Simple J-Pole attic BEAM

[hertzian]

Here's a fun project I'm playing with and thought I'd share.

I recently picked up a "roll up" twinlead J-pole half-wave vertical antenna that is just hung from the rafters and fed at the bottom end. Works fine, and I started to make my own for the aircraft band. But this isn't a j-pole article.

I got to thinking, why not use the j-pole as the driven-element of a simple 2-element vertical beam, and just hang another half-wave of wire down from the rafters a quarter-wave away as a reflector and see what happens?

I still have the reflector wire hanging from a tie-wrap at the top, and a small roll of electrical tape as a weight to keep it straight. This reflector is exactly the same length as the half-wave part of the j-pole (ie, the length above the notch)

Sure enough, now I've got about 10dbd or so front-to-back, and just a barely noticeable forward gain of about 3dbd. Or at least enough to pull a weak station just out of the noise in the favored direction.

Spacing wasn't critical, nor was the reflector element length for this quick setup, although I did my best to cut it (468 / f Mhz). The SWR analyzer did show that the resonant frequency dropped about 2 percent - no big deal for my rx-only setup.

To be totally honest, this quickie setup came from a book I just added to the library, although they didn't mention using a j-pole as the driven element:

Basic Antennas
"Understanding Practical Antennas and Design"
Author: Joel R. Hallas, W1ZR
ARRL publishers - 2008

Yep - I should probably optimize it for spacing, element length, tuning, etc, and that is much easier when the elements are just hanging down from the rafters. Not to mention cheap! Just a half-wave of wire hanging a quarter-wave away from the j-pole itself. Makes it real eazy to play with spacing.

I'm moving it around and playing with it. Real-world up in the attic means that I have a squirrly pattern, but hey, if it works well enough, I think I'll keep it.

Obviously I could build or buy a more purpose-built beam antenna, but I like to make incremental improvements to my setup - as much as I hate crawling through the attic space.

There are a lot of beam threads here for further enlightenment - but for now, my hanging roll-up twinlead j-pole and simple reflector wire are putting a smile on my face - even though a 2-element beam isn't exactly a powerhouse antenna.

[ampulman]

Any chance of posting a picture of your setup?

I have an EDACS system in the next town, which drops out as soon as you cross the border to my town. Sounds like it might be an easy fix.

Amp

[hertzian]

Sure - here is a shot before taking it to the cramped attic.

On the left is the twinlead j-pole as the driven element feeding a scanner below (not in frame). On the right, is the just slightly longer reflector element, about one quarter-wave away.

You'll notice my patented temporary electrical tape weighting system to keep the reflector straight while testing. Ugly, but it works nicely up in the attic where nobody will see it. Sure is easy to work on.

[ampulman]

Thanks. Since the system I mentioned is in the 850 mhz range, I'm going to play around with a 'hard' version.

Amp

[N_Jay]

Originally Posted by ampulman

Any chance of posting a picture of your setup?

I have an EDACS system in the next town, which drops out as soon as you cross the border to my town. Sounds like it might be an easy fix.

Amp

I would doubt 3 db is going to get you much.

[hertzian]

800 mhz will be a little tougher to build correctly, and yes, 3db isn't much but it is enough to bring a noisy system just out of the noise where copy is tolerable.

Most importantly, the 2-element beam turns the vhf j-pole into a unidirectional antenna real fast, with a very useful front-back ratio which helps with my simulcast situation. In that case, I actually point the antenna away from the weaker simulcast transmitter causing me some grief. The main lobe isn't pointed at the transmitter I want, so most of the benefit is coming from the rearward attenuation.

Note that my design criteria was not to affect the driven element too much as I still desire to use the j-pole in the field occasionally as an omni. Therefore, I chose to stick to the wider spacing of .25 wavelength calcuated from free space, where the reflector element length is nearly the same as the driven - although in this case, due to velocity factor differences between twinlead (0.83 in my case) and straight coated wire (0.95 estimated), the reflector is very near the usual calculation with VF taken into account (468/f mhz * 0.95), the reflector wire is definitely physically longer than the radiating element on the j-pole.

Had this been a typical center-fed wire dipole as the driven element, and if using the same wire for the reflector, the reflector would have been physically the same half-wave length, or perhaps just a smidgen longer. Move the reflector inwards (< 0.25 wavelength) and interactions start to become more critical.

