Antenna in a Tree

[DylanMadigan]

I have a 4ft long GMRS antenna, it is within a pvc pipe. Right now I have both the tx and rx antennas on the barn, but i cant get them too high (theres no good place for guy wires, and i dont have much money or need a large coverage area). The issue is that while the tx antenna gives pretty good range, would like to increase the receive range for handheld radios.

So I am fairly new to all of this, and I've seen it done, but I've never done this myself. I think i can mount the mast to the tree using a few stainless steel bands (stainless mostly so i can adjust them over time so i don't just choke the tree), but this would make the antenna the highest point on the property by about 10ft, so now my concern becomes lightning. I'm thinking its a good idea to ground the steel mast (with an appropriate 8ft grounding pole in the ground), but is there something more i can do for the antenna it's self that wont hinder the signal more then having it 15ft lower? Its encased with PVC with a grounded steel mast under it, but im not sure that PVC will hold back the EMP given off by a bolt of lightning hitting it.

The two biggest reasons for the move to a tree is to increase the height, and get it further away from the tx antenna (this has caused some issues with our handhelds transmitting to it from town).

 

[mmckenna]

PVC pipe isn't going to stop the energy created by either a direct or a nearby strike. Remember that lightning travels thousands of feet through the air. A 1/4 inch of PVC isn't going to even slow it down.
As for induced energy, the PVC is transparent to a pulse like that, so again, not going to help.

You need to have a proper ground system. That may consist of one or more ground rods. The number needed depends on how well the ground in your area dissipates energy. Wet soil may only need one, dryer or rocky soil may require a network of interconnected ground rods.

You do need something like a PolyPhaser module in all your coax feedlines. This will help protect your equipment.

Adding 10 feet will help coverage a bit, but what really matters is how much coax cable you'll use to get the 10 additional feet. Since coax cable has loss, and that loss increases the longer the cable is, you could be negating your gains.

Wouldn't hurt to try it out as a temporary set up on a nice clear day. See if it's worth the trouble.

 

[DylanMadigan]

I think (and hope) 1 8ft pipe should be good, its basically really damp sand after like 4 or 5 ft (2 miles from the south shore of Long Island, NY).

I'm assuming I could get this and it would work? Just stick it on the back of the radio - PolyPhaser 50 MHz to 700 MHz Lightning Protector, IS-50UX-C1-ME | ProtectionGroup.com - Polyphaser

I have a 100ft LMR400 cable. I do have a bit of excess cable (i'm not to great with cables, so i just ordered premade), and there's enough extra to move the antenna to this location. I'm more worried about what resistance the lightning protection could cause, the research ive done shows that it will cause some loss, but im not sure what that loss is compared to the gain from the height.

 

[gewecke]

Dylan, I'm going to suggest 2 or 3, 10ft steel ground rods spaced evenly in Solid earth around that tree with 10awg. Steel ground cable, Because your using a tree for support! Plus the Polyphaser, because lightning LOVES tall trees! I'm sure you'll hear other suggestions as well, All based on your safety. It only takes lightning Once! 73, n9zas

 

[jim202]

Dylan, I'm going to suggest 2 or 3, 10ft steel ground rods spaced evenly in Solid earth around that tree with 10awg. Steel ground cable, Because your using a tree for support! Plus the Polyphaser, because lightning LOVES tall trees! I'm sure you'll hear other suggestions as well, All based on your safety. It only takes lightning Once! 73, n9zas

Let me point out that the Motorola grounding specs requires the use of a minimum of a #2 AWG sized wire. If you go look at any of the cellular tower grounding, you would also see this size wire being used.

The use of a #10 wire is like a fuse for lightning grounding protection. It's not what I would suggest or have seen used in the some 45 plus years of working at tower sites. It might be what some insurance companies tell you to use, but you might want to take a peak at the National Electrical Code (NEC) grounding and see what is said there.

Another point to take a look at is in sandy soil, the use of just a 10 foot ground rod is probably not going to provide a low resistance ground. Your more likely to need to stack 2 or 3 rods on top of each other vertically to be able to obtain a ground resistance of 5 Ohms or less. The rods should be exothermically welded together at the joints to insure long term low resistance.

 

[gewecke]

Yes, 2 awg I suppose if you have access to it and if you're a purist, but I suggested what I did for two reasons. Ease of install for the average hobbyist, and availability of materials. My local ACE hardware doesn't even carry #2 Ground wire. I suppose they could get it, but Ive use #10 for years with good results. 73, n9zas

 

[mmckenna]

I think (and hope) 1 8ft pipe should be good, its basically really damp sand after like 4 or 5 ft (2 miles from the south shore of Long Island, NY).

Not pipe, Ground Rod. Use the right materials if you want proper results. If you are concerned about lightning strikes, then you need to do this correctly.

And I agree, a single 8 foot rod isn't going to be enough. I think if you did some research, you'd find that sandy soil isn't that good a ground.

I'm assuming I could get this and it would work? Just stick it on the back of the radio - PolyPhaser 50 MHz to 700 MHz Lightning Protector, IS-50UX-C1-ME | ProtectionGroup.com - Polyphaser

That will work. Ideally, for UHF, you need to move away from UHF/SO239 series connectors and start using N connectors. They are better suited for higher frequencies.

I have a 100ft LMR400 cable. I do have a bit of excess cable (i'm not to great with cables, so i just ordered premade), and there's enough extra to move the antenna to this location. I'm more worried about what resistance the lightning protection could cause, the research ive done shows that it will cause some loss, but im not sure what that loss is compared to the gain from the height.

So, I'm going to point out something here, don't take offense to it:
1. You are willing to use excess cable and ignore the feed line losses.
2. You are concerned about the 0.1dB that the polyphaser will introduce.
That doesn't add up in my book.

Cable loss is important, and you want to use just what you need. I understand not being comfortable terminating cable. It takes practice, skill, and the right tools. I'd recommend figuring out what you need and ordering a proper cable. If you are running a repeater and are concerned about coverage, then you need to look at using something like 1/2 inch heliax or better. That'll probably make a bigger difference that raising the antenna a few feet.
Antenna height is important, but a 15 foot hight difference isn't going to make that much an impact on your coverage.

What I think you are actually experiencing is receiver desense.
Since you said you are using a TX antenna and an RX antenna, I suspect the two antennas are too close to each other. The RF energy from the TX antenna is getting into the RX side of the repeater and desensitizing it. This alone will impact system coverage greatly.
There are two ways around this:
1. You need a lot more separation between the TX and RX antennas. It works better if it's vertical separation. Horizontal separation will work, but you'll need a lot more cable.
or
2. You need to get a duplexer tuned for your system and that will combine the TX and RX side of the repeater into one antenna.

Option 2 is a much better way to do this, but it'll cost more. In return, you put one antenna up as high as you can get it, fed with proper cable, not LMR-400, use Heliax. Ground it all per National Electric Code at minimum.
If the duplexer is tuned properly, you should see a pretty good bump in coverage.

Using two antennas and chasing around the issue is going to end up costing you more in the long run.