Antenna gain

[gts1007]

I know this is probably a dumb question, but I am trying to decide on antennas for a tower.
The tower is 100 feet tall and I want to put a VHF repeater on it.
My question is regarding antenna gain. I know that gain drastically effects your transmit power. I have selected an exposed dipole 7 DB gain for that, but does gain have any effect on receive? I am weighing the costs of using two high gain antennas or using a duplexer, or just using a unity gain for my receiver. Can anyone offer any help with my desicion?

Thanks

G Smith

[Bruce1807]

depnds, urban or rural.
flat or hilly?

[gts1007]

it is pretty hilly but the tower is on the top of a hill on the east side of town. The tower site elevation is 1254 feet, the valley dips to 856 feet and back up to 1208 feet on the west side of town. The town from hill to hill is 6 miles with really no plateaus in the middle, just straight down and straight back up.

Thanks

GS

[Wowbagger]

... but does gain have any effect on receive?

Yes - every dB of gain will be an extra dB of received signal, which is why the first and best place to improve any radio system is the antenna. Gain there will not only improve transmit coverage but receive coverage as well.

As to using 2 antennas vs. 1 antenna and a duplexer: you are trying to keep the transmitted signal out of the receiver - not just in the sense of "Not putting 50 watts into the front end of your receiver" but also in the sense of "not putting a -80dBm signal into your receiver and desensing it." With 2 antennas, mounted one over the other, with tens of feet of vertical separation, you can get about 50 dB of isolation - but that still means that if your repeater is putting out 50 watts, your receiver is seeing .5mW of signal (or about -3dBm). That's WAY more than enough to desense the receiver by a good 20 dB.

Now, a good six can, bandpass/band reject duplexer will give you over 110 dB of isolation - taking your +46dBm repeater down to -64 dBm, enough to not desense it so badly.

Now, if you REALLY want to get fancy, and can afford it, having 2 antennas, separated by 50 feet, and feeding into the 2 halves of a 6 can duplexer (in other words, split the duplexer in half and use it as two separate 3 can pass/notch filters) and you can get that cross-over signal down to -100 dBm or less (assuming you are using GOOD, double-shielded cables). You won't have any desense. You will also have a spare so that if weather knocks out one antenna, you can reconnect the halves of the duplexer and get back on the air (albeit with a bit more desense) until you can fix the antenna.

[Jim202]

There is a down side to a high gain antenna. They tend to flatten out the vertical radiation pattern and put more power into the horizontal plane. What this does is to concentrate your power out on the distant horizon in laymans terms. In doing this you loose the ability to fill in under the antenna and near in. You thus create weak or dead zones below and near the base antenna. This effect will go out several miles in most cases.

Kind of depends on several factors as to just how pronounced this null or dead zone may be. How high the antenna is compared to the region below the antenna your trying to cover. The gain of the antenna. The gain of the user unit antenna. Weather or not the base antenna has any down tilt built into it. What kind of obstructions you have between the repeater and the user radio.

Good luck on you endevers with a new antenna.

Jim

[Wowbagger]

There is a down side to a high gain antenna. They tend to flatten out the vertical radiation pattern and put more power into the horizontal plane. What this does is to concentrate your power out on the distant horizon in laymans terms. In doing this you loose (sic) the ability to fill in under the antenna and near in. You thus create weak or dead zones below and near the base antenna. This effect will go out several miles in most cases.

I don't think you are really going to see much effect from this, and I speak from pratical experience. I have a very high gain antenna 1200 feet up on a tower - there is no noticable "null" at the base of the tower, or within 1 mile, or ten miles.

Remember, while a 20 dB antenna is really flattening the radiation pattern, and might be -20 dB out of the gain area (although that is not a given - it could be only -10 dB), the delta in space losses going from 1 mile from the tower to 10 miles to the tower are over 20 dB - so what you <i>lose</i> in antenna gain you make up in being close.