Why your Homebrew 5/8 wave antenna isn't working as designed.
- n4cly7
- Dec 7
- 5 min read
A 5/8λ (five-eighths wavelength) antenna is very popular on GMRS (462–467 MHz) because it offers more gain than a 1/4λ or 1/2λ whip, especially when mounted high and in the clear. In the UHF band (GMRS is ~462 MHz), a 5/8λ vertical is roughly 15.5–16 inches long physically (not counting any loading coil at the base that some commercial designs use).
Radiation Pattern Characteristics (Free-Space, Ideal 5/8λ over Perfect Ground Plane)
Maximum gain: ≈ +3 to +3.5 dBi in free space, but ≈ 5–6 dBd (7–9 dBi) when mounted over a real vehicle roof or large ground plane. This is where the reputation for “great range” comes from.
Main lobe: The strongest radiation is downward from the horizon, typically peaking at –10° to –18° below horizontal (the exact angle depends on height above ground and ground-plane size). This is why a 5/8λ works so well on vehicles or hilltops — it throws the signal toward the horizon and slightly downward, which is perfect for reaching distant repeaters or mobiles that are lower.
Take-off angle: Very low (good for DX). On a car roof at 462 MHz, the main lobe is usually 8°–15° below the horizon.
Nulls: There is a deep null straight up (almost zero radiation overhead). This is the classic 5/8λ trade-off — terrible for airplanes or very close stations directly underneath, but excellent for everything else on the ground out to the radio horizon.
2D Elevation Pattern (Vertical Plane)
Here’s an approximate polar plot (dB scale, normalized) for a 5/8λ vertical on a vehicle roof at 462 MHz:
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↑ 90° (zenith)
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. null (~ -30 dB or worse straight up)
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/ \
/ \
/ \
/ \
/ \
/ \
/ \
/ \
/ \
/ \
0° -----------------------→ 0° horizon
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.'
↓ -90° (straight down)The fattest, strongest part of the lobe is tilted downward about 10°–15° from horizontal. That downward tilt is what gives the extra 2–3 dB of useful gain on flat or rolling terrain compared to a 1/4-wave.
Real-World Performance on GMRS
Simplex mobile-to-mobile: Often 50–100% farther range than a 1/4-wave mag-mount in open terrain.
Repeater access: Excellent because most repeaters are on towers or hilltops; the low take-off angle hits them perfectly.
Close-in coverage: Slightly weaker directly underneath the antenna (inside ~1–2 miles if the antenna is very high), but rarely a problem on GMRS.
Handheld with 5/8λ whip (e.g., Nagoya NA-771G, etc.): Still shows noticeable gain over a stock rubber duck, especially when you’re outside and waving the radio around — the lobe compresses downward and you get 3–5 dB more toward the horizon.
Summary
The 5/8λ on GMRS gives you:
Highest practical gain of any simple mobile whip
Very low radiation angle → excellent for distance
Classic “cone of silence” straight overhead (usually not an issue on GMRS)
If you ever see side-by-side range tests between a 1/4-wave and a good 5/8λ (same height, same radio), the 5/8λ almost always wins beyond 5–10 miles line-of-sight.
Would you like a proper NEC-model plot or comparison with 1/4-wave and 1/2-wave patterns? I can generate those too.
There are TWO different designs
Name People Use | What It Actually Is | Physical Length on GMRS | Real Gain | Why the length difference? |
“True” or “full” 5/8 wave | A single 5/8λ radiator over a real ground plane (or with radials) | ≈ 25–26 inches (sometimes as short as 24" with end-loading) | ~3–4 dBd | This is the 16–26 inch antennas you sometimes see advertised as “5/8 wave” (e.g., some cheap Amazon no-name sticks, or the Ed Fong roll-up J-pole cut for 5/8λ) |
5/8 wave collinear (the famous 44-inch ones) | 5/8λ radiator on top + a 1/4λ decoupling stub or lower radiator below it + a phasing coil in the middle | 42–48 inches total | ~3–4 dBd (same as above, sometimes a hair more) | This is the design used by Midland, Laird, Comet, Harvest, etc. |
Why the commercial GMRS ones are ~44 inches long
The classic mobile/base 5/8 wave antenna sold for GMRS is NOT just a single 5/8λ whip.It is an end-fed collinear that consists of:
Bottom section: ≈ 1/4 wave long (about 16–17 inches) — acts like a 1/4-wave counterpoise so you don’t need radials on a car roof.
