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There are several forms of transmission line.
The most popular type in Amateur radio is coaxial cable, consisting of a central conductor of solid or stranded wire, surrounded by an insulator, also called the dielectric. Around this is a metallic shield, consisting of braid, or foil and braid; or of a corrugated or smooth copper or aluminium tube.
The nature of the dielectric has some affect on the characteristic impedance, and significant impact on the velocity factor of the cable.
A fairly popular alternative consists of two parallel conductors, such window line, a large version of TV ribbon, with large windows cut out, which are often 450 ohms. The next step up is ladder-line, with some form of spacer, often with un-insulated wire, and an impedance of 600 ohms. High power HF broadcasting often uses open wire feed-lines strung tightly between posts with a pair of glass or ceramic insulators, similar to power or more accurately, old-style open-wire telephone lines. These are also 600 ohms. 300 ohm TV ribbon has some application as well. Little known, but ordinary figure-8 speaker wire, or mains zip-line can be used, especially at HF, to feed dipoles.
Off the exam, there are also a range of more complex open wire arrangements, use on broadcast site, both things like 4-wire balanced systems; and unbalanced ones with a single centre signal wire, and 4 grounded wires, sort of emulating coax.
RF energy travels through different cables at different speeds, termed the velocity factor, a factor which represents the proportion of the speed of light at which the signal travels. The slowest propagation, and the lowest VF is in coax using solid polyethylene, a tough translucent plastic, as the dielectric, at around 0.66. PTFE (Teflon) is similar if solid, at 0.695. If either is in foam form, then signals propagate faster, around 0.78 to 0.88, depending on the nature of the foam. Air-spaced cable is listed on Wikipedia as having a VF of "about" 1. As we know, signals cannot exceed c, the speed of light in a vacuum.
Occasionally the VF is written as a percentage.
This velocity factor might not matter if you are connecting a 2 metre rig to a resonant antenna on a vehicle, or connecting to a simple dipole at home, but comes into play when you do cool stuff using fractional wavelengths of coax.
An example of this is the quarter-wave transformer, made using a length of coax. The most common is using a piece of 75 ohm cable to transform from a 100 ohm antenna, such as a loop, to 50 ohm feedline. For a specific frequency, how long this is depends on the VF of the cable.
Thus, if we have RG-59/U, with solid PE, and a VF of 0.66 we multiply the free-space quarter-wave figure by 0.66. Thus, if we are in the CW sub-band at 50 MHz, 300 / 50 x 0.25, we get 1.5 metres. In this case we cut the cable to 1.5 x 0.66 = 0.99 metres. If we have RG-59A/U with a foam dielectric, and thus a VF of 0.78, we need 1.5 x 0.78 = 1.17 metres of cable.
You can see one on my Delta Antenna page which opens in a new window.
The parallel line discussed above has a fairly high VF, and reverse engineering the question about it, the examiner is now assuming the figure is so close to 1 as to not be calculated.
VF is also applied in the following cases:
If a quarter-wave can transform, as above, then if the end is shorted, then the opposite, an open will appear at the other end; and if opened it appears as a short. Note that this is termed as a very low, or very high impedance. Note that this only applies at the frequency at which it is a quarter-wave.
Off the exam, these can be used as "stub" filters, meaning a shorted quarter-wave, which appears open provides a band-pass filter. An open stub, which appears as a short, can be used as a notch filter. A downside of these filters is that while they can be effective for blocking a harmonic at twice the fundamental, they tend to pass signals at 3 times the fundamental, as the stub appears as a 3/4 wave.
A half-wave length of cable passes the impedance it "sees" in the load to the source.
In the past, it was said that a VHF feedline should be a multiple of electrical half-wavelengths long. I expect that if an odd multiple of a quarter wave of 50 ohm coax was connected to a 102 ohm loop antenna a low impedance would be presented to the radio.
Two further cases are the shorted and open lengths of 1/8 wavelength. A shorted piece appears inductive reactance, while an open section appears as a capacitive reactance.
In various situations we need to divide power from a single feedline into two loads, typically two antennas. An example is feeding two folded dipoles in a repeater system. Originally designed by Ernest J. Wilkinson for microwave work, and published in 1960, they can be constructed using quarter-wave lines in PCBs, or using coaxial cable. These lines or cables act as transformers, as previously discussed, and as such, you do need to take VF into account.
Optionally, a resistor of twice the system impedance is placed between the two output ports, and this helps cancel noise, and provide isolation between the outputs. This is probably more important if the splitter is feeding amplifiers.
A common configuration takes a 50 ohm input, and uses two λ/4 lengths of 70 to 75 Ω and outputs at 50 ohms. Given a 50 ohms load, coax of around 70 ohms, the driven end of EACH coax will present a 100 ohm impedance. Place two in parallel, and we see 50 ohms. Nice!
For a repeater a fairly thick 75 Ω old stock LDF4-75A 1/2” Heliax, or the current option, LMR600-75. Something like Belden 9292, a quality RG-11 may be an option.
Thus the resistor between the outputs, if used, would be 100 ohms.
Another use is to feed the two feedpoints spaced 90 degrees along the boom on a × or + format circularly polarised Yagi, used for VHF or UHF satellite work. The is different to the system on the previous page. CP Yagis, or arrays of them, are also useful for EME / Moonbounce.
Two additional features of the Wilkinson is that the levels on both outputs are at the same level (that is, the division is equal), and the same phase. The exception is when unequal impedance branches are used in a PCB splitter, by using tracks of different width. PCB dividers at high microwave frequencies can be just 2 × 3 mm. A web search will show