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Amateur Radio Info & Exams - Feedlines and SWR

Feedlines connect radio equipment and antennas. They have a characteristic impedance, based on the type of feedline, and on diameters and spacing.


Coaxial cables, meaning the round conductors are centred on the same axis. The characteristic impedance of coaxial is related to the ratio between the outer diameter of the inner conductor, and the inner diameter of the shield. Thus for a cable of ~5mm shield inner diameter, then the 75 ohm version (RG-59) will have thinner internal wire than the 50 ohm (RG-58). Off the exam, 93 ohm coax has very thin internal wire, as does the special high impedance cable used for car radios, required as the antenna is a very small percentage of a wavelength for MW and LW AM signals.

A fair compromise between power handling and receive signal loss, 50 ohm coax is generally use for Amateur and CB transceivers, and in most other two-way systems.

75 ohm cable is used in receive-only systems, such as terrestrial TV, satellite TV, FM and DAB+ radio, and cable-TV feed-ins. If a length of 75 ohm coax, such as the heavy street cable for pay-TV, there are ways to use this for transmitting. This involves using a twelfth wavelength of 75 ohm cable, then a twelfth of a wavelength of 50 ohm cable, before joining the length of 75 ohm cable, before repeating at the far end to transition to the final link to the 50 ohm antenna. A quarter wavelength of something line RG-59, RG-6, or maybe RG-11, can be used to connect a 102 ohm delta loop to a 50 ohm feedline to the radio.

An upside of coax is that it can be taped to metal poles, or tower legs. For flexible plastic coated braid cable, zip-ties or cable-ties should not be used, as they distort the cable, and affect the impedance. For Heliax these can be used, ideally the stainless steel version, as they are not affected by UV light. Black is much preferable to natural or white nylon.

Coaxial, along with shielded audio or video cable with a single internal wire is termed "unbalanced".

Parallel feedline

TV antenna "ribbon", "ladder line", and "window line" all have two lines in parallel. The distance between the centres of the conductors, and the radius of these conductors, determines the characteristic impedance. TV ribbon, with only around 10 to 12 mm spacing has 300 ohm impedance. Larger, wider-spaced cable can have a 450 ohm impedance, some 600 ohm.

Rarely mentioned, apparently speaker cable has an impedance of around 200 ohms, and can be used up to around 200 MHz.

These cable must be spaced away from metal, by several times the spacing between conductors.

For these feedlines, it is the diameter of the wires and distance between them, NOT the length or the frequency of the signal, which determines impedance.

These feedlines are termed "balanced". The term is also applied to professional audio cable with a pair of internal wires, and a shield; and to "twisted pair" wire.

Mismatch (SWR)

When an antenna is not resonant, there is a mismatch, such as using a 40 metre antenna on 20 metres. Likewise, if an antenna IS resonant, but the impedance is different, such as because it is a delta loop, or other loop, and you just connect 50 ohm cable to it, then a mismatch will also occur.

The mismatch is expressed as an SWR ratio.

Thankfully, determining the ratio is quite easy: If we have a 50 ohm system and the antenna's impedance is 100 ohms, then the ratio is a 2:1. Likewise, if the impedance is 25 ohms, the SWR ratio is 2:1. The larger number also goes on the left of the colon, and always a 1 on the right.

Oversized Coax

For each size of cable there is an upper limit for its use, after which point starts to behave simultaneously as both cable and waveguide, a bad thing. While normally applying to microwaves, ~20 cm diameter hardline used for UHF TV transmission is specified by highest channel it can be used on.

Examples of Coax

These are cables useful for hams, provided as practical info only. RG (Radio General) series, URM67, and H-series cables can be made by made by any manufacturer. LMR is a trademark of Times Microwave, hence other letters on competing products. These are 50 ohms, unless noted otherwise.

RG-58/U, RG-58C/U, and H155 is good for short jumpers and mobile use on 2 metres, and medium runs at 100 watts on HF. The /A version has a single-strand core which will fail from flexing. RG-8X is similar. These have a ~6 mm overall diameter.

