1) Attach a quarter wave (234 / freq mHz * 12 = inches) of wire to the center conductor of the transmission line. Here, I used the center conductor of an F-chassis connector to solder to, and screwed the RG-6 into the chassis connector. Nothing is connected to the shield.

This wire slopes down towards the ground at a 45-degree angle. Be sure you don't short it out to the braid when you bend it downward. If you need to, you can support the sloping wire with cord, dental floss (no kidding!) whatever to maintain at *least* a 45-degree downward angle. You can bend it downwards even further, perhaps 60 degrees downwards max before things start to fall apart, as the impedance drops when you bring the sloping wire closer to the vertical section.

(antenna height, angle of the slope, element lengths are all tunable for transmitting duties that need absolutely low swr.)

2) Hang the top of the coax feedpoint near the top of the ceiling, rafters, eaves, whatever.

3) At the quarter-wave point down from the end of the coax, (234 / f Mhz * 12) I attached about 6 snap-on RF chokes (RS #273-0105). This tunes the common-mode of the coax to emulate a quarter-wave element. Forget about any sort of "velocity factor" compensation, as VF only applies to the differential-mode, or inside part of the coax. Yes, there is a very small amount on the outside due to the jacketing, but for rx-only we're starting to split hairs.

Compared to a full-sized vhf dipole, this really saves a lot of space up in the attic, and the feedpoint being at the top helps keep the antenna further away from attic clutter.

It was resonant in the air-band when I used 22-inches for both the sloping wire and the start of where the chokes snapped-on. However, I'm using a short run of 75 ohm RG-6 and my SWR meter is designed for 50 ohms so very precise measurements are off. The scanner likes it just as much as my dipole however, although now I have some directivity and a bit of gain.

I don't know how to model a ferrite-choked transmission line used as an antenna element in EZnec, but it does seem to be holding to the textbook descriptions for HF half-slopers.

Note that I tried using a separately cut quarter wave wire attached to the shield at first, but the system was just too unstable on the swr meter, and I didn't feel like devoting my life to it. So on go the chokes instead.

Ok, it isn't a world-beater antenna, but it might be a nice alternative if you need the features.

 

[kb2vxa]

Sorry OM, what may hold true at HF goes right out the window at VHF. The term "sloper" only says the element is neither horizontal nor vertical and polarization is unimportant on HF to begin with. The waves get all twisted up once they leave the near field, ionospheric refraction causes unpredictable polarization shifts so what hits the antenna may be vertical one minute, horizontal the next or anything in between which accounts for cross polarization fading on the higher frequency bands where antenna polarization becomes a significant factor. The slight directional characteristic is caused by wave interaction with the ground at an angle, the physics of it are a bit too much to be explained here.

When you get above 30MHz polarization becomes critical, cross polarization accounts for approximately 20dB signal loss up around 100MHz. The higher you go the more it becomes a factor so it becomes an advantage, two signals in the GHz range may occupy the same frequency at the same time with no interference when polarized at right angles. That having been said you're beginning to see how your antenna is acting being neither horizontal nor vertical. Frankly you'd do better with a ground plane or discone being they're vertically polarized to match the polarization of the signals you want to receive.

 

[nanZor]

Build one and see.

You'll find just by listening that it is predominantly vertical in polarization without a huge hit in cross-polarization, has directivity and slight gain, and has a very high feedpoint which can help avoid ground clutter indoors or in the attic.

Or very simply I could just run the sloping element straight up for a more classical vertical dipole.

I am very mindful of the differences between HF and VHF+ antennas, and have built and tested many versions of "snake-oil" antennas that are nothing more than fancy random wires that use the common mode of the coax to do all the work - poorly.

This is not one of them.

And due to the space savings, this version could be just the ticket for those that want to do low-band vhf, but don't have the vertical room, nor the space for a groundplane - not to mention getting that feedpoint higher.

Indoors and the attic are always not optimal, but when in a pinch, this could be hidden behind a curtain, up in the eaves, in a glass window etc - compromised of course like any other antenna close to reflective surroundings.

It is just one of many antenna options - and I don't post anything that I build that is based on wishful thinking.

I guess the challenge is out - time to pull out EZnec and use say a 1K ohm load on that vertical section.

 

[nanZor]

BTW, my apologies to KB2VXA. I Had kind of a rough day before posting, and I know you are trying to help, which I appreciate.

I finally did get a quick rundown in EZnec, and once I get my laptop back, I'll upload what I found here.

The interesting part of the modeling where I used just two quarter-wave elements 45 degrees apart, one totally vertical and one shooting off to one side, fed at the top, shows that the pattern has some gain, but it is more BI-directional, rather than having a predominant gain towards the sloping wire - which would be the case for a grounded tower on HF. It seems to have a nice combination of usable low-level lobes as well as high-level lobes - not useful for some, but for my penchant for dx-ing high-level artcc comms, it is doing well.

So KB2VXA was right! The antenna has a slightly different pattern than one modeled on HF - probably due to the fact that we aren't compromizing with an electrically short tower and the legs are full-size at VHF.

The fact that this antenna has been up for more than 5 days at my location must mean I like it!

 

[kb2vxa]

No need to apologize, you were polite about it.

"Build one and see."
Been there, done that.

"It is just one of many antenna options - and I don't post anything that I build that is based on wishful thinking."
By your post that's pretty obvious, you had most of it right.

