Comments on the Multiband Antenna in the Wiki

[nanZor]

This looks like a fun project, and although I haven't built one myself, I have used stubs in my designs and some things really stand out for possible improvements. I ran it through EZnec modeling software, and came up with some altered lengths.

(My stubs tend to run in the other direction, but I'll leave that for other projects)

Scanner Antennas - The RadioReference Wiki

In the homebrew section it is a pdf link under "The Multiband Scanner Antenna"

Essentially it wants to be a 150/450/800 mhz groundplane antenna. The problem is that the stubs DO affect the element lengths when the stubs are non-resonant and require compensation to get the other bands back to where you wanted them to be. There is quite a bit of interaction here, but for the typical homebrew, it looks to be an easy way to get into the ballpark.

Suggestion:

1) With the lengths shown, the main VHF element is actually loaded by the 450/800 stubs down to 130mhz. To compensate and bring the resonant frequency up to 150, I suggest using a total length of about 16.5 inches, rather than 19 for the main vhf element.

2) The UHF stub appears to be resonant at 446mhz. You may want to cut the open end down to about 5.25 - 5.5 inches to get it up into the 450/460 band.

3) The 800mhz stub seems resonant near about 740 mhz. I'd bring the solder-connection point of this stub to just above the UHF stub. Having the stubs on opposing sides is fine. Lowering the soldering connection will raise the resonant freq and if you need to raise it some more, you can trim the open end of the stub from the original 3.5 inches to just a smidgen less height for best response.

Note that the 800mhz stub is listed as being at the "3/4" decoupling node. While you may be able to obtain a match for it, the angles that the antenna is looking at at 800mhz is like a cloverleaf. It just may be pointing right over the system you are trying to hear.

Still, this one is on my to-do list. If anyone has built one, and especially if they can compare it to one with the changes in the dimensions listed here, I'd love to hear about it!

 

[nanZor]

After putting on my glasses I noticed that the 800mhz stub has a spacing of only a half-inch. That actually makes things a lot easier. The end result is even simpler:

1) Cut main VHF element length to 16.5 inches.
2) 450 stub is 5.5 inches long with 1-inch spacing
3) 800 stub is 3.5 inches long with half-inch spacing

You can attach the 800 stub exactly like the article. No need to move it down.

EZnec plots to follow!

 

[nanZor]

SWR Plot at 75 ohms

Here's the plot of the swr at the various frequencies. Don't get too excited over the mil-air peak as the directionality is facing downwards. The resolution of the screen grab looks like a peak at 120 mhz, but it is really at 150 mhz...

 

[nanZor]

Elevation plots

Heres the elevation plot for 150 mhz. pretty standard.

 

[nanZor]

Mil-air - don't get excited

Here is the plot for Mil-Air even though the antenna wasn't designed for it. Not very inspiring.

 

[nanZor]

450 UHF plot

And now the elevation plot for 450 mhz. Getting a little squirly.

 

[nanZor]

Finally, the 800 mhz elevation plot. Like most antennas that are a bit too long, the pattern starts to shoot upwards...

 

[smason]

Interesting results, thanks for sharing!

 

[jambo]

So has anyone here made one of these, and how well does it work? I'm in the process of getting all the goodies to make one myself. Just curious if it's just a plan, or someone really made one a tested it. I've been looking for a long time for something inexpensive, but effective, to put with my PRO-197 without having to install some huge tower or mounting anything to the new home.

PS: If someone has one of these multi-band antennas made and in use, can you post some pics please?

 

[nanZor]

Note that the pdf shows 20-inch radials only - unless my eyesight is going again.

As built, that really lands on or very near 800 mhz! This thing really benefits from adding 3.5 inch radials which help raise the resonant point to about 820 mhz. You still have a cloverleaf elevation pattern though. For a quickie, you could just run with two 20-inch radials, and two 3.5 inch radials - equal lengths opposing of course.

If you want to take it further, cut radials specific for each band.

 

[FoeHammer]

I realize this is an old thread but ,..I was to build this tribander ,...for say 142mhz , 300mhz & 800mhz What would my element & radial lengths be ? & where would the stubs be placed? I would assume it wouldn't be far off from original plans shown SCANNER ANTENNA

 

[LtDoc]

FWIW
Just a warning.
EzNec is not 'perfect' by any means. The author will tell you that himself. It can give you a general idea of what you -might- expect, but that's about it.
Do I use it? I certainly have, and it's certainly close. Would I stake the 'farm' on it? No, and neither would the author (ask him!). There are other more elaborate (and expensive) programs that will yield a more 'real life' result. Wish I could afford it/one!
- Paul

 

[nanZor]

I heartily agree with LtDoc about real-world differences.

