Seeking Advice on my Antenna Setup for far, and Expanding it

[arudlang]

Hello all!
Last fall I put up a used UHF/VHF ARX-270 that I got for free (used, broken). I repaired it as best I could and stuck it on a 20' piece of fence top post on my porch, attached with a decent foot mount on the porch roof and secured about 6-7' up on the peak of my house's roof. It's a little noodly in big winds but it stayed up there all winter.

Today we had our first little thunderstorm, didn't really worry about lightning over the winter here in Minnesota but I thought maybe some folks could weigh in on my efforts to lessen the risk and damage of a potential lightning strike re: having this antenna attached to my favorite wooden building here...

Partway down the mast I have a plastic box with a typical lightning arrestor. It is attached as best I could to a 4 gauge pure-copper solid ground wire I wrapped around the mast and then looped down and over to my service ground where the electric enters the house next to the porch. Not relying on the service ground, I did drive down my own 8' ground rod right next to the service ground and bonded my 4ga wire to both.

Other info, the coax (after the lightning arrestor in the box) goes into the upstairs window. I am putting together a large plastic cabinet inside the house to hold my gear (radios, duplexer, power supply, computer+SDRs, etc)

I have big ideas of DIY repeater stuff with the antenna. First up I plan to experiment with a DIY low power GMRS repeater. I have a lot of family within 6 miles and want to see what kind of family comms are possible. I also have a mini-PC and a couple of SDRs to play with, maybe implement some monitoring for local repeater activity. Also thinking of doing a part-time parrot repeater to aid in range tests. Won't go into deep details about that now, suffice to say I want to try all sorts of things with the ARX-270.

At the same time, there is one local ham repeater that is pretty active and I try to stay active on it myself. I have a "spare" radio with crossband repeat functionality and I've found it very useful to have that running, as I can take any cheap HT from my collection and wander around my property and even across town (if I'm using the ARX-270) and easily access this repeater which is ~30 miles away and not usually reachable from the HT on the ground.

The other day I was thinking "what if I put another VHF/UHF antenna somewhere on my property and just run the crossband repeater off of it while doing whatever other experiment with the ARX-270?

The thing is if I get a successful GMRS repeater going on the ARX-270 it might be used a fair amount as I have a lot of local interested family. I'm trying to gauge how I could potentially have both the GMRS repeater going and the crossband repeater to the local club tower without them causing each other too much interference.

Once up on my roof it doesn't take much to hit that club repeater. 5 watts is plenty. I wonder if I had a basic antenna, even an HT antenna, down towards the peak of my house on that same mast would the NULL of the two antennas and the 12(ish) feet of separation be enough to avoid them overloading each other if both were active? The GMRS repeater would have 10(ish) watts to the ARX-270 but with the high gain hopefully it throws so much of it to the horizon hardly anything would hit another VHF/UHF antenna below... Meanwhile a crappy HT dual band antenna below running 5 watts VHF/UHF crossbanding hopefully wouldn't dump an excessively harmful amount of RF into the ARX-270. Thoughts or opinions on that? I'm pretty sure it comes down to "I just gotta try it and see."

I'd kinda like to keep everything on that mast, as I already have the provisions for lightning mitigation and a short run to a nice equipment cabinet inside the window, with a big power supply to run everything I might want to run. I could consider putting the crossband repeater somewhere else nearby but I only have a half an acre to work with, nowhere on the property is very far from the ARX-270. I assume putting something on my garage roof 25 feet away would put them more at odds with each other than trying to stack them into each other's null zones(?)

But anyways, that's the latest around here. Sorry to cram two things into one thread. Still just looking for any glaring flaws in the lightning mitigation efforts, and then any thoughts about stacking two dual band antennas with active(ish) low power repeaters running on them like that.

Thanks in-advance for the thoughts and advice!

 

[mmckenna]

A couple of things I'd suggest:

1. From the looks of your drawing, the 4 gauge ground comes down on the opposite side of that pushout from the utility ground. I'd do one of the following, 1. run the ground lead down the side closest to the utility ground to shorten the distance and give the straightest path possible. If you cannot do that, put a ground rod where the ground down lead first comes down on the back side of the pushout, and then keep it bonded to the utility ground (required by NEC). You want to give any unwanted energy the straightest path possible to ground. Routing it around the building like that isn't doing you any favors if you get a direct or even nearby strike.
But don't trust me as some random dude on the internet. There's a lot of "opinions", and ideally you should consult an expert (not an electrician, but someone who actually does this stuff for a living), or at least take a look at the R56 Motorola document that covers proper grounding.

2. Unwrap the ground wire from around the mast. You want a straight path down to ground. Wrapping it around the mast is not giving it a straight path.

3. I'd strongly recommend using a basse antenna that is actually tuned for GMRS, and not a ham radio 70cm antenna. You'll get much better performance from a properly tuned antenna. 70cm band is far enough away from GMRS that a higher gain antenna like yours may cause high SWR.

You could mount a GMRS antenna below your ham antenna, but give it as much vertical separation as possible to increase the isolation.
You'll also need to stand the antenna off the mast about 2 feet or more to keep the mast from detuning the antenna and making it directional.

 

[arudlang]

Thanks mmckenna!
Re #1: I will add another ground rod on the opposite side there, good suggestion.

Re: #2: The thought process behind wrapping the ground wire was increasing contact between the copper and the galvanized steel, as I was unsure how to get the best transfer of energy from the steel mast to the copper. I could make it straight and just use several clamps if it really makes a difference.