On VHF at .25 spacing, it only affects the resonant frequency by dropping it about 2mhz or so. At UHF or 800 mhz, this would be significant enough to require careful pruning of both the director, reflector, and spacing. I suppose I could just fold back about an inch or so of the top of the twinlead for VHF to raise the resonant frequency again, but for rx-only purposes it still works ok.

So for an 800mhz situation, I suppose that one would expect to be cutting BOTH elements once the driven and reflector are in place. OR, what I would do if I wanted an 860 mhz 2-element beam would be to cut a dipole for 880 mhz first. Then calculate a half-wave reflector for 860 mhz, and place it a quarter-wave away.

Of course EZnec would make this all much easier.

Overall though, what I was searching for was an easy way to walk up to a omnidirectional VHF j-pole in the field, and be able to easily turn it into a temporary unidirectional antenna with some spare wire and a pair of dikes. And a ruler.

[hertzian]

For 800 mhz and nearly the same result, 3db forward, 10 db or more f/b, it might be easier just to make a plane-reflector, or corner reflector. I haven't done it so maybe a new thread is in order.

At 800 mhz, a plane reflector could be easily found - cookie sheet, foil on cardboard, etc. An entire cookie sheet would be considered and infinite plane I suppose!

Hmmm... so for 860 mhz usage, I'd probably cut a dipole for 880 mhz, then move the cookie sheet close to the dipole at about .25 wave or so away. Next up would be the corner-reflector etc. Sounds like fun!

[N_Jay]

Make a 120 to 180 degree curved reflector.

[hertzian]

Update --

I should have checked this beam out with EZnec modeling software first.

Using a simple wire for the reflector is out. I got it to work with the normal pattern but only on the design frequency. I didn't realize just how quickly the pattern would change to a distorted mess when moving less than 5 mhz up or down with a slim wire for a reflector.

Solution is to use tubing for the reflector, but I lack tubing so I used an alternative that I can also use in the attic - REAL metallic duct-tape. I have 2-inch wide metallic tape, and dividing that by 4 serves as a simulated half-inch tube. Hung that from the rafters as a reflector instead. I'm still going to have to listen carefully as I have no idea what the velocity factor of metallized duct-tape is.

Still it seems to have the 3db forward gain and something near 10db f/b on the design freq. it will take me awhile to see if this is playing out right.

[ampulman]

Interesting information, guys. I've got lots of time (retired) to play around with various ideas. Thanks.

Amp

[N_Jay]

How are you measuring you F/B gain?

[john_in_lc]

Just wondering if someone can tell me if a scanner is affective in an attic when the roof is metal? My attic is very high in the middle as all four sides of the roof slope up to a peak. I can actually get the antenna higher in the attic then outside but wonder about the efficiency???

[john_in_lc]

Doh! I meant is an antenna more affective in attic......

[hertzian]

Originally Posted by N_Jay

How are you measuring you F/B gain?

Not very scientifically!

I have a handful of 3-db inline attenuators supposedly good up to L-band.

I'll line up on a signal, (or move the element to the other side) and compare it to 3 of the 3-db attenuators (as close as I can get) and listen to the amount of quieting. I have no s-meters on the receiver I'm using, and the other ones I have use unknown deflection charecteristics.

However, the last time I checked those attenuators on a calibrated s-meter, where each s-unit corresponded to a 6-db difference, the 3-db pad did reduce the level by half an s-unit.

Not precise to be sure, but it is enough to tell if it is somewhat in the ballpark in comparison to quoted claims of the 2 element beam.

For very small gain, such as 3-db, I normally take a signal to the noise floor the best I can with inline attenuators and built-in scanner attenuation, and see if i can detect a minimal improvement in s/n. Here I have to assume that a barely noticeable difference is about 3db.

I sure wish I had the equipment to do it right.

Or I could just hang a quarter-wave tube or flat strip a quarter-wave away and see if it makes a difference. It does! However, I'll try to assure that it is doing reflector duty by making it just a *tad* longer.

Thing is, the pattern is for a narrow band of freqs. Outside of this, and you can have the pattern do funny things like reverse itself!

So while this worked for me, in practice it turns out to be a bit too narrow-banded for my tastes.