RG-8/U, RG-213/U, and URM67 are good for home and field-day VHF and UHF stations; and at HF, including for higher powers. These are ~10 mm overall. RG-214 is the double-shielded version.

LMR- series cables have 3 or 4 digits specifying the size in 1/1000 of an inch, and are comparable with similar sized products. There are several alternatives, such as CFD-400.

RG-223/U is a nice double-shielded cable, with a 5.4 mm overall diameter. RG-400/U is a teflon insulated cable with a silver-plated copper core, and double silver-plated shields, with a tan FEP sheath. It is used in aircraft, and can withstand 200°C, but is costly.

Thin (0.1" or 2.5 mm) coax may me used at moderate power for hike-in operations, due to its small size. RG-58 sized "CELL-FOAM" cables, designed for in-car mobile 'phone installations is also light in weight, and fairly low in loss at UHF.

RG-59/U is a 5mm 75 Ω cable, useful for matching sections at low power. It is also good for composite video, etc. Various RG-6 cable are around 7 mm overall, and is probably best terminated into F-connectors. RG-11 series cables are 10.5 mm overall 75 Ω cables.

If you are offered cable which you don't recognise, you can always do a web search to see its specifications. Also, provided you can get suitable connectors, or they are fitted, there is nothing wrong with using better than necessary cable.

Heliax, from Andrew, is a range of cables with a corrugated copper shield. The inner can be copper-clad aluminium or corrugated copper. In between is foamed plastic or Teflon spacers, making the dielectric mostly air. The smallest is FSJ1-50 (or its cousin FSJ1-75) with a roughly ¼" shield internal dimension. The sheath is a tough black plastic. Overall size is from 10 mm to around 50 mm. LDF4-50 is another example. Special connectors are needed. These are great for high performance VHF and UHF systems, such as moon-bounce and weak signal DX work; and in repeater systems, which may have feedline lengths of over 100 feet (30 m) to a tower mounted antenna.

Relevant Questions

These are actual questions from the General exam pool.

Which of the following factors determine the characteristic impedance of a parallel conductor antenna feed line?
A. The distance between the centers of the conductors and the radius of the conductors
B. The distance between the centers of the conductors and the length of the line
C. The radius of the conductors and the frequency of the signal
D. The frequency of the signal and the length of the line

This is the distance between the centre of each conductor, and the radius of those conductors, answer A.

What are the typical characteristic impedances of coaxial cables used for antenna feed lines at amateur stations?
A. 25 and 30 ohms
B. 50 and 75 ohms
C. 80 and 100 ohms
D. 500 and 750 ohms

The most usual one is 50 ohms, with 75 ohms having various uses, answer B.

What is the typical characteristic impedance of "window line" parallel transmission line?
A. 50 ohms
B. 75 ohms
C. 100 ohms
D. 450 ohms

Feedlines consisting of spaced parallel wires have higher impedance than coax, in this case 450 ohms, answer D.

What might cause reflected power at the point where a feed line connects to an antenna?
A. Operating an antenna at its resonant frequency
B. Using more transmitter power than the antenna can handle
C. A difference between feed line impedance and antenna feed point impedance
D. Feeding the antenna with unbalanced feed line

If the antenna feed point has a different impedance to the feedline, this will cause a reflection, answer C.

Any step in impedance can cause reflections.

How does the attenuation of coaxial cable change as the frequency of the signal it is carrying increases?
A. Attenuation is independent of frequency
B. Attenuation increases
C. Attenuation decreases
D. Attenuation reaches a maximum at approximately 18 MHz

For a certain cable, attenuation increases with frequency, answer B.

In what units is RF feed line loss usually expressed?
A. Ohms per 1000 feet
B. Decibels per 1000 feet
C. Ohms per 100 feet
D. Decibels per 100 feet

For RF feed-line, the loss is in decibels per 100 feet, answer D.

These are published for spot frequencies, so you might use the 10 MHz one for low-mid HF, the 30 MHz one for 10 metres, and the 150 MHz one for 2 metres. Overseas a metric distance will often be used, even if this is 30 metres.

