Building my own dual band (at least) antenna

[treydawgmt]

Hello all,

I am looking to build an antenna. (Or two, or three down the road!)

I currently monitor mainly High VHF and UHF, with a greater focus on VHF. These are both for public safety transmissions on my Pro-163 scanner. The Pro-163 has the ability to scan 700 mHz trunked, though there are only a few people I could maybe get with a better antenna. This is not too important to me.

I would like to get a Pro-197 down the road to do 800 mHz digital scanning.

I should have my Technician license tomorrow (I really hope at least. I passed, waiting for it to show up in ULS). I'd like to begin using some 2m repeaters with this.

So this brings me to some questions about my antenna. I am probably going to build a OCFD antenna. I am looking at something either like dmg1969's http://forums.radioreference.com/bu...567-my-homemade-off-center-dipole-antenna.htm or http://mysite.verizon.net/cpthaines/id14.html by N1GY/KV5R. (So sorry for the links looking like absolute poop, I can't figure out how to code them correctly. I've read the Wiki coding rules, and doing it exactly like it says isn't working. Why don't they just use HTML??)

Anyway, here are my questions:

dmg's looks promising. However, this is going to be an outdoor antenna, on my roof (somehow, still have to get there,) in north central Illinois, meaning lightening, storms, snow, ice, possible tornadoes, the whole works. Comments on his link mention spraying with KleerCoat, using Coax-Seal, and grounding. So basically the KleerCoat makes the copper piping and PVC essentially "water proof" and the tape makes the connections virtually "water proof." If this makes sense, I can do that.

The one by N1GY/KV5R looks slightly different. I don't see a balun, It looks like they use just the coax itself, strip it back, and attach it to those screws at point C. Could I used a blaun inside the PVC at point C, screw the screws into the PVC, the blaun, and then the metal pipes? The logistics of this seem a touch difficult, but would the end result be the same and/or better? Maybe it would be better to attach the blaun to the outside of the PVC, and screw them into the metal that way. Seems much easier and I think it would work just as well. When I finish this one I still KleerCoat the metal, tape exposed ends, etc...?

Is 1/2" copper or 3/4" copper better for either design? Does it matter?

Grounding. Okay, this is where I'm really curious. My initial thoughts were run 15-20' of coax cable from the antenna to a grounding block. Have the grounding block attached to a grounding rod in the ground. Attach another cable (this one probably in the 5-10' range) into my house from the outside, where it connects to a splitter of some sort so I can connect more than one scanner. This seems to not hit the mark with this picture though. http://www.hdtvprimer.com/ANTENNAS/grounding.gif Do I need to ground the mast as well, and what is this red line? I have seen lots of comments about grounding, especially since I'd like to use this to transmit as well as receive in the not too distance future.

Thanks for any thoughts everyone give me! This is a new adventure for me, and I think I probably have just enough information to be dangerous right now!

 

[737mech]

The OCFD antenna is a fun project but only does so much for your scanner. There is a ton of discussion about it in these forums. Most people like them because they hear alot more than the stock duckie or telescope antenna that comes with the scanner. That's a good thing. The experts here usually start to discuss the tech specs and that gets very confusing. I'm not the expert just trying to keep this simple. The common thought process I took was more metal in the air = more stuff on the scanner, not so. In the end most people end up buying or building much better antennas suited for certain freqs in their areas. The best advise I can offer is try it, the hold off boom can house the balun without a problem, the thicker tube is supposed to be better for bandwidth. In city areas where there is 100 FM radio stations and rf noise some people have found out less metal in the air is better, and then start adding traps and filters etc etc etc. Good luck with it, and be sure to unplug the scanner from it before using it for transmitting.

 

[jim202]

Hello all,

Grounding. Okay, this is where I'm really curious. My initial thoughts were run 15-20' of coax cable from the antenna to a grounding block. Have the grounding block attached to a grounding rod in the ground. Attach another cable (this one probably in the 5-10' range) into my house from the outside, where it connects to a splitter of some sort so I can connect more than one scanner. This seems to not hit the mark with this picture though. http://www.hdtvprimer.com/ANTENNAS/grounding.gif Do I need to ground the mast as well, and what is this red line? I have seen lots of comments about grounding, especially since I'd like to use this to transmit as well as receive in the not too distance future.

Thanks for any thoughts everyone give me!!

