Multiband Vertical Antenna Construction Questions

[rgchristy]

I'm looking to build a multiband ground plane vertical or j-pole antenna. I'm looking at a tri-bander with the center frequencies of 154, 507, and 867. I've been able to find the design specifications for the verticals, ground planes and J-pole, but I'm having trouble understanding the "trap" portions of the antennas. First, I'm not sure if I'm labeling this coil between the vertical elements correctly. I can't tell if it is a trap, loading coil, phasing coil or what. I'm trying to figure out how to design the "traps" based on my frequencies. I own and have searched through the ARRL antenna book (20th) and have been up and down the web. With the ARRL book, I'm trying to understand the design as it relates to the VHF/UHF frequencies. I can't seem to convert the trap information for multiband 40, 80 meter bands, etc. to the higher frequencies (coil diameter, number of turns, wire diameter, etc.) The reason that I'm doing this is because I am limited to an attic mounted antenna with a height of less than 6 feet. I hope that I've explained this correctly. Any help would be appreciated. Also, any alternatives would also be welcomed.
Thanks much,
Rich

 

[N_Jay]

I'm looking to build a multiband ground plane vertical or j-pole antenna. I'm looking at a tri-bander with the center frequencies of 154, 507, and 867. I've been able to find the design specifications for the verticals, ground planes and J-pole, but I'm having trouble understanding the "trap" portions of the antennas. First, I'm not sure if I'm labeling this coil between the vertical elements correctly. I can't tell if it is a trap, loading coil, phasing coil or what. I'm trying to figure out how to design the "traps" based on my frequencies. I own and have searched through the ARRL antenna book (20th) and have been up and down the web. With the ARRL book, I'm trying to understand the design as it relates to the VHF/UHF frequencies. I can't seem to convert the trap information for multiband 40, 80 meter bands, etc. to the higher frequencies (coil diameter, number of turns, wire diameter, etc.) The reason that I'm doing this is because I am limited to an attic mounted antenna with a height of less than 6 feet. I hope that I've explained this correctly. Any help would be appreciated. Also, any alternatives would also be welcomed.
Thanks much,
Rich

I would make a simple ground plane with three parallel vertical elements of the appropriate length for each band.

 

[kb5udf]

No trap needed

Jpoles for 144 mhz and up don't usually use 'traps.' Typically, in my experience, traps are devices (coils) used to electrically lengthen an antenna element. Ie, so traps are usually used on HF/Shortwave antennas. Since a 5/8 wave Jpole at 150 mhz is well under 6 feet, you should have no problems, and have no need to resort to traps, in order shorten the antenna.

I agree with Njay's solution above, no need to make this overly complicated.

 

[Al42]

The "traps" on singleband J-poles are phasing loops.

Multiband Js are possible, but 150, 500, 850 is probably not - the frequencies have to be harmonically related.

Js don't take ground planes - a J is a half-wave rod (which, being a full half wavelength long, needs no counterpoise), end-fed and the bottom section is a matching stub to match the ~5,000 ohm end impedance to the 50 ohm cable. (High E/low I gives high R.)

 

[rgchristy]

How far apart should the 3 parallel verticals be from each other and should I use 1/4 or 5/8 wave or something else? Should they be mounted on a flat aluminum plate? thanks.

I would make a simple ground plane with three parallel vertical elements of the appropriate length for each band.

 

[N_Jay]

How far apart should the 3 parallel verticals be from each other and should I use 1/4 or 5/8 wave or something else? Should they be mounted on a flat aluminum plate? thanks.

1/4 wave works fine

15 degrees separation is a good guess.

just use 3 or 4 ground radials hanging down at about 22 degrees or so.

 

[nd5y]

I think he means the separation of the parallel vertical elements like on
the old Radio Shack scanner antenna. It had 3 elements that were about 2 1/2" or 3" or so apart.

I made a 146/223/440-MHz ground plane out of wire and a chassis mount connector once.
I made the elements about 1/2 to 1" apart and the spacing didn't seem to make much difference.

The Austin Metroploitan is a 2m/220/440 antenna
http://www.austinantenna.net/multiband.htm (at the bottom of the page)
that consists of a 1/4 wave 2m whip with 1/4 wave tubing sections for 220 & 440 that are all
connected at the base. Those elements are really close together. I had one of those once and it
worked great.

 

[rgchristy]

I'm sorry, but I don't understand what 15 degrees separation means in inches, etc. The way I understand it from the ARRL book, each vertical element should have 4 ground radials each equal to the 1/4 wave of the frequency used.

