Reinforcing a Ground Plane Antenna

[Phil_KD0SCJ]

I recently made a prototype of the following antenna for receiving UHF/VHF, and it is performing very well for me. However, my prototype was made from a BNC connector and used 16 gauge copper (stuff I had lying around). I have now gathered the parts for a more permanent version. The SO-239 has a larger solder cup, allowing me to use larger copper wire. So, before I run this thing up a 30 foot mast, I have a question:

The drawing is not to scale otherwise you might wonder this yourself... The 19" vertical and other vertical elements are supported only by the solder connection at the SO-239. I am in Northern Minnesota, and this antenna will see significant wind, snow and occasional ice loads. My gut tells me I need to reinforce that solder joint in some way. I am thinking about encapsulating the area in green with a block of epoxy (clear, non-conducting, of course). In order to be effective, it will have to come up past the bottom of the 450MHz element. Maybe this will impact reception on the 70cm band, but I'm ok with that because the 70cm band is pretty quiet where I am anyway. Does anyone see any other pitfalls or have other solutions for this?

Yes, I could try it and see. I fully believe in the experimental find-out for yourself philosophy. I just can't undo epoxy so if this is a dumb idea, I'd really like to find out from people with more experience. And, bringing a 30 foot mast down for repairs in the winter here is just not a fun learning experience.

Yes, I have searched the forum and the internet on this. I couldn't find anything definitive, although there is mention of using epoxy to pot RF circuits, so that's why I chose epoxy as a candidate.

I am not transmitting with this antenna, and I can tolerate a minor reduction in signal. Actually, my prototype is at 10' and this will be at 30' so the net will probably still be a gain.

What else can I tell you... RG6 for cable to a pair of RTL-SDR's. (Trunking/P25 plus UHF/VHF exploration). Here is the antenna:

You can view the entire antenna at http://debandphil.com/pictures/ham/coathanger_antenna.pdf.

I found this quite some time ago, and I wish I had noted the source. I have gone back and tried to find it again, but no luck so far.

Thanks for your comments!

~Phil

 

[mmckenna]

It should work just fine. The mechanical strength will be increased, as will protection from water ingress. The only issue you'd have would be if the epoxy had any conductive properties, but unless you were using something like J-B weld, or similar, you'd be OK. Standard 2 part epoxy should be RF transparent and non-conductive for this application.

Other options would be:
Enclose the radiating element in a length of capped PVC pipe. This would address some water intrusion, but would increase wind loading.
Use a PVC pipe cap to add mechanical strength. Use epoxy underneath it to seal everything up.

Mechanical damage is a concern, no doubt, but water ingress into the cable and connector is a concern too. Copper, solder and the connector will likely quickly corrode over time. Any water getting into your coax will cause major issues too.

Interesting looking antenna. Let us know how it works.

 

[AronDouglas]

You could always do something like this. https://sites.google.com/site/j19sac/pvcmast

This would allow you to use PVC as an offset mount from the mast. Plus you could run the coax inside the PVC until it got to the mast for longer UV protection and weather proofing your connection.

 

[Phil_KD0SCJ]

Thanks for the words on the epoxy mmckenna, and the PVC.... well, capsule, I guess... pod? It is perfect and I am going to do both. Yeah, that pod idea is brilliant, it's going to look even more funky with my little stud sticking out. This is exactly the response I was looking for... thank you both!

 

[ronhl]

Are they just using (2) 20" grd radials, or is the other 2 not shown or called out? Think if be more effective if they would use 4, the So-239 would allow for it

 

[flythunderbird]

Aron, I love that PVC mast idea! Thank you for posting it!

Phil, here's an alternative way of doing your SO-239 ground-plane ... this looks like it could take a LOT of wind. I plan to build one after I get a mast installed next year.

http://forums.radioreference.com/bu...854-1-4-wave-ground-plane-noaa-broadcast.html

 

[Phil_KD0SCJ]

Thanks for the link flythunder... food for thought!

 

[Phil_KD0SCJ]

And we shall call it Ping!

A novel method came to me while I was daydreaming at work. Here is the end result:

I'll get a better picture outside during the daytime. So, the problem was how to package this epoxy business. I was going to try and tape up a form around the SO-239 and pour it. Then, I thought, ping pong ball.

It's pretty obvious.... half a ping pong ball threaded on top. Seal the penetrations with super-glue and then invert it and fill it with epoxy,

You have to arrange the ground plane wires under the SO-239 to make it fit:

Here is the underside after filling:

By the way, you can get a 2-part epoxy syringe with a mixing tube. It's a lot smaller than the normal ones, 0.47 fluid ounces. It makes it really easy to squirt in, and one package fits perfectly. You can use the 5 minute stuff, it will not melt the ping pong ball. 12 gauge copper almost fit my SO-239 center conductor, I had to take a couple of swipes at it with a small file, then it slid in tight.

One more thing just because:

But, how does it work...

For tonight, I have it 10' up outside, 50' of RG6 to an RTL-SDR. I don't have much to compare it to, but these are some spots I checked:

110MHz - Local airport weather, pretty decent, loud and clear, not bad given this is a little low for this antenna. My NOAA station at 162 MHz literally pushes 0dB over -65dB noise floor.

400MHz - I found a bunch of new beacons I have not tracked down yet, I think they are GSM somethings. Very strong. At 468.5465 MHz there is a carrier with data on USB, getting -35dB signal on -60dB noise floor.

800MHz - At 859MHz, I am getting my local P25 control tower at -25dB over a -60dB noise floor.

I'll get it up on a 30' mast next week, and I just got a NLA4ALL in the mail today. Then, I will try some more difficult stuff. But, for the antenna itself, I'm very happy and don't plan to change a thing. I think the bottom line on this one was about $15 with all new parts.

Again, thanks for the comments!

 

[mmckenna]

A happy antenna is a good antenna.

Nice job, great idea on the ping pong ball.

So, that -technically- leaves you with another half a ping pong ball to make a second one…..

 

[Phil_KD0SCJ]

They come in six-packs... yeah I have a few extra... maybe I will make a Sputnik model.

 

[AronDouglas]

I love it, simple and easy. I'll have to see the Sputnik antenna, that sound interesting. Maybe an eggbeater antenna with 4 ground planes sticking down...and you could use it to monitor satellite traffic. I think I try that design for my satellite antenna

 

[prcguy]

Are you thinking an eggbeater for amateur satellites or UHF military?
prcguy

I love it, simple and easy. I'll have to see the Sputnik antenna, that sound interesting. Maybe an eggbeater antenna with 4 ground planes sticking down...and you could use it to monitor satellite traffic. I think I try that design for my satellite antenna

 

[AronDouglas]

Right now I just want one for ISS APRS traffic (what little there is). But who knows, I might make one for military.