Diamond X300A Review: Ten Feet of Gain for a Home Station

The short answer

Buy the X300A if you have a mast or a gable end that can take a taller antenna and you want more gain than a short collinear gives on both 2 metres and 70 centimetres. Skip it if the mounting point is low or awkward, because a shorter antenna ten feet higher will beat a taller antenna at eave height nearly every time.

Bands: 144 to 148 MHz and 435 to 450 MHz
Stated VSWR: Typically below 1.5 to 1
Connector: UHF female, SO-239
Mounting: Towers, masts and rooftops

Roughly 6.5 dBi on 2 metres and 9 dBi on 70 centimetres, pre tuned, on an SO-239.

Gain when you have the height

Diamond X300A Dual Band Base and Repeater Antenna

Around $177

Pre tuned dual band vertical for 144 to 148 MHz and 435 to 450 MHz with roughly 6.5 dBi gain on 2 metres and up to 9.0 dBi on 70 centimetres, in a fibreglass radome with a UHF female connector.

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Specifications

Bands	144 to 148 MHz and 435 to 450 MHz
Gain	Approximately 6.5 dBi on 2 m, up to 9.0 dBi on 70 cm
Tuning	Factory pre tuned, no field adjustment
Stated VSWR	Typically below 1.5 to 1
Connector	UHF female, SO-239
Construction	Fibreglass radome with corrosion resistant hardware
Mounting	Towers, masts and rooftops
Approximate price	Around $177

Specifications above come from Diamond's published specifications for the X300A. Treat pricing as approximate, since Amazon pricing moves.

What it does well

Real gain on both bands from a single feedline, with nothing to adjust after installation.
The 70 centimetre figure of up to 9.0 dBi is genuinely useful where UHF repeaters are marginal.
Factory pre tuned, which matters when the antenna will be at the top of a mast you would rather not climb twice.
Fibreglass radome and corrosion resistant hardware, built for permanent outdoor service.
Standard SO-239 connector, so any normal coax and adapter arrangement works.

Where it falls short

Gain on a vertical comes from flattening the pattern. If your repeater sits on a mountain directly above you, more gain points energy away from it.
It is a long antenna. Wind loading and the mounting arrangement need real thought, and a flimsy mast will not do.
Feedline choice can eat the gain. At 440 MHz, thin coax over a long run can lose more than the antenna gains.
Height beats gain. If the mounting point is low, a shorter antenna higher up is the better buy.

Gain, height and what actually reaches the repeater

A vertical antenna produces gain by squashing its radiation pattern toward the horizon. For terrestrial VHF and UHF work that is exactly right, since every repeater and every simplex station is near your horizon. But it is only useful if the horizon is where the signal needs to go, and it is only useful if it survives the feedline.

Do the arithmetic before you buy. Use the VHF range calculator to see what your mounting height reaches, and the coax loss calculator to see what your planned cable costs at 440 MHz. If the cable eats 3 dB, you have thrown away half the gain you paid for. RG-8X versus RG-213 versus LMR-400 covers the choice. Antenna and mast work carries real fall and power line risk, so read the installation safety guide first.

What owners report

Published owner reports on the Diamond X series are consistent and long running. Owners describe the antennas as working exactly as specified with no adjustment, surviving years of weather, and delivering an obvious improvement over a shorter vertical when the mounting height allows. The 70 centimetre performance draws specific praise, since UHF is where most home stations are weakest.

The recurring cautions are mechanical and about feedline. Owners emphasise a solid mast and proper clamping because of the length, and several report that their improvement only appeared after replacing thin coax with a low loss cable. At UHF that is not a marginal effect. Run your own numbers on the coax loss calculator before you order cable.

The other honest theme from owners: height did more than gain. Several describe moving the same antenna higher and gaining more than they did by moving from a shorter antenna to a longer one. That ordering holds nearly everywhere. Check what your planned height buys on the antenna height calculator.

Who it suits

Home stations with a mast, tower or gable end that can carry a long antenna properly.
Operators who work UHF repeaters at the edge of reliable range.
Anyone replacing a short vertical who has already got the height they can get.
Club and repeater sites where a permanent, pre tuned antenna is wanted.

Who should skip it

Anyone whose mounting point is low. Buy a shorter antenna and spend the difference on height.
Stations in a valley talking to a repeater directly above them, where a flatter pattern hurts.
Anyone using thin coax on a long run at UHF, who will lose more than the antenna gains.
Installations where wind loading on a long antenna is a structural problem.

Alternatives worth considering

Budget Pick

Diamond X50A Dual Band Base Antenna, 5.6 ft

Around $123

The shorter sibling at 5 ft 6 in, roughly 6 dBi on 2 m and 8 dBi on 70 cm. Easier to mount, less wind load, and the better buy when the mounting point is modest.

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Step-Up Alternative

Comet GP-9M High Gain Dual Band Base Antenna

Around $324

A 17 foot fibreglass collinear for operators who want the most gain a vertical can reasonably give and have a mast that can carry it.

Check price on Amazon

Cross Shop

Comet GP-6 Dual Band Base Antenna

Around $198

The obvious competitor at a similar length and price, from the other main Japanese antenna maker. Choose on availability, mounting hardware and connector preference.

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Keep reading

This review draws on published specifications, manufacturer figures and the consensus of verified owner reviews. We have not tested this product ourselves, and we say so rather than implying otherwise. As an Amazon Associate we earn from qualifying purchases.

Common questions

Questions people ask about this

Diamond X300A vs X50A: which should I buy?

Buy the X50A unless you have somewhere solid to put a longer antenna. The X300A gives modestly more gain on both bands, and it is longer, heavier and harder to mount, and it needs a mast that will not flex. If your mounting height is fixed and modest, the shorter antenna is easier to install well, and installing well matters more than the gain difference.

Does more antenna gain always help?

No. Gain on a vertical comes from compressing the radiation pattern toward the horizon, which is right for almost all terrestrial work and wrong in one specific case: when the station you want is significantly above or below you, such as a mountain top repeater seen from a valley floor. That case is uncommon and worth knowing about before you assume more gain is better.

What coax should I use with the X300A?

Low loss cable, and the longer the run the more it matters. At 440 MHz, thin cable such as RG-58 over a long run can lose several decibels, which is more than the antenna gains, so the upgrade cancels itself out. LMR-400 class cable or better is the usual answer for a permanent base installation. Calculate your specific run rather than guessing.

How high should I mount it?

As high as you can do safely and legally. On VHF and UHF, range is close to a line of sight problem, so height beats gain and beats power. Ten more feet of mast will usually change more contacts than a longer antenna at the same height. Mast, roof and tower work carries real fall and power line risk, so plan the job and get help where it is needed.

Do I need a lightning arrestor?

If you are running coax from an outdoor antenna into your house, yes, along with proper bonding of the mast and the coax shield to your ground system. This is a safety and property protection matter governed by the National Electrical Code and local rules, not an optional accessory. Hire an electrician if the bonding work goes beyond what you can do correctly.

Keeping your own cut lengths, SWR sweeps and exposure record? The Station Build & Antenna Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.

Disclaimer Researched guidance for planning purposes, not professional engineering, electrical or legal advice. Antenna work, RF exposure, mast and tower work, grounding and battery handling all carry real risk that depends on your specific site and installation. Verify anything safety-critical against current FCC rules, the National Electrical Code and the manufacturer's own documentation, and hire a professional where the job calls for one. We research equipment from published specifications, regulatory documents and verified owner reviews rather than claiming hands-on testing we have not done.