Grounding is like asking your grandmother a question. The same thing goes when talking to the so called grounding experts. If you ask 3 people, your going to get at least 4 answers to the same question.

Having spent the last 18 years building cellular communications sites, I can give you all sorts of war stories. But the bottom line boils down to how much you think you want to spend on a GROUNDING SYSTEM. Yes I said system, not a ground.

To start with, I would check with the insurance company you have the house insured with. Find out what they expect you to do with any antenna system you put on the roof. It doesn't matter if it is a TV antenna, a sat dish or a ham antenna. They understand a TV antenna, so I would restrict the words you use to a TV antenna.

You will probably find a statement from the insurance company that says you will need a ground wire run from the antenna support to the ground. No sharp bends and don't run it along side any metal objects. In other words, stay away from aluminum gutters, air conditioners, metal storm windows and the like. The wire should be no smaller that a number 10 solid. Larger is better. Don't use a wire with insulation on it. The insulation will only hold any moisture that will collect and start the corrosion faster. Plus the insulation will crack and fall off over time from the sun's UV rays. I would like to see at least a number 8 or even a solid number 2 wire used for you down lead run to the earth.

On the run between the antenna support and your earth connection, you want to be careful to avoid going near any metal objects. Reason being, if you do take a strike, you don't want the high voltage jumping from the ground down lead to the metal object and causing damage. This would include items like a metal gutter, air conditioning units, metal window frames, metal doors, metal fences and the likes. Make sure the run has no sharp bends. Think of a sharp bend as anything smaller than the radius of a basket ball. Lightning doesn't like to make turns and will jump off the wire to what it thinks is a better path.

Then you need to consider just what you need for a ground rod or rods to get a low impedance ground to tie this down lead wire to. Wet clay soil is the best and dry sand is the worst. My guess is you fall someplace between the 2 types of soil. It might take connecting 2 or more rods on top of each other to get the rod to find moist soil for a low resistance ground. All connections should be by the exothermic weld process or by a heavy compression connection.

Bear in mind that you don't want any copper wire touching a galvanized surface. Over time, the copper will leach out the zinc and it will rust. The best thing to do for connecting the ground wire to the antenna support is to get a bronze grounding clamp. This allows the copper and galvanized surface to stay apart, but provide a low resistance connection.

The grounding rods or rods generally are spaced twice their length apart. This is due to the "cone of influence" around the rod. If you space the rods closer, you will impinge on the rods cone with the one next to it. This will reduce the ability to obtain the lowest resistance ability of both rods. I like to see at a minimum of at least 2 rods used on any antenna installation. Again the connection to the ground rods should be made with the exothermic weld process. If you can't do that, then a good heavy duty compression connection is next in line. The use of the common hardware store grounding clamp is the least desirable connection. It comes loose easily and corrodes the connection.

You mentioned the electrical panel ground. Normally this is not on the side of the house from where your grounding wire will come down to the ground. Plus most electrical inspectors frown on seeing any other ground attached to this. Most of the time they are not familiar with the section of the NEC (National Electrical Code) that deals with telecommunications. Your TV antenna really sort of falls under this section. But you will generally get a bunch of push back from your local wiring inspector in this area. I use to cheat in this area and would make my cellular site connection on the electrical meter ground about a foot below where the meter wire would connect to the ground wire. I knew no inspector would take a shovel and dig down to see if there was any other ground connected to that ground rod. This provided us with a common ground between the electrical service and my antenna system.

Hope this gives you some helpful hints on the issues of trying to ground any antenna on your house. You just need to do some home work to see what and who control what your going to do.

 

[nanZor]

I am probably going to build a OCFD antenna.

I'm a fan of OCFD's, but only if one understands its limitations.

I highly suggest rigging up a temporary prototype made out of nothing but wire and a 75:300 ohm balun first. 18 inches (1.5 feet) to one side of the balun , and 48 inches (4 feet) to the other. If the balun has flexible leads coming out of it, stretch the leads out away from each other and take into account that they will be part of the antenna length. Run the coax horizontally for a few feet before bringing it down horizontally.

Hang this up and test it to see if building a bigger version out of pipe will be a total waste of time, or useful.

The bandwidth is not the problem, but the *directivity* of what are essentially long wires at UHF with directivity lobes that look straight up. These upward-looking lobes are going to be a problem if your targets are down low and/or weak.

Doing it with wire temporarily will save you a lot of time and frustration.