General Coaxial Feedline Considerations

A high-level summary is presented here in quick, bulleted form.  More details are provided below. It is important to understand that the recommendations for antennas really apply to any simplex FM station, whether it is used for voice or data communications.

• The feedline consists of cable, connectors and adapters.
• Use the lowest loss components possible and practical for the situation. This is especially important when operating simplex, and most especially, simplex at 440 MHz.
• To connect to existing antennas
  • Make sure you have the appropriate adapters to connect your radio to your coax and your coax to the following types of antenna couplers
    • BNC (male and female)
    • UHF (male [PL-259] and female [SO-239])
    • N (male and female)
• When setting up your own field site antenna
  • For VHF-UHF:
    • 25 feet of LMR-240-UF (or equivalent) to reach from the antenna down to the bottom of a 32 foot push-up mast
      • LMR-240UF coax is higher-loss but well shielded and lightweight, making it useful for the vertical run up lightweight masts.
    • 50 feet of LMR-400-UF (Ultra-Flex) to go from the antenna mast to the operating station.
      • Lower-loss, double-shielded coax, such as LMR-400UF is best for the rest of the feedline (typically 25-50 feet).
  • For HF:
    • Coming soon.
• When installing a fixed site antenna
  • For VHF/UHF:
    • 1/2 Heliax (Andrew LDF4-50A) will provide the lowest loss, highest shielding; use for runs 100 feet or longer
      • Jumpers: Superflexible Heliax (FSJ4-50B) is best if the jumper doesn't need to be moved. Otherwise, LMR-400-UF is best.
    • LMR-400 (Times Microwave or equivalent) is good enough for runs 100 feet or shorter
      • Jumpers: LMR-400-UF (Ultra-Flex)
  • For HF:
    • Coming soon.

Cable Loss

• Every 3 dB of attenuation (loss) in the cable represents a loss of HALF of your transmit power (and half of the received signal's power, too)
  • For example, 6 dB is a loss of half and then half again (3 dB + 3 dB), for a total of 3/4 of your power being lost.
  • This loss is less important when communicating via a repeater. The repeaters are usually located up high, with excellent antennas and filtering so they can hear even weak signals.
  • But cable loss is VERY important when communicating simplex. And simplex operation is critical for emergency communications!
• For a given cable type, attenuation (loss) increases with frequency
• What works fine for HF, may be just OK for VHF, and terrible for UHF
• Cables with lower attenuation typically cost more than cables with higher attenuation (you get what you pay for)
• Choose the lowest loss cable that fits your needs and is within your budget

Cable Type	Approx. Diameter	Shield Type	Attenuation (db/100 ft)
			10 MHz	30 MHz	50 MHz	150 MHz	220 MHz	450 MHz
LMR-240-UF	0.240"	foil + braid	-	1.6	2.1	3.6	4.4	6.3
LMR-240	0.240"	foil + braid	0.8	1.3	1.7	3.0	3.7	5.3
LMR-400-UF	0.405"	foil + braid	-	0.8	1.1	1.8	2.2	3.3
Belden 9913	0.405"	foil + braid	0.5	0.8	1.0	1.6	1.9	2.8
LMR-400	0.405"	foil + braid	0.4	0.7	0.9	1.5	1.9	2.7
LMR-600	0.6"	foil + braid	0.2	0.4	0.5	1.0	1.2	1.7
RG-8X	0.240"	braid	1.0	2.0	2.3	4.7	-	8.6
RG-8/U	0.405"	braid	0.5	0.8	1.1	1.8	2.2	3.3
RG-213/U	0.405"	braid	0.6	1.2	1.5	2.8	-	5.2
RG-214/U	0.425"	braid x 2	0.6	0.9	1.3	2.3	-	4.5
1/2" Heliax Superflex (FSJ4-50B)	0.5"	solid	0.3	0.6	0.7	1.3	1.5	2.3
1/2" Heliax (LDF4-50)	0.5"	solid	0.2	0.4	0.5	0.8	0.9	1.4
7/8" Heliax (AVA5-50)	0.875"	solid	0.1	0.2	0.2	0.4	0.5	0.8

"-" means no data available from the manufacturer. Interpolate to get a rough idea.

