Mast-Mounted Preamplifier for VHF and UHF: When It Helps
On HF, the noise arriving from the antenna is so much stronger than the noise generated inside the receiver that a preamplifier rarely helps and often hurts. On 2 metres and above the picture reverses. The band is quiet, the receiver’s own noise matters, and every decibel of loss in the coax between antenna and radio is a decibel of weak signal thrown away before the receiver sees it. A mast-mounted preamplifier, a low-noise amplifier fixed at the antenna rather than in the shack, is the standard cure. It amplifies the signal before the feedline loss, so the loss no longer sets the noise performance of the station. This guide explains when a preamplifier genuinely improves a station and when it only makes the S-meter move, how to read noise figure and gain specifications, why the preamplifier must be protected from your own transmitter with a sequencer, and how to mount and power one properly.
Why Feedline Loss Matters So Much Above 144 MHz
A receiver’s sensitivity is set by its noise figure, the amount of noise it adds to the signal, expressed in decibels above the noise of an ideal receiver. Everything in front of the receiver’s first amplifier adds to that figure, and a length of lossy coax adds its loss directly: ten metres of cable with 3 dB of loss turns a receiver with a 3 dB noise figure into a system with a 6 dB noise figure, as far as the antenna is concerned. On HF this does not matter because the atmospheric and man-made noise coming down the antenna is tens of decibels above the receiver noise on any band, and losing a few decibels of both signal and noise changes nothing. On VHF and especially UHF the external noise is low enough that the receiver’s noise figure becomes the limit, and feedline loss translates directly into weak signals you cannot hear. The noise figure reference page gives the formal definitions; the practical point is that at 432 MHz the coax is usually the biggest single contributor to a station’s noise figure.
The cure follows from the cascade formula for noise figure: the noise figure of a chain is dominated by the first stage, provided that stage has enough gain. Put a low-noise amplifier at the antenna, with a noise figure well under 1 dB and 15 to 20 dB of gain, and the coax loss and the receiver’s noise figure are divided by that gain in the calculation. The system noise figure becomes almost that of the preamplifier alone. That is the whole point of mounting it at the mast rather than in the shack, where it would sit after the loss and amplify the loss along with everything else.
When a Preamplifier Helps and When It Does Not
| Situation | Preamp at the mast | Preamp in the shack | Why |
|---|---|---|---|
| Weak-signal SSB, CW and digital on 2 m, 70 cm, 23 cm | Large improvement | Small improvement | Feedline loss and receiver noise figure set the limit |
| EME and meteor scatter | Essential | Not sufficient | Every tenth of a decibel of noise figure counts |
| Satellite downlinks on 70 cm | Clear improvement | Modest | Downlink signals are weak and the band is quiet |
| Local FM repeaters and simplex | Little benefit | None | Signals are already strong; noise figure is not the limit |
| Urban site with strong nearby transmitters | Risk of overload | Risk of overload | Extra gain in front of the receiver raises intermodulation |
| Very short feedline, modern low-noise radio | Small benefit | None | Little loss to recover |
| HF bands | None, often harmful | None | External noise is far above receiver noise |
The rows that matter are the weak-signal ones. For the pursuits described in our guides to EME moonbounce and meteor scatter, a mast-mounted preamplifier is part of the basic station. For an FM-only station it is money better spent on feedline or a higher antenna.
Reading the Specifications
Noise figure
The headline number. Modern preamplifiers for 2 metres and 70 cm offer noise figures of a few tenths of a decibel. The difference between 0.3 dB and 0.8 dB is real for EME and almost irrelevant for terrestrial work, where the external noise from the sky and the ground sets a floor around 1 to 2 dB on 144 MHz anyway. Pay for very low noise figure only if you need it.
Gain
Enough gain to overcome the feedline loss and swamp the receiver’s noise figure, which usually means 15 to 20 dB. More than that does harm: it compresses the receiver’s dynamic range and makes strong-signal overload more likely. Many radios have an attenuator or a gain setting intended for use with an external preamplifier; use it to bring the total gain back to what the receiver can handle. Our article on receiver dynamic range explains what happens when the gain in front of a receiver is too high.
Selectivity
A preamplifier with a band-pass filter at its input, or a high-Q helical front end, rejects strong out-of-band signals from broadcast, paging and cellular transmitters before they reach the amplifier. At an urban site this is the difference between a usable station and one that hears intermodulation products everywhere. A wideband preamplifier with no filtering is cheaper and is fine on a quiet rural hill.
Power handling and switching
A mast-mounted unit sits in the transmit path, so it must either contain relays that bypass the amplifier during transmission, rated for your full transmitter power, or be used with an external relay at the mast. The relay rating must exceed the actual power at the antenna end; check both the continuous rating and the switching method.
Power feed
The preamplifier needs DC at the top of the mast. The two methods are a separate cable, which is simple and robust, or DC fed up the coax through a bias tee at each end, which saves a cable but requires that nothing else in the line blocks DC.
Sequencing: Protecting the Preamplifier From Yourself
The most common way to destroy a mast-mounted preamplifier is to transmit into it. The relays that bypass the amplifier take a few milliseconds to move, and if the transmitter produces power before they have finished, the full transmitter output hits the input of a device designed for microvolts. The cure is a sequencer, a small controller that switches things in a fixed order with delays between them: first the preamplifier is bypassed, then any external relays, then the amplifier is enabled, and only then is the transceiver allowed to transmit. On release the sequence runs in reverse so that the transmitter stops before the preamplifier is switched back into the line.
