Batboard

1st Thanks to Slim Bob for the help on previous question. I still have a couple questions concerning antennas if anyone would again care to share their expertise. The 1st being what would the danger be to the wearer of a backpack mounted 45 watt radio with antenna right by their head? I need to protect the guys but don't know how dangerous or what danger. Next is we have 2 antennas on a 50' pole both are 151 - 153 MHz and 1 is upsidedown (pointed to the ground) to prevent interference. Won't this harm the radio by sending the power back into the radio? Is that a legitamite application?

When you say pointed towards the ground, are you refering to the actual tip of the antenna being pointed at the ground, or are you saying that it is mounted sideways on the mast?

What types of antennas are these?

If you have omni antennas, the radiation pattern isn't going to change all that much if they are inverted, being as they are horizontally polarized.

If you're using yagi type antennas, then the polarization is going to be off and you'll get decreased range.

Separation is best, not changing the way an antenna is mounted. How close are the two antennas?

As far as 45 watts right at your head on VHF, I wouldn't put that thing on. It would have to be cut it back to 10 watts or less before I would even think about it.

Are you carrying car batteries in the backpack too? How on earth are you powering a 45 watt radio in a backpack!

I would also agree there are better ways of working with antennas than inverting them....

It's not unusual, especially on rural cell towers, to mount two omni antennas on the same tower sidearm mount, one mounted upright and one mounted upside down directly under it. That's provided that there's no true downtilt radiation pattern from the bottom antenna, because then it would be "uptilt."

Thanks for replys. The antennas are a 1/2 wave with a ground plane and the one is pointed towards the ground, and they are about 18' apart. I'm not sure if theyu are omni or what.
The back pack still worries me. Can anyone tell me what is the danger fron these antennas. I don't want to put them at risk but need some ammo if I'm going to make some changes. Thanks again, MP

why do you need a 45w backpack radio?

The typical drag along radios like lunch box style or manpacks and VHF and UHF freqs are 5-10w max.

Well, aside from the supposed risks of harm caused by RF at close distances, there's the very real risk of being burnt by RF at those power levels.

If that antenna starts whipping around a bit, or the user bumps into a tree branch and bends the whip against him/herself why transmitting, it'll be ugly.

RF causes burns. The higher in frequency, the deeper the burn. (This is why a microwave oven heats from the inside out.) Ten watts on high band will give a heck of a burn. And it will feel DEEP, like a needle that hit a deep nerve. And they take WEEKS to heal.

A 45 watt high band RF burn . . . no thank you, I don't even want to remember the last time that happened to me.

As for an inverted omni antenna, there's nothing wrong with it. It'll work just fine - as long as there's no tilt as mentioned above. A standard high band or UHF stationmaster for example will be just fine, just the the height above ground calculations have to consider where the antenna really is, not where the base of it is. The antenna will still be vertical. Think about all the antennas on the bottoms of aircraft. They work just fine. And vertically polarised too.

I knew there was a reason why the circuit breakers for the VHF, UHF, and HF radios were always pulled during maintenance...lol. Just kidding, I knew why they were pulled...I just wonder how many watts one of our transmitters put out. I know I could talk to Charleston AFB Command Post on 349.4 thirty minutes away at FL250 (25000 feet) flying at about .75 Mach. Figure several hundred miles. I can only imagine the RF Burn from one of the antennas during transmit. (The HFs were flush-mounted on the vert. stab. of the T-Tail, and the VHF I know were on the underside)

WA2ZDY - I used to live in Freehold....across from the Nescafé plant on what was it.. Route 34 in those townhouses. Do you live close enough to get the roasted coffee smell (stench)?

I'll echo what some of the others have said, 45W that close is a bit iffy. If you really want to do that, I'd come up with a pole mount of sorts to get it up about a foot (at least) above the user's head.

These may be bailout radios, in which case power isn't a bad thing. But 10W should just about do it for anything short of 30 miles or mountains. Your battery life will be fairly compromised at 45W. That's about 12A under transmit.

Hiya Loadsmasher. . .

I live in the township, a mile north of downtown on 537. But I'm sure close enough to be grossed out on Tuesdays when they burn the slag off the ovens. That's just disgusting! The coffee smell is ok, but the slag burn . . . that's another story.

As for that long range on UHF, that's a factor of the altitude. At FL250, your horizon is 213 miles! I wouldn't be surprised to find your transmitter power is on the order of 10 or 20 watts or so. In fact, I think VHF airband radios put out 10w max. And from high enough, you can be heard FAR away!

Altitude is not such a factor on HF. For direct to ground, yes, but skywave - all the same.

I still can't see the need for a 45 watt backpack. WAY too dangerous.

As for an antenna on a mast, you might look at the Antenna Specialists ASP-358 (seen here at: http://www.antenna.com/lm_cat/lmrpg10.html )

Good luck.

Edit: I see looking at Part 87, the max power output for an aircraft station on VHF is 55 watts. So that answers that question. I've never seen such a transmitter in private aircraft. I think the King in my Godfather's plane is 5 watts, but won't swear to it. I know for sure it's not 55!

wx4cbh wrote:It's not unusual, especially on rural cell towers, to mount two omni antennas on the same tower sidearm mount, one mounted upright and one mounted upside down directly under it. That's provided that there's no true downtilt radiation pattern from the bottom antenna, because then it would be "uptilt."

Just be aware that some antennas are made to be only mounted pointed up, other wise , they can collect water / ice in the ray dome.

Yep, what he said about the water and ice thingie.