Re #3: My self-repaired ARX-270 tunes up great on GMRS, better than 70cm actually, almost a flat 1.0 SWR. Maybe a result of me having to replace parts of the radiating element when I repaired it, I had to guess at lengths. I might well have mess up the gain or pattern in some way but it at least does not present a high SWR to the transmitter. Still has low SWR on 2m and 70cm too luckily. My data comes from my nanoVNA and my Surecom SW-102S meter.

The GMRS repeater project might or might not turn into a longstanding thing, it depends on how well it works. Being the highest frequency vs family members with low power HTs I want to use my highest and best antenna for it, as experiments so far show marginal coverage at the edge of where my parents and my brother live. Dropping to a lesser antenna lower on the mast will likely mean project failure.

Meanwhile, I need almost nothing to reliably reach the VHF ham repeater from the roof. That's why I'd like to add a small antenna lower on the mast just for the crossband repeat to that active club repeater. I'm just unsure what will happen running both at the same time (bad desense?)

In a perfect world the GMRS repeater is on all the time on top of the mast (ARX-270), and the crossband repeat radio (doing both VHF and UHF) is also running all the time on a minimal antenna lower on the mast, and I can set my cheap HT to dual watch and keep an ear on both family chatter and the local ham chatter while wandering around my property and even perhaps at family member's houses within a mile or so.

All that with ideally both antennas on the same mast and having short coax runs in through the upstairs window to my centralized equipment cabinet so I can run everything off the same power supply and backup power.
If the crossband radio is going to brutally desense the GMRS repeater and vise-versa during normal usage though, I would want to change it up. Other variations I've thought of is doing a VHF/UHF diplexer to split the output of the crossband repeat radio to two antennas, with one being a VHF yagi aimed at the ham tower and one going to a UHF antenna closer to ground level to get more separation from the crossband UHF and the GMRS stuff, maybe put an attenuator on it (but then I lose the neighborhood range I enjoy now). There is already going to be 15+MHz of space between the GMRS repeater activity and the UHF-side of my crossband ham activity, it's just the proximity of the antennas that worries me on things like the desense/overload of one into the other. Again, I feel like I just need to try it and find out, but trying not to outright waste effort either if failure is certain.

Here is an updated graphic to try to better illustrate the idea/goal:

 

[mmckenna]

With corrosion, the copper and galvanized steel are not going to get along very well, and may cause some RF noise. I'd stretch it out straight and keep it as short as possible. If you want to connect them at the base, use a proper clamp designed for the job.

 

[prcguy]

You can make an arm to hold one or several antennas out from the mast using Speed Rail or Nu Rail fittings and some tubing. For typical 1 1/4” OD mast use the 1” pipe size fittings and they come in different sizes and seem to be based on steel water pipe sizes.

Here is an 8ft arm I made using all aluminum components to hold a UHF and 800 antenna either side of a mast. That puts each antenna 4ft out from the mast which is fine for 800 and ok at UHF but further would be better. I would suggest using a 10ft steel fence top rail which is 1 1/4” OD and works great with the 1” fittings and places both antennas 5ft out from the mast which is about the limit for this hardware. You can also get 90deg elbows for the ends instead of using a T like I did.

If you want to go real crazy make two of these and stack them but 90deg apart so you can mount four antennas around the mast.

 

[arudlang]

Sounds good I will straighten out, unwind, and shorten the copper ground wire, and put a few clamps on it.
Those are some pretty nice standoffs @ prcguy, probably overkill for me. I think it would actually be OK if the lower antenna was somewhat directional, if I aim it, as I am almost always west of my property about a mile away when I am generally using the crossband option to access the ham repeater. The explanation there is that my Grandmother is a mile west of me, I visit her every Sunday and sometimes I am still there when the local Sunday night net is happening. I can't reliably make the club repeater from her house with an HT, but I can easily get back to my crossband unit and have successfully checked into the net many times that way now (using the ARX-270 at home, GMRS repeater is not done/operation yet). I am hoping that if I do another antenna lower on the mast for this crossband purpose that I will still be able to reach it from Grandma's on Sundays or any other time I am visiting her.

One of the things I am trying to learn here is what positioning gives the two antennas the most possible isolation from each other? Having seen images of the typical "donut" pattern of many vertical antennas, I notice the "null" in the center and it makes me assume if I (nearly) stack them with some vertical spacing, they will have better isolation than if, say, one antenna was on one end of the roof and the other placed on the opposite side about 26 feet away horizontally. They would still have about 10-12 feet of vertical separation too but it feels like they would be pretty much blasting each other, no? Plus there are the other conveniences of short easy cable runs and grounding and such if they are stacked... but if horizontal separation helped more I could put one of them on my garage about 30ish feet away.

 

[prcguy]

You will get a lot more isolation from vertical spacing vs horizontal. 10ft of vertical might be as good or better than 50ft horizontal.

 

[JustinWHT]

With corrosion, the copper and galvanized steel are not going to get along very well, and may cause some RF noise.

Years ago I came across a Ring 25 galvanized tower and #4 gauge copper wire attached under a bolt. The electrolyte corrosion was created miscellaneous metallic junction intermodulation. I wire brushed the tower leg and isolated the copper wire with stainless steel washers and looped the wire under the washers such that it wouldn't touch the galvanized bolt.

Then there was a guy that was going to paint his bare copper ground wires with zinc rich paint. Bad idea.

On a side note I'll start a new thread on

cold galvanizing paint​

 

[JustinWHT]

I could consider a tripod bracing using EMT. Two legs to opposite sides of eaves and third on roof line. You can use some imagination and repurpos chain link fence hardware attached ten feet up the mast (not half way of total length as its resonate in the wind).

On side note... Home improvement stores sell thin wall top rail, visit a commercial chain link fence company for a heavier gauge.