However, you can see the obvious trend here in that this antenna makes a poor milair antenna due to directionality, and also being somewhat directional at 800mhz with the majority of the lobe being on the side of the stub itself. So while impedance matching may be ok, on milair it would explain poor results.

One real-world modeling issue is that many fail to include one important additional wire. Let's say you are mounting your antenna up at 30 feet. You should add a 30 foot long "radial" that runs from the antenna feedpoint to ground. Run your plot again. Also look at the current distribution. This 30 foot long radial is due to the outer skin-depth of the coax braid which is part of the antenna, and also possibly conductive mast coupling.

Depending on a number of factors, this may totally upset your directional pattern, or improve it. Either way, it is one reason I like to choke the feedpoint when I can.

But, back to the real-world - this antenna may be all you need, even with it's poor downward directionality if your milair signals are nearby. You may be able to get a slight improvement by incorporating 9 inch radials along with the others. Just know that multibanders are sometimes a compromise in either impedance matching, or directional pattern.

 

[FoeHammer]

The reason I wanted to include a milair tuned stub was because I have tried a few different antennas & my old ( And now severely degrading & falling apart) radio shack sputnik type antenna has outperformed them all even on milair ,Im not sure what factors make it that way , surrounding objects , mounting location or whatever , but basically I figure if it works , then why not build a similar type design actually tuned for what I monitor ,..Milair , 800Mhz & Fleetnet 142.0Mhz , however at those frequencies Im not sure I understand the formula for spacing location of the stubs & what factors affect that

 

[FoeHammer]

ok I have just made one of these per these instructions SCANNER ANTENNA I cut the elements for 142.0mhz 300mhz, & 866 Mhz thats the only change I made ,...
I have only tested it very briefly , on my living room floor ground level it pulled on the weatherradio station from ohio very clear , Im in windsor ontario , also the detroit 800mhz system seemed to have an improvement , I still need to weatherproof it , but it looks promising that out in the open on a mast this thing should do pretty good ,..my old rs sputnik has got to be over 20 years old now & through the years I have had other antennas including the popular stainless discone they either fell apart ,got storm damage , or simply didnt perform , the sputnik has been the best performing antenna I have used , but with its age I can see the plastic parts are now cracking up & falling apart so this is going to replace it. Now that I have one built , I have 3 chassis connectors left to make 3 more variations ,..any suggestions for this design to be better at those frequencies?

 

[mrkultra]

I've made a couple of these

I made one of these multi-bands with Hertzian's recommend corrections. 16.5 inch main element, 5.5 inch 450 stub, and 3.5 inch 800 stub. I also employed the two 20 inch radials, and two 3.5 inch radials. I made the whole thing from copper coated steel welding rod, 3/16 inch. It seemed to work very well. However, now that I have a whole bunch of actual brass/copper brazing rod, I want to make more. I would really like to refine it to 150 mhz, 460 mhz, and 860 mhz. I have the construction method down pat, all I really need is the measurements. Hertzian, you mentioned to make radials for each band, which I want to do, but how long for each band. I plan to have 4 radials for each band, for a total of 12. Believe it or not, I found the easiest way to connect the radials is to solder them to automotive wiring eyelet connectors, the ones with the blue insulators. I take off the plastic insulators, solder them, then use a small piece of heat shrink tubing. Works perfect. I hope you can see what I mean in the pics. They are pics of a single band version I made for 860 mhz, so I can pick MA State Police. I live in Westfield, MA and pick up the Mt. Tom MSP repeater crystal clear from this little unit I have mounted in the rafters in my garage. Any help you guys could give me will be greatly appreciated. Thanks.

 

[FoeHammer]

well for cutting for certain frequencies I think you can use the standard formula or this calculator
Amateur Quarter Wave Ground Plane Antenna Calculator , What I am not sure on, is once you start deviating from the original plan ,...is the spacing & placement of the stubs , Im not sure if it changes at different frequencies ,..also from what I have read ,& I could be totally wrong ,..but it seems the more radials the better & it also seems that it would be good to have some radials cut to match each of the elements or stubs you are using , we are building these as receive only so I dont know how critical some small devations may be ... Also for weatherproofing ,I think im gonna try some paint ,lots of silicone & maybe cut the bottom out of a ping pong ball & see if he main connector will fit in there,...