What must be done to prevent standing waves on an antenna feed line?
A. The antenna feed point must be at DC ground potential
B. The feed line must be cut to a length equal to an odd number of electrical quarter wavelengths
C. The feed line must be cut to a length equal to an even number of physical half wavelengths
D. The antenna feed point impedance must be matched to the characteristic impedance of the feed line

The antenna needs to be adjusted so that its impedance matches that of the feedline, answer D.

For antennas where we are using a section of 75 ohm to match a 102 ohm loop, then the end of this matching piece is where we must have 50 ohms.

Answers B and C conflate an old idea of using a number of electrical half wavelengths.

If the SWR on an antenna feed line is 5 to 1, and a matching network at the transmitter end of the feed line is adjusted to 1 to 1 SWR, what is the resulting SWR on the feed line?
A. 1 to 1
B. 5 to 1
C. Between 1 to 1 and 5 to 1 depending on the characteristic impedance of the line
D. Between 1 to 1 and 5 to 1 depending on the reflected power at the transmitter

All the tuner (matching network) does is stop the nasty impedance damaging the transmitter, or causing it to fold-back its output power. The high voltage, or high current caused by the mismatch is still present on the feedline, along with the loss this cuases, so 5:1, answer B.

What standing wave ratio will result when connecting a 50 ohm feed line to a non-reactive load having 200 ohm impedance?
A. 4:1
B. 1:4
C. 2:1
D. 1:2

One is four time the value of the other, so the ratio is 4:1, answer A.

The large number is always to the left, so 12.5 ohms would also cause a 4:1 mismatch.

What standing wave ratio will result when connecting a 50 ohm feed line to a non-reactive load having 10 ohm impedance?
A. 2:1
B. 50:1
C. 1:5
D. 5:1

The ratio of the largest value to the smallest is 5:1, answer D.

What standing wave ratio will result when connecting a 50 ohm feed line to a non-reactive load having 50 ohm impedance?
A. 2:1
B. 1:1
C. 50:50
D. 0:0

Both are 50 ohms, so the ratio is 1:1, answer B.

What is the interaction between high standing wave ratio (SWR) and transmission line loss?
A. There is no interaction between transmission line loss and SWR
B. If a transmission line is lossy, high SWR will increase the loss
C. High SWR makes it difficult to measure transmission line loss
D. High SWR reduces the relative effect of transmission line loss

High SWR can cause a greater loss, answer B.

What is the effect of transmission line loss on SWR measured at the input to the line?
A. The higher the transmission line loss, the more the SWR will read artificially low
B. The higher the transmission line loss, the more the SWR will read artificially high
C. The higher the transmission line loss, the more accurate the SWR measurement will be
D. Transmission line loss does not affect the SWR measurement

If you have something like 10 metres of RG-58 going to an HF antenna, and you were to connect a 2 metre transmitter to this line, then the loss in the cable means that a limited amount of power reaches the antenna's feedpoint. A certain amount of power will be reflected back to the radio, but again, at 147 MHz, this cable has a fair amount of loss. Thus the possibly high SWR of the antenna is masked. The loss makes the SWR meter read artificially low, answer A.

This does actually work, as while the power radiated may be low, the antenna is at least outside, and somewhat elevated. The losses mean that the SWR the radio sees is acceptable, and no expensive smoke will escape.


For your PC and/or Raspberry Pi, as well as connecting home entertainment equipment, and various projectors, HDMI (High-Definition Multimedia Interface) is a digital protocol and cable standard. It supersedes DVI (Digital Visual Interface), with some compatibility. The most important thing to note is that, being digital, any cable complying with the standard for the bit-rate, and thus the resolution and frame rate, will give quality video and audio. And it can't make up for excessive compression on video streaming services or pay-TV.

Why an I raising this? Because, if you head off to the Hi-Fi warehouse shop, with their fake hand-written price posters, and get a TV or player for a good price, the salesperson will then "helpfully point out" that there are only basic cables in the box, and that you need quality cables, which are their own brand, made in China for a dollar or two, but selling for $25 or $80, or whatever.
Affordable examples: A 2 metre HDMI version 2.0 from E14 or a 3 foot HDMI 1.4 from Core Electronics.

Now to connect your feedlines to some: Antennas!

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, April 2022.

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