Ben’s Blog

Back in October I mentioned that my dad had come down to help me put up a ham radio antenna. I never got around to saying any more about it. Well, now I am.

The antenna he had come to help me put up was a Diamond X50NA. It’s a 2m/70cm antenna. 2 meters is a popular VHF ham band, and the local club has a 2m repeater. The club also has a 70cm repeater less than a mile from my house. I don’t currently have a radio that supports 70cm, but when I get one, I don’t have to worry about an antenna.

The X50NA is not a small antenna. It’s over five and a half feet tall. I needed a substantial support to install it on. Also, at VHF frequencies and above, propagation is essentially line of sight, meaning that height is very important.

Luckily, when my grandmother moved a few years ago, she didn’t need all of the tower her TV antenna was on. My dad had taken the parts she didn’t need and stored them for a “rainy day.” (God bless pack rats.) So the base of my antenna support would be two sections of what appear to be Rohn 25G. On top of that we mounted a 10′ section of 2″ pipe using muffler clamps. Inside that, we placed a 10′ section of 1 1/2″ pipe restrained using a hose clamp. And on top of all that went the antenna. I estimate the base of the antenna is up about 25 to 30 feet.

I feed it with around 60 ft. of LMR-400 (PDF), a low-loss coax. Coax tends to be pretty lossy at VHF frequencies, so I wanted to go with the good stuff. At 147.33 MHz, the frequency of the local repeater, LMR-400 has a loss of 1.52dB (29%) per 100ft. The next best stuff, RG-213, has a loss of 2.50dB (44%) per 100 ft.

As you may recall, the only radio I have at the moment (that’s put together anyway) is a Yaesu VX-170, a handheld 2m transceiver. To reliably talk on the local repeater I had to walk up on the hill behind my house. When the repeater was having a bad day, as it sometimes does, I would even have to hook it up to my quarter wave ground plane antenna that I built to get into the repeater.

Now I can reliably talk on the repeater, even with my radio turned down to low power (500mW). I can also talk on the Pontotoc and New Albany repeaters, though I’m told I’m a little scratchy on the New Albany one. I had hoped to be able to hit the Batesville, Mooreville, and maybe even the Grenada repeaters. I still feel like I ought to be able to hit the Batesville repeater, I but I don’t even receive it well here for some reason. The Mooreville repeater was always a little optimistic, as Mooreville is all the way on the other side of Tupelo, but it’s on a tall tower, and I can hit it from inside my car halfway between Pontotoc and Oxford.

I have two theories as to why my antenna isn’t working as well as I expected it to:

1. My coax connectors weren’t installed correctly and I’m experiencing loss. I’m using N connectors, which are better than the standard UHF connectors, but I have no experience installing them. (Not that I have any more experience installing UHF connectors.) A couple of guys around here have equipment I could have borrowed to measure the loss through my coax, but I didn’t make those arrangements in time, and now I’d have to get up on my roof and lower the antenna. I’ll definitely do it when I move one of these days.
2. Power makes a lot more difference at VHF frequencies than I gave it credit for. At high power, my radio only puts out 5 watts. Your average mobile VHF radio can do 50W, a 10dB difference. One of these days I’ll remedy this “problem,” but its not near the top of my list.

So, did I spend entirely too much time and money to get an antenna up for 2m? Probably. I could have done it a lot cheaper and with a lot less fuss. However, this setup should be all I need for a while.

This entry was posted on Wednesday, February 11th, 2009 at 11:14 pm and is filed under Amateur Radio, Family. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.