A phasing coil in the middle (adds 4–6 inches of length).
Top section: ≈ 5/8 wave long (another 25–26 inches).
Total electrical length inside the fiberglass: roughly 7/8λ stacked in phase → about 3 dB more gain than a 1/4 wave and a very low radiation angle.Physical length on 462 MHz: 42–48 inches (depending on velocity factor of the coil and fiberglass).
So when you see the Midland MXTA26 (44"), Comet CA-712EF (41"), or Laird B4505C (44"), those are 5/8 over 1/4 wave collinear antennas, not a simple single-element 5/8λ whip.
The rare 16–26 inch “pure” 5/8 wave antennas
You do see some very short whips (16–20 inches) marketed as “5/8 wave” for GMRS. They achieve the electrical 5/8λ length by:
Heavy base or center loading coils (lots of loss), or
Very thin wire with a top hat (fragile).
They work, but they have narrower bandwidth, are more affected by poor ground plane, and usually perform worse in the real world than the longer collinear designs — which is why almost nobody serious on GMRS uses the short ones.
Bottom line
16–26 inches → a single-element 5/8λ (rarely used on GMRS mobiles)
42–48 inches → the 5/8 over 1/4 wave collinear that everyone actually recommends and uses on GMRS
That extra length is what gives you the good ground-plane-independent performance and the gain everyone loves.
Short summary: Why a “pure” (single-element) 5/8λ antenna should only be used at home over a proper ground plane
A true 5/8 wave vertical (≈25–26" radiator with no lower 1/4λ section or phasing coil) requires a large, symmetrical ground plane (at least 1/4λ radials or a solid metal surface) to:
Provide the missing return currents,
Keep the radiation pattern low and omnidirectional,
Prevent high SWR and common-mode currents on the feedline.
On a vehicle, the typical small mag-mount, trunk-lip, or mirror mount does not provide an adequate or symmetrical ground plane. The result is:
High SWR (often >3:1),
Distorted/drooping pattern (signal goes upward instead of toward the horizon),
Much of the power wasted heating the coax shield.
That’s why the short “pure” 5/8λ whips work great on a fixed mast with radials or a big metal roof at home, but are a poor choice for mobile use. Vehicles need the longer 5/8-over-1/4λ collinear designs (the 42–48" ones) that build their own counterpoise and work properly even with imperfect vehicle ground planes.
NOT Sponsored by any of these products! They are simply good products with a long life span in my use of them.
Great Antennas for your GMRS needs.
Suggested antennas for your GMRS use.
Midland - MXTA26 MicroMobile 1.5 inch Base Diameter 6DB Gain Whip Power120W Boost Signal Output - 5/8 Wave - 32” Antenna
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Midland – MXTA27 MicroMobile Universal Lip Mount with NMO Connector
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Midland Micro-Mobile MXTA12 Antenna Mag Mount
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Laird Technologies - B4505CNS - 450-470 Antenna w/Spring,
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Comet CA-2X4SR Dual-Band Mobile Antenna
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Gmrs Antenna 15.3-Inch Whip GMRS (462MHz) Antenna SMA-Female
**This antenna changed my GMRS HT results greatly!
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Retevis RT97, GMRS Base Antenna,7.2dBi High Gain 462-467MHz
Saving the best for last. Great antenna I have it on a pole 50 feet up. Great antenna.
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Thanks for reading my blog. Hope to hear you on the air sometime.
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