Why your wireless drops out near a TV tower

You scan, you find a hole, you assign a frequency, and the range is still terrible. Talent walks fifteen feet and the receiver mutes. Nothing on the scan explains it. Somewhere within sight of the location there is a broadcast tower putting out tens or hundreds of kilowatts, and your transmitter is putting out 50 mW. That is a difference of roughly a million to one before either signal has travelled anywhere, and it is the single most common reason a technically legal, apparently clear frequency does not work.

Two failures that look identical from the cart

Sound mixers tend to file every RF problem under interference, but the two big ones behave differently and respond to completely different fixes. Getting the diagnosis right is most of the work.

Why a signal on a different frequency still hurts

A receiver is not a perfect filter followed by a perfect demodulator. Between the antenna connector and the decision-making part of the radio there is an amplifier, a filter of finite sharpness, and a mixer, and all three are analogue devices with a limited linear range. Front-end problems happen in that chain, before any of the receiver's clever digital processing gets a vote.

Blocking (desensitization)

A strong off-channel signal drives an input stage out of its linear range, so every signal passing through that stage — including yours — gets less gain. The receiver is still tuned correctly. It simply cannot hear.

Reciprocal mixing

Every tuner's local oscillator carries some phase noise on either side of its nominal frequency. A very strong nearby signal mixes with those noise skirts and lands on top of your channel as broadband hiss. This is the mechanism by which a transmitter on a completely different frequency raises your noise floor.

Receiver-generated intermod

Push two or more strong signals into a non-linear stage and it manufactures new signals that were never on the air. One of them can land on your carrier. See intermodulation, explained.

Third-order intercept (IP3)

The figure of merit for how much off-channel signal a front end tolerates before it starts generating its own garbage. Higher is better, and it varies enormously between designs — which is why two receivers in the same bag, on the same antenna, can behave completely differently in the same spot.

What to do about it, in order of how often it works

1. Move the receive antennas. Distance from the tower helps, but so does geometry. Put the building, a hill, or the truck between your antennas and the transmitter site while keeping line of sight to talent. Ten feet in the right direction can be worth more than any accessory in the bag.
2. Take gain out of the chain. Bypass the amplified distro, switch the antenna amp to its lowest setting, or insert attenuation ahead of it. If range improves when you reduce gain, you have confirmed overload, which is a useful thing to know.
3. Add filtering ahead of the amplifier, not after it. A band-pass or notch filter in front of the first active stage stops the tower from reaching it at all. Filtering after the amp is too late — the damage has already happened inside the amp.
4. Change blocks, not frequencies. Receiver filtering rejects signals better the further away they are. Moving from one end of a block to the other rarely helps; moving to a block a hundred megahertz away often does.
5. Get the transmitter signal up at the antenna. Overload is a ratio problem. Anything that raises your own signal at the receive antenna — shorter cable runs, antennas closer to the action, better transmitter antenna placement on the body — buys back headroom that filtering cannot.
6. Check the cable before you blame the air. Long thin coax at UHF throws away a startling amount of signal. A soft dropout that follows a specific antenna is often a cable or connector, not the tower.

Knowing before you get there

Wireless microphones are secondary users in the TV bands. The broadcaster was there first, has orders of magnitude more power, and is not moving. All the leverage you have is in choosing where to work in the spectrum and where to put your antennas — which is much easier to do the night before than at call time.

That is what RF Scout is for. It reads the FCC's own engineering records for the stations around an address — callsign, RF channel, ERP, antenna height, distance — and scores each TV channel Recommended, Caution or Avoid, with the specific stations that drove the score. It is a planning proxy, not a measurement and not a terrain-aware propagation prediction, so it never replaces a scan on location. What it does replace is the discovery, at 7 a.m., that the channel you packed for belongs to a network affiliate nine miles away. More on the maths in how RF Scout scores a location.