Shielding

• At VHF and UHF frequencies, it is important to have as close to 100% shield coverage as possible. This not only prevents leakage of your signal out of the cable, but also prevents intrusion of other signals into the cable.
• LMR-type
  • A 100% coverage aluminum foil shield surrounded by a tinned copper braid shield
  • Advantage:
    • 100% shield coverage with lower weight and cost than other 100% coverage options
    • An excellent choice for most home and field stations
  • Disadvantage:
    • Over time, any imperfection in the cable can cause the braid and the foil to become oxidized.
    • The oxidation creates very small disconnects between the two shields wherever the oxidation occurs.
    • This is usually not a problem for simplex operations. Installations can operate for 10 or 20 or more years without an issue.
    • But this type of cable is not used for repeater installations, where both transmit and receive are happening at the same time. As the cable moves, such as when subjected to wind, those oxidized areas connect and disconnect, and the transmit power in the cable causes the cable to produce "crackling" sounds on the receive side. Well constructed repeater sites use hardline instead.
• RG-type
  • One or more braided shields, usually silver-plated copper braid
  • Each braided shield has about 95% coverage.
  • RG-214 is essentially RG-213 with two braided shields, yielding about 98-99% coverage
    • Oxidized silver is still a conductor. So even if some oxidation does occur, the "crackling" sounds usually don't occur as with LMR-type cables. Therefore, RG-214 is often used in repeater site construction, where flexibility to make tighter turns is needed, such as when connecting duplexers together.
• Hardline (Heliax is a brand name)
  • 100% solid copper or aluminum shield
  • As it's name suggests, hardline is not very flexible and, in fact, has a limit to the number of times it can be bent/unbent
  • "Superflex" hardline is more flexible than standard hardline, but is till very stiff compared to regular coax cable. It is also not meant to be constantly flexed, such as would happen at a field site. It's added flexibility is used for making tighter turns than are possible with regular hardline, such as inside a rack or cabinet.

Center Conductor

• A solid center conductor will be lower loss, but the cable will be very stiff.
• A stranded center conductor will be higher loss, but the cable will be more flexible. LMR-type "Ultraflex" (UF) cables use a stranded center conductor.

Use With HTs

• HTs (handheld transceivers) usually have SMA or BNC antenna connectors.
• The connectors on HTs are usually not designed for any significant stress and can snap off if a heavy feedline is pulling on them.
• It's best to use a short jumper of very thin coax (see below) between the HT and the feedline to provide strain relief, which will help avoid snapping off the connector on the HT.

Power Handling

• Power handing for a given cable decreases with frequency
  • For example: a given cable may be able to handle 1500 Watts at 50 MHz, but only 600 watts at 450 MHz
• Power handling for VHF/UHF FM is less of an issue for most feedlines since the power levels tend to be lower
  • For example: at a site with three radios, each transmitting at 50 watts, sharing the same tri-band antenna (144/220/440 MHz), the feedline (coax, connectors, adapters) needs to support at least 150 watts.
• Power handling for HF is a problem for smaller coaxes
  • Verify that all parts of the feedline (coax, connectors, adapters) are capable of supporting the chosen power level, which could be up to 1.5 KW.

Connectors

• UHF (PL-259 male or plug; SO-239 female or socket)
  • Most amateur radio gear uses a UHF connector.
  • The UHF connector is not a 50 ohm connector and the name is deceptive; it is not designed for use at UHF and above frequencies.
  • This means that each connection made with a UHF connector will suffer somewhat more loss than with other N-type connectors, especially when used at UHF frequencies, such as 440 MHz (70 cm band).
  • Nevertheless, because the connector is so common on amateur gear, if the site has few connections, such as a field site or a simple base station site, it makes sense to purchase coax with UHF (PL-259) connectors.
• N-type connectors (N, BNC, TNC, ...)
  • N-type connectors are 50 ohm connectors
  • The standard N connector (named after Paul Neill of Bell Labs) is about the same size as a UHF connector.
  • The BNC (Bayonet Neill-Concelman") is a smaller version of the N connector with a Bayonet-style attachment. BNC connectors are quick and simple to attach. But they can wiggle somewhat and leak RF. BNC is commonly found on some older HTs.
  • The TNC (Threaded Neill-Concelman) is about the same size as the BNC, but is threaded, like a standard N connector, making it more secure than a BNC and less susceptible to leakage. The TNC connector is not common with amateur radio equipment but is often found on commercial radios and RF test gear.
  • Although UHF-type connectors (PL-259, SO-259) are available for Heliax/hardline, the N connector is more common by far when terminating Heliax/hardline. After all, if you're going to the trouble and expense of installing hardline, it makes sense to use a low-loss, 50-ohm connector.
  • In sites with many connections in the feedline, such as stations that have bulkheads, filters, and switches, consider using N-connectors throughout the system, and converting to UHF only at the radio and/or antenna as needed. This reduces the number of sub-optimal UHF connectors to just one or two.
• SMA
  • The SMA (Subminiature version A) connectors are commonly found on HTs
  • SMA connectors are 50 ohm connectors
  • The connector is small and not suitable for use with RG-8/LMR-400 or larger cables.

