Multiple antennas on mast - interference?

We are surrounded by hills, so we get very few wireless signals on our property (not even cellular!). So, anything we want to send or receive needs an antenna on our roof mast that's about 30ft above the ground.

I currently have a Yagi connected to a cellular booster on that mast, and I'm planning to add a Wi-Fi Access Point to the mast to provide Wi-Fi outdoors. Next, I need to add an antenna for the scanner. Primarily, I need to be able to pick up the P25 system at 769 - 775 MHz from a tower about 3 miles away, though I'd like to be able to pick up as much as I can.

Everything was very simple with the one cell booster antenna. But now that I want to add more, I need to make sure I'm not doing anything stupid and ending up with the antennas interfering with each other. Is there anything I should do to keep that from happening?

Separation is the key.

You want to antennas that transmit away from antennas that receive (or transmit AND receive). Even if the frequencies are different and even if you turn off the receivers, too much transmitted RF can get into the front ends of the receivers and either:
1. Overload, so it will not receive well.
2. Overload, so it's totally deaf.
3. Overload to the point of damaging the receiver.
Simply turning off a scanner or transceiver will not disconnect the antenna from the soft expensive bits on the inside.

You want to keep any vertical antennas spaced away from the vertical mast. If the vertical element is too close to the mast, it can do a number of things:
1. Make the antenna directional away from the mast.
2. On a transmitter/transceiver, reflect enough power to raise the SWR.

Since the Yagi for your cellular booster is already set up to be mounted on a mast, put it as low as you can that it still works and is far enough away from the indoor antenna to prevent it from self oscillating.
Put the WiFi antenna where ever it works and covers what you need.

Put your scanner antenna on the top.

For the scanner, a 700MHz Yagi would improve performance off the P25 system, but at 3 miles, it's probably overkill unless the signal is really weak. Get an omnidirectional antenna that covers the bands you want to listen to, even a discone.

Make sure you use good coax for the scanner antenna. Something with a good shield that will lessen the effects from the Cellular booster and the WiFi antenna.

There's quite a lot more to consider than I had expected. Definitely not a simple subject.

I'll need to keep the cellular booster at the top of the mast. Even with the booster, our signal is pretty limited, so I definitely don't want to make the reception any worse. The scanner antenna would be second in priority. A discone will likely not work, primarily because the length of the radials will limit the space on the mast. I've been looking for a more compact omni, but I'm thinking the bands may be too far apart -- VHF High Band (city and state) and 700 MHz (county) bands. Have you seen anything other than a discone that would cover both bands?

As for the wi-fi, I can push it down to the bottom or move it somewhere else -- it's less of a concern than the scanner antenna.

The discone might be your best bet.

Depending on the type of mast, you might be able to fabricate a stand-off bracket to move it away from the mast. Using some 1" emt and a conduit bender, I've done this for small antennas.

You could use a length of Uni-Strut, too.


But, for a scanner, I wouldn't get too hung up on it. Do the best you can.

Doesn't a discone have almost zero reception straigh down? Mount it to the top of the mast and have the cellular booster right below it. Depending of how that cellular antenna looks like you could put a metal sheet or chicken net above it to isolate it almost completly from the discone.

/Ubbe

No - there is a null in the pattern immediately above and below - but so of course has a simple dipole. The simplest solution for all these things are filters. You can get reject filters for most frequencies - so you select one for the cellular frequencies and anything outside this band is attenuated - the actual amount depending on size and expense usually. You can find types that notch out unwanted frequencies too. Comms towers have to use this kit to make things stable.

A scanner complicates things because you want it to hear everything, and if too much level at any frequency comes down the feeder, Performance suffers with weird images and phantom signals. What you have to do is draw out a graph showing frequency on one axis and block in and out what you need, and see where a filter could help by including or excluding certain frequencies. It is usually possible. This all said - I've got a UHF repeater, a marine band antenna, a dual band ham vertical and a discone all very close, and the discone works fine on my scanner - theres just a bit of noise when the repeater fires up.