1/4 wave works fine

15 degrees separation is a good guess.

just use 3 or 4 ground radials hanging down at about 22 degrees or so.

 

[Bill2k]

For the ground plane you only need 4 radials cut to 1/4 wave of the lowest frequency you're using.

 

[Al42]

The radials should be at least 1/4 wavelength on the lowest frequency used. Think of it this way - the planet is a great counterpoise (which is what the radials are). Do people put up different sized planets for different frequencies? Of course not - as long as the frequency isn't so low that the planet is too small (about 10 Hz), it's a good groundplane.

As far as the separation - none. Connect the bottoms of all the elements together - then space them so they're only touching at the bottom. 15 degrees is a lot more than you'll need. Unless the verticals are harmonically related (like 150 MHz and 450 MHz), they don't interact much. (And you don't need both 150 and 450 elements - a 1/4 wavelength element on 150 is a 3/4 wavelength element on 450. Any odd multiple of 1/4 wavelength is okay - 1, 3, 5, 7, etc.)

 

[rgchristy]

Here's the radial information I was going by in section 7-18 of the ARRL book: "From a practical standpoint, the customary number of radials is four or five. In a multiband configuration, λ/4 radials are required for each band of operation with the ground-plane antenna." If I only have to do 4, that's fine with me and less work. As far as the vertical elements are concerned, does this mean that they don't have to be parallel to each other?
thanks again. Also, by the way, I'm really enjoying this discussion as it is a real learning lesson for me.

The radials should be at least 1/4 wavelength on the lowest frequency used. Think of it this way - the planet is a great counterpoise (which is what the radials are). Do people put up different sized planets for different frequencies? Of course not - as long as the frequency isn't so low that the planet is too small (about 10 Hz), it's a good groundplane.

As far as the separation - none. Connect the bottoms of all the elements together - then space them so they're only touching at the bottom. 15 degrees is a lot more than you'll need. Unless the verticals are harmonically related (like 150 MHz and 450 MHz), they don't interact much. (And you don't need both 150 and 450 elements - a 1/4 wavelength element on 150 is a 3/4 wavelength element on 450. Any odd multiple of 1/4 wavelength is okay - 1, 3, 5, 7, etc.)

 

[rgchristy]

I think I found a antenna project that might work. Here's the link:
http://www.qsl.net/na4it/dbgp.html
I was thinking something more like the vertical section of the RS 20-032 with ground plane radials. Here's the link:
http://www.radioshack.com/product/index.jsp?productId=2102469
Here's more questions:
If I use RG6 cable (75 Ohm), how do I calculate the angle of the radials to match the cable?
Does anyone have any information or have knowledge of "trap" design and construction?
Any other ideas would be much appreciated.
Thanks again, Rich

 

[Al42]

I think I found a antenna project that might work. Here's the link:
http://www.qsl.net/na4it/dbgp.html
I was thinking something more like the vertical section of the RS 20-032 with ground plane radials. Here's the link:
http://www.radioshack.com/product/index.jsp?productId=2102469

Stick with a multi-vertical configuration. It works much better than a trap vertical, and is infinitely easier to "desigh" (all ypu need is the 1/4 wave lengths).

Here's more questions:
If I use RG6 cable (75 Ohm), how do I calculate the angle of the radials to match the cable?
Does anyone have any information or have knowledge of "trap" design and construction?

Anything from 22 degrees to 45 degrees (down from the horizontal) should work fine for receiving.

 

[rgchristy]

Well, with the help of my wife, nephew and son, we got the antenna together. The first go round using soldered connections and brass didn't work. We switched to zinc plated steel and aluminum and it worked. Everything was threaded to #6-32 using a tap and die set. The parts included:
1 - RS SO239 female socket
5 - "Sign stakes" (see picture)
1 - 1/4" fender washer
1 - 90 degree bracket
11 - #6-32 nuts
7 - red rubber caps
1 - old antenna base from an old mobile scanner antenna
Here's the pictures:
Thanks to everyone for their comments and support.
Rich

 

[rgchristy]

Here's the rest of the pictures:

 

[rgchristy]

Now for the performance results over my RS Discone (both mounted in same location in attic):
46 MHz - Don't know why, but much clearer.
154 MHz - No longer need to use ATT on Pro-2096. Sounds much clearer.
507 MHz - Weaker channels that had static before are much clearer.
867 MHz - About the same. Still 95 - 99% on control channel for 2 systems. 1 is about 10 miles away and the other is about 20.

All parts came from Home Depot except for the RS SO-239 socket, which came from RS.