Making a dipole from two mobile antennas…

[BMDaug]

Hey everyone, I just picked up a Harris xg-100m. I’m planning to use for disaster relief interops and search and rescue.

I have two Sti-Co rfmt-nt-v/u/c triband antennas (136-174, 380-520, 762-870mHz). They are very light, ultra flexible, and are perfect for the radio. I want to set them up as a dipole that I can hang from a tree limb or whatever for use as a base station. It’s very easy to remove the antennas from their bases, ending up with a threaded hole at the base of each antenna…

Is there anything wrong with mounting the antennas on opposing sides of a slab of poly (where the bolts simply include a ring terminal on each, one positive, one negative…) to create a dipole? Any help is greatly appreciated!!!

-B

photo is a repost from another thread… wasn’t getting any responses about the antenna idea.

 

[prcguy]

You would have to make a custom dual sided NMO mount where the top side connects to coax in a conventional way and the bottom side has the center pin grounded making the other half of the dipole. I might work ok or the match could be way off as this antenna is designed to match when placed over a large piece of sheet metal.

I would simply use an NMO base adapter with ground radials cut to the recommended length for VHF hi band and be done with it?

 

[redbeard]

Dipole will be directional which may be good or bad depending. Might as well just buy a small base antenna. I mean if you build the dipole you will still need a roll of coax, and maybe a tripod and pipe to mount it to. How else would you mount the antenna and make sure it wasn't near anything conductive that would afect the SWR? If you really want something ultra portable, just build a ladder line wire j-pole.

The base adapter @prcguy mentions would still need to be mounted to something and have coax run to it. I suppose you could hang it by string from a tree.

Motorola portable suitcase repeaters and bases would come with a mag mount mobile antenna meant to stick on the removable lid of the metal suitcase. Maybe that will give you some ideas.

 

[BMDaug]

So I have one antenna already and this was initially a concern, the two sided nmo mount… but I got a nice surprise upon receiving the antenna… it unscrews from the nmo mount leaving just a threaded hole. So I can just use a bolt with a ring terminal on each bolt, one for the positive and one for the negative, through a piece of poly cut to size.

Also, the spacer mounted about an inch and a half up the antenna serves as an excellent hang point while ensuring enough of a weight offset to keep the antenna vertical. It’s all metal and the setup is very light. My plan is to toss one end of the paracord over a suitable tree branch and pull the antenna up reasonably high.

I’ve thought about stuff like the cantenna heavy duty counterpoise, which includes an nmo mount, but it doesn’t pack well. The Sti-Co deployable ground plane system looks awesome but it’s cost prohibitive and also doesn’t pack well.

I might give it a go and at worst, I’ll have two Sti-Co antennas, one for each vehicle and then I’ll build a wire J pole or try a mag mount, that’s good food for thought for sure!

Thanks everyone for the suggestions! If I do go this route, I’ll report back with a completed build and some swr measurements. At least nobody thinks it’s the worst idea!

-B

 

[BMDaug]

Ok! Got the second Sti-Co… This is what I built. I’m waiting for the feed line to come in later today so I can wire it up and test it. Both antennas screw right off and are super flexible so it packs down really small. There’s a hole in the center of the white disc so I plan to pull the shield back about an inch and a half, then put the core with some added heat shrink through the center hole to the ring terminal on the other side. A couple of crimps and some solder and it’s ready for the swr meter.

-Brian

 

[BMDaug]

So I initially thought this project was dead, but it turns out my old second hand SWR meter was failing… testing this design with my new RigExpert yields some pretty awesome results, especially considering I’m measuring indoors in a fairly small room (it’s cold and dark outside right now). The dipole is mounted to a carbon fiber mast with the negative side of the dipole about an inch from the mast (this distance will grow when I rebuild the antenna using a larger plastic disc). Check it out!

136-174
380-520
762-870
430-450 (uhf ham)
144-148 (vhf ham)

The ham bands are where I’ll do the most of my long duration transmitting while chatting just for fun. The rest of the range is commercial and public safety so those transmissions, when authorized of course, will be short and to the point.

 

[BMDaug]

Here’s the reworked antenna mounted to the pack. Both halves of the dipole are now in the same exact plane. Vhf swr is even better, but uhf and 7/800 suffered a bit as I was forced to move the two halves 3/4” further apart to bring them into alignment.

I’m working on ways to tune the dipole. The entire thing (base to element and cross beam to element) is held together with set screws, so I could disassemble it and use music wire to find the best length and then either cut the factory elements or leave the music wire and cover that in heat shrink and preserve the Sti-Co OEM elements…

-B

 

[BushDoctor]

With the coax running beside the ground element u might get coupling since the coax needs to run away from a dipole at a 90 degree angle. If u could use a pipe as the ground element or maybe even tinfoil somehow and run the coax up inside the pipe or tinfoil

 

[surfacemount]

Or maybe some ferrite. I'd like to know how this ended; I didn't have much hope but when I saw the plots... dang

 

[BMDaug]

Or maybe some ferrite. I'd like to know how this ended; I didn't have much hope but when I saw the plots... dang

I trimmed the elements, but I could really only get the thing to tune all bands the way I wanted when holding the coax away from the antenna. It would have been perfect if I was needing horizontal polarization or wasn’t worried about a compact footprint, but most systems I need to work are vertically polarized so I would be putting myself at a disadvantage. I’ve added a stand-off so I can mount it to a mast and appropriately space it from the mast while keeping the coax away from it. I’m gonna try it as a mobile base station antenna with a fiberglass paint pole as a mast. I don’t really have a need for such a setup at the moment, so it’s just waiting right now!

-B

Edit: I’ll add that the system collapses quite small, as both radiators unscrew from the core, as does the stand-off, so it should actually be really cool as a mobile base, I just needed something that was operational on the move.

 

[surfacemount]

I bet, hence my interest. Try getting some ferrite beads, and read a little on how to decouple your feedline. I am thinking it won't take a great deal; perhaps even a change of coax length or quality (quad shielded like for satellite purposes).
very interesting experiment at any rate!

 

[BMDaug]

I bet, hence my interest. Try getting some ferrite beads, and read a little on how to decouple your feedline. I am thinking it won't take a great deal; perhaps even a change of coax length or quality (quad shielded like for satellite purposes).
very interesting experiment at any rate!

Ya it’s really cool that the stico antennas unscrew from the the NMO mount and just leave a 5/16”-24 threaded hole. It makes it really easy to make them into a dipole. Also, I think the element is just music wire and could theoretically be replaced, but I think they use thread lock on the set screws, requiring heat to get them loose, which would remove the paint and heat shrink coating from the elements. If I ever needed them to be longer (as in I ruined them by trimming too much), I’d definitely attempt to remove those machined pieces before tossing them in the bin. They are really cool triband antennas.

-B

 

[surfacemount]

I have some experience with those. Next time I run across one, I'll see destructively how to remove the aerial elements for you.

 

[BMDaug]

I have some experience with those. Next time I run across one, I'll see destructively how to remove the aerial elements for you.

On the spacer, there are two painted over hex head set screws and two more behind the label on the base. I tried to remove them on one of mine but I know better than to try too hard or it’ll just round the hex head and then you’re drilling. I was gonna first try penetrating oil, and if that doesn’t work, it’ll have to be heated.

Thanks for offering to give it a shot! Technically mine are still useful as a dipole, but if that goes unused for long enough, I may try and replace the elements. Thanks again!

-B