The short version: get the antenna high and clear first, keep the cable short and thick, and treat antenna gain as a way to aim your signal, not to add to it.
Research-based. The figures below are math from published formulas and manufacturer cable data, not field measurements. We will not publish range numbers until we test with a documented method.
Height comes first
At 915 MHz, radio behaves a lot like light: hills, buildings and dense trees block it. Two things decide whether two antennas can reach each other over open ground.
The curve of the earth. Using the standard allowance for the atmosphere bending radio slightly, each antenna sees about 4.12 x √h km to the horizon, with h in meters. Two radios at 6 ft see each other to about 6.9 miles over perfectly flat ground. Put one at 30 ft and that grows to about 11.2 miles.
The clear zone. The signal does not travel along a thin line. It fills a football-shaped region around it, the first Fresnel zone. RF references advise keeping at least 60 percent of it clear. At the midpoint its radius is 17.3 x √(D / 4f) meters, with D in km and f in GHz.
That clear zone is bigger than most people expect. Worked through our line-of-sight calculator at 915 MHz:
| Path | Clear-zone radius at the midpoint | 60 percent of it | Equal heights that just clear flat ground |
|---|---|---|---|
| 2 miles | 53 ft | 32 ft | about 33 ft each |
| 5 miles | 84 ft | 51 ft | about 54 ft each |
| 10 miles | 119 ft | 72 ft | about 84 ft each |
So two handhelds at chest height, two miles apart, are inside a partly blocked zone even over a parking lot. LoRa can still get a message through a partly blocked path, with less margin. That is why a single roof or hilltop node changes a mesh more than any handheld upgrade.
What antenna gain does
Gain, in dBi, compares an antenna with an ideal one that radiates equally in every direction. A higher-gain omni antenna does not create power. It squeezes the pattern into a flatter disc, reaching farther toward the horizon and less above and below.
- On a tall mast over open ground, a flatter pattern sends more signal where the distant radios are. The 5.8 dBi omni is aimed at this.
- On a roof with radios close below, a flat pattern can pass over their heads. A lower-gain antenna such as the 3 dBi fiberglass keeps more signal pointed down.
- On a handheld, the antenna it shipped with is matched to the radio. Swap it only for a tuned 915 MHz antenna with the right connector.
The 6 dBi line
Under 47 CFR 15.247(b)(4), if you use an antenna above 6 dBi in this band, conducted output must drop by the amount the gain exceeds 6 dBi. Both outdoor antennas we list are under that line. See the US 915 MHz rules for the rest.
What the cable costs you
Coax loses signal along its length, and the loss counts both ways: what you send and what you hear. Every 3 dB lost is half the power. Times Microwave’s figures at 900 MHz, run through our coax loss calculator:
| Cable | dB per 100 ft | 25 ft run | 50 ft run |
|---|---|---|---|
| LMR-400 | 3.9 | 1.0 dB, about 80% delivered | 2.0 dB, about 64% |
| LMR-240 | 7.6 | 1.9 dB, about 65% | 3.8 dB, about 42% |
| LMR-195 | 11.1 | 2.8 dB, about 53% | 5.6 dB, about 28% |
| RG-58 | 16.5 | 4.1 dB, about 39% | 8.3 dB, about 15% |
Connectors and adapters add more; take their figures from their data sheets. A 50 ft run of RG-58 can throw away more than the antenna gains.
A useful trade: for a fixed node, put the radio up at the antenna in a weatherproof box and run power up the mast instead of coax down it.
Connectors that match
- SMA and RP-SMA look alike and do not mate. Check type and gender.
- IPEX (U.FL) jacks on bare boards are small and fragile. Use a short pigtail to a panel-mount SMA for anything that moves, as Heltec’s own caution for the WiFi LoRa 32 V4 suggests.
- N-type is common on outdoor antennas. Both outdoor antennas we list are N-male.
- Every adapter adds loss and a place for water to get in. Fewer is better.
Outdoors: water, wind and lightning
- Seal outdoor connectors and leave a drip loop so water runs off before the radio.
- An outdoor antenna mast and its lead-in fall under National Electrical Code Article 810. An EC&M summary of it lists a listed antenna discharge unit for each lead-in, grounding the mast, a copper grounding conductor of at least 10 AWG run as straight as practical, at least 2 ft from exposed power conductors, and never using the electric service mast as an antenna support. Check the code edition your area enforces; an electrician can do the grounding.
- Never mount near power lines.
Checklist
- Highest safe spot you have permission to use.
- Clear zone checked with the line-of-sight calculator.
- Antenna gain chosen for the site, at or under 6 dBi unless you reduce power.
- Shortest, thickest coax, or the radio at the antenna.
- Connectors matched, sealed, with a drip loop.
- Mast grounded.
Bill of materials
| Part | Choice | Need | Why | Evidence | Where to get it |
|---|---|---|---|---|---|
| Outdoor 915 MHz antenna, low gain (roof) | RAK 3 dBi N-male fiberglass outdoor antenna | Optional | Fatter pattern for radios close below the roofline. | Manufacturer specs only | Not yet available |
| Outdoor 915 MHz antenna, mid gain (mast) | 5.8 dBi N-male outdoor omni antenna, 915 MHz | Optional | Pattern pushed toward the horizon for a tall, open site. Under 6 dBi. | Manufacturer specs only | Not yet available |
| Low-loss coax, as short as the mount allows | Your choice (see checklist) | Required | Loss rises with length; thin cable on a long run wastes the antenna. | Buying checklist | Any seller meeting the checklist |
| Adapters matched at both ends | Your choice (see checklist) | Required | N-type antennas, SMA or IPEX radios. | Buying checklist | Any seller meeting the checklist |
Scroll the table sideways to see every column.
No total price: we only show prices from approved, current merchant data. Parts may come from different stores; there is no combined checkout.
Questions
Will a higher-gain antenna on my handheld double my range?
Not reliably. Gain narrows the pattern rather than adding power, and a handheld at chest height is usually limited by what is in the way, not by the antenna.
What coax should I buy?
The thickest low-loss cable that suits the run, kept as short as possible. At 900 MHz, LMR-400 loses 3.9 dB per 100 ft per Times Microwave; thin RG-58 loses several times that.
Can I use a 9 dBi antenna?
Under 47 CFR 15.247(b)(4), antenna gain above 6 dBi in this band means reducing conducted output by the excess. A 9 dBi antenna means 3 dB less output. Check your radio's authorization too.
Sources
- Line of Sight (RF glossary), RF Essentials, read 2026-10-26.
- Fresnel Zone, RF Cafe, read 2026-10-26.
- Times Microwave LMR-400 data sheet, Times Microwave Systems (via RF Parts), read 2026-10-26.
- Coax Selection Guide (Times Microwave LMR and RG cables), Times Microwave Systems (via RF Parts), read 2026-10-26.
- 47 CFR 15.247: Operation within the bands 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz, eCFR (US Government), read 2026-10-26.
- 5.8 dBi N-Male Omni Outdoor 915 MHz Antenna, Rokland, read 2026-10-26.
- RAK 3 dBi N-Male Fiberglass Outdoor Antenna and Bracket, Rokland, read 2026-10-26.
- Article 810: Radio and Television Equipment, EC&M (Mike Holt), read 2026-10-26.
Update history
- 2026-10-26: First published.
Spotted an error? Corrections are logged here with the date and what changed. Editorial policy.