Many transceivers have a dedicated output for controlling an external preamplifier, and some include a sequenced delay internally; the manual will say. If the radio does not, a stand-alone sequencer between the footswitch or the PTT line and the radio does the job. Whatever the arrangement, test it with the transmitter at minimum power and an oscilloscope or a sensitive detector on the preamplifier side before trusting it, and never run a mast-mounted preamplifier with a transmitter whose PTT is not sequenced.
Installing the Preamplifier
- Mount it as close to the antenna as possible. The length of coax between antenna and preamplifier is the only coax whose loss still counts. A short jumper of good cable is ideal; mounting the unit on the boom or on the mast just below the antenna is normal.
- Weatherproof it. Use a unit in a sealed enclosure, mount it with the connectors facing down, seal the connectors with self-amalgamating tape and a final layer of ordinary tape, and leave a drip loop in the cables. Water in a preamplifier is the second most common failure after transmitting into it.
- Use good connectors. N-type at UHF. A poor connector on the input side adds loss in exactly the place where it hurts most.
- Protect against lightning. A mast-mounted unit is the first thing in line from the antenna. Follow the usual grounding practice for the mast and feedline, and accept that a direct strike will take the preamplifier whatever you do.
- Route the control and power cables away from the feedline. If DC comes up the coax, install the bias tees and check that nothing in the line, such as a lightning arrestor with a DC short, blocks it.
- Set the receiver’s gain. With the preamplifier running, reduce the radio’s RF gain or engage its attenuator until the noise floor rises by only a few decibels when the preamplifier is switched on. That shows that the preamplifier’s noise, not the receiver’s, now sets the floor, which is the goal, without excess gain.
Checking That It Works
The test is simple. Point the antenna at cold sky, note the noise floor with the preamplifier bypassed, then switch it in. The noise floor should rise by roughly the preamplifier’s gain minus a little, and a weak signal that was at the noise should now be readable. Then point the antenna at the ground or at the Sun: the noise should rise further, which shows that the system is now hearing external noise rather than its own. If the noise floor rises by the full gain but weak signals are no better, the receiver was already quiet enough and the preamplifier is only adding gain; reduce the gain in the radio. If the noise floor does not change at all, the preamplifier is not in the line, is unpowered, or the relay has failed. If reception is worse with the preamplifier on, strong signals are overloading something; add filtering or reduce the gain. The feedline loss that the preamplifier is overcoming can be estimated from the cable tables in our coax comparison, and comparing that number with the measured improvement is a good sanity check.
Common Mistakes
Mounting the preamplifier in the shack. It then amplifies the coax loss along with the signal and improves little. The whole benefit comes from placing it before the loss.
Too much gain. Overload, intermodulation and a receiver that cannot cope with strong local signals. Use the radio’s attenuator and choose a modest-gain unit.
No sequencer. The preamplifier survives until the first time the relay is slow or the operator keys up while a switch is in the wrong position.
Forgetting the relay rating. A preamplifier rated for 100 W in the bypass path does not survive a kilowatt.
Wideband preamplifier at an urban site. Paging and broadcast transmitters mix in the amplifier and fill the band with phantom signals. Use a filtered unit.
Poor weather sealing. A season of rain finds its way into the enclosure and the noise figure quietly rises until the unit fails.
The Bottom Line
A mast-mounted preamplifier is the single most effective receive upgrade for a weak-signal VHF or UHF station, because it moves the first amplifier in front of the feedline loss and makes the coax almost irrelevant to the noise figure. It does nothing useful for local FM, it is dangerous without a sequencer, and too much gain is as bad as too little. Choose a unit with a modest noise figure, 15 to 20 dB of gain, filtering if your site is noisy and relays rated above your power; mount it at the antenna, seal it properly, sequence it, and set the radio’s gain to match. Then point the antenna at the sky and listen to what you have been missing.
Frequently Asked Questions
Does a mast-mounted preamplifier help on FM repeaters?
Rarely. Repeater and simplex signals are usually far above the noise floor, so lowering the system noise figure changes nothing you can hear. The benefit is for weak-signal CW, SSB, digital modes, satellites, meteor scatter and EME.
Why mount the preamplifier at the antenna instead of in the shack?
Because the coax loss between antenna and radio adds directly to the noise figure. A preamplifier before the loss makes the loss almost irrelevant; one after the loss amplifies the degraded signal and gains little.
How much gain should a VHF preamp have?
Enough to overcome the feedline loss and dominate the receiver’s noise, typically 15 to 20 dB. More gain increases overload without improving sensitivity.
What is a sequencer and do I need one?
A controller that switches the preamplifier out of the line before the transmitter is allowed to produce power, and back in only after it stops. Yes, you need one; transmitting into a preamplifier destroys it.
Can I power the preamplifier through the coax?
Yes, with a bias tee at each end to inject and extract the DC, provided nothing in the line blocks DC. A separate power cable is the simpler alternative.
What noise figure do I need?
Under 1 dB is good for terrestrial VHF and UHF work. A few tenths of a decibel matters for EME and for 70 cm and above, where sky noise is low.