"I finally did get a quick rundown in EZnec, and once I get my laptop back, I'll upload what I found here."
Nah, most wouldn't understand what they're looking at being it's a rather advanced bit of software designed for engineers.

"So KB2VXA was right!"
FYI I've been around a lot longer than PCs and NEC software, you kids have it SO easy. (;->)

"The antenna has a slightly different pattern than one modeled on HF - probably due to the fact that we aren't compromising with an electrically short tower and the legs are full-size at VHF."
That's the half of it, the other half is the waves being MUCH shorter and the antenna elevated you have eliminated ground interaction. Plug that into the equation and things will come out quite differently.

"The fact that this antenna has been up for more than 5 days at my location must mean I like it!"
A bit short tempered aren't we? (;->) I'm glad you like it and in keeping with the Amateur philosophy, "damn the slide rule, what works is all that matters".

 

[kb2vxa]

deleted duplicate post

 

[nanZor]

VHF Half-Sloper EZnec models

Here's the antenna modeled with the feedpoint up at 16 feet. I chose this as an optimistic height for a single story attic location.

A major low-level elevation lobe with about 4dB gain over a dipole. To be conservative, let's say 3dB real-world.

For most, the other lobes are not very useful, however for my aircraft needs, they make a nice mix of low and high-level with a small amount of gain.

The az plot shows a small amount of bidirectional gain, again about 3db to be conservative, with about -6dB squeezed on the sides.

Do I trust this model completely? NO. The only thing it has done is let me know that it seems to correspond to what I'm observing without getting too far out of the ballpark.

The model is not an exact duplicate of what I am using with chokes. It is only two quarter-wave elements spaced and angled at 45 degrees with respect to one of them being straight up, and fed from the top of the sloped wire. When I get a chance, I might try modeling with lumped loads to simulate chokes on a longer vertical feedline.

But so far, so good! Would I put this on a tower with hard-line? Probably not, but that is not what I was looking to do with it.

 

[webley445]

Reminded me of these (when I saw 146 Mhz) though not scanner related.
As stated above, damn the slide rule....

2 METER ANTENNAS - CHEAP AND EASY!

KJ4LXW Cheap and Easy 2 Meter Antenna Project

 

[nanZor]

Ah, very inspirational!

I suppose the first thing I'd do to those V-dipoles is choke the feedline at the feedpoint, and move it further away from the tripod among other things. But it looks like a fun project. I might model those things with and without a feedline and tripod, move it up 10 feet, and see what's up.

With the half-sloper shown above in the pics, one could easily have three antennas with simple bending:

1) Run the sloping element straight up as a center-fed dipole. (even though it appears to be end-fed, it isn't)

2) Run the sloping element out at 90 degrees for an "inverted-L". This would not be useful in the majority of cases for us scannists except for one:

Low-band VHF skip, where you might encounter non-vertical signals by the time it reaches you! Having an inverted-L standing by that you can switch to when the band opens might be very useful.

3) and of course the half-sloper, where you need the additional feedpoint height and possibly other features.

I did find that while no coax braid makes a great substitute for tubing or thick wire, the braid's common-mode choke-tuning on the RG-6 is doing much better than the scrap RG-58 I had lying around. Of course hardline would be even better, but I doubt I'll be using hardline for a simple dipole or relative. Then again.....

Tuning is easy - just snip the sloping element and/or move the chokes up or down a bit. Once I get my hands on some decent 50 ohm coax, I'll try the choke routine on those. Plenty of references in the threads for that.

Now to find out how to put my tripod to use somehow so it doesn't affect the antenna, nor react to the closeness to ground on that other antenna project you found - head spinning......

 

[kb2vxa]

Dizzy, I’m so dizzy my head is spinning
Like a whirlpool it never ends
And it’s you who's makin’ it spin
You're making me DIZZY

Tommy Roe, I really wouldn't want a name that means fish eggs.

 

[nanZor]

Well, thanks again for having me look further into this KB2VXA !

I've learned how to model lumped choke impedances a bit better in EZnec, and am pretty excited about it.

During modeling, I found that if one wants to emulate a quarter wave wire using the common mode of the coax, you are going to need about 3K ohms or more of choking impedance! Anything less and you are losing both your gain and elevation angles are higher.

I doubt that the 6 RS type-43 material chokes are up to the job *completely*, although I haven't measured their individual choke impedances. They got me in the ballpark.

BUT, modeling also revealed that a more effective solution is to just use TWO choking sections - one at a quarter wave, and another one at a half-wave. The nice thing is that only about 500 ohms choking impedance at each section gives me the same result without having to snap on a million chokes to get up to 3K or so.

Soooo.. I have only 3 of the RS chokes down the line from the feedpoint at the quarter wave point. From the top of these three upper chokes, I measured down another quarter-wave and placed three more of the chokes starting there. Wala! Brought some of my weaker stations out of the noise.

In fact, I scrounged for some other type-43 chokes I had, and now have placed 4 chokes at the quarter wave and again also at the half-wave point. At this stage I'm pretty confident that the antenna is a much closer fit to the model that uses wires for both elements.

So where do we go from here?

Revisit the "RFD" HF dipole, which these are but on a VHF scale. I had poor results with the HF RFD's, but now I think I see that there was no way my little coax loop was providing enough choking impedance to do the job. Might have to sneak up a 20-meter version of this sloper in one section of the house that can accomodate it, once I measure the impedance of the HF chokes I have and see how that goes!

Always something around here.

 

[kb2vxa]

"Always something around here."
THAT sounds familiar.