Adapters

• Radio to coax
  • For mobile and base station radios, the coax can usually connect directly to the back of the radio
  • For HTs, usually a jumper of thin coax is used for strain relief, with SMA or BNC on one end to connect to the HT, and SO-239 on the other end to connect to the coax feedline. Some examples:
    • ABR Industries model 23316-SM-SO-3 (3 ft.)
    • ABR Industries model 23316-SM-SO-6 (6 ft.)
    • Comet-NCG model HS-5 (18 in.)
    • Comet-NCG model HS-10 (39 in.)
• Coax to coax
  • Usually a female-to-female UHF coupler (SO-239 to SO-239) to connect two feedlines together, each section of feedline having male UHF (PL-239) connectors.
• Coax to antenna or antenna jack
  • Assuming your coax is terminated with a male UHF (PL-239) connector, then:
    • SO-239 to SO-239 (female-to-female coupler to connect to another coax that is terminated in male UHF (PL-239)
    • SO-239 to BNC male
    • SO-239 to BNC female
    • SO-239 to N male
    • SO-239 to N female
  • If your coax uses other than PL-239 connectors, you will need adapters to connect it to the standard six types of connects we typically see in the field:
    • Male and female UHF (PL-239 and SO-239)
    • Male and female BNC
    • Male and female N
• Sources for RF coax adapters (partial list)
  • Ham Radio Outlet (https://hamradio.com)
  • RF Parts (https://www.rfparts.com)
  • DX Engineering (https://www.dxengineering.com)
  • Pasternack (https://www.pasternack.com)
  • Newark (https://www.newark.com)
  • Allied Electronics (https://www.alliedelec.com)
  • RF Coax Connectors (https://rfcoaxconnectors.com)

Weatherproofing Connections

• Weatherproof ALL permanent outdoor connections, regardless of connector type
  • N-series connectors are considered weatherproof. But no competent installer would install them outdoors without additional weatherproofing.
• If you're operating a temporary field station in the rain, you may need to apply at least some weatherproofing to any connections that are subject to water.
  • A single wrap of electrical tape is usually sufficient. Leave a flag (short piece of folded-over tape) on the end for easier removal.
• For permanent outdoor installations, more comprehensive weatherproofing is needed to protect from water and condensation. The weatherproofing itself must be resistent to UV deteriorization.
  • Self-fusing splicing tape is used underneath to block water and fill in the gaps so the outer layer can go on smoothly. An example is 3M Temflex 2155 Ruber Splicing Tape.
  • High quality electrical tape is used on the outside, to futher protect from water and to resist UV deteriorization. Examples are Scotch Super 33+ or Super 88.
  • Professional installers typically use three layers of electrical tape, with the last layer wrapped going upward (if the cable is vertical), so that the overlaps look like shingles on a house.
• DX Engineering has an excellent paper on Weatherproofing Coaxial Cable Connections

Cable Assemblies

• Many operators choose to purchase pre-terminated cable assemblies rather than soldering and/or crimping the connectors themselves
• It's probably hard to beat the quality and consistency of cable assemblies from reliable vendors, unless you are really, really good
• Some sources of coaxial cable assemplies (cables pre-terminated with connectors) are (partial list):
  • ABR Industries (https://abrind.com)
  • Cablexperts (https://www.cablexperts.com)
  • DX Engineering (https://www.dxengineering.com)
  • Pasternack (https://www.pasternack.com)
  • Talley (https://www.talleycom.com)
  • Ham Radio Outlet (https://hamradio.com) resells some of the above

Notes on Some Specific Feedline Configurations

• Fixed sites
  • The SCCo EOC and the packet BBS locations use 1/2 inch Heliax (LDF4-50A), with jumpers made from 1/2 inch Superflexible Heliax (FSJ4-50B) or LMR-400 Ultraflex (or equivalent).
• Field sites
  • LMR-240UF (or equivalent) is very high loss, but it is useful for the vertical portion of the cable run with lightweight masts used in portable field stations. Assuming a 32 foot mast and a roll-up j-pole of about 6 feet in length, a 25 foot length of LMR-240UF will reach down to the bottom of the mast where it can be connected to lower-loss cable. An additional 25 to 50 feet of high-quality, low-loss coax will likely be needed to reach from the base of the mast to the radio, especially if the radio is located indoors and the antenna is outdoors. LMR-400 Ultraflex (or equivalent) is popular and provides a good compromise between flexible deployment and low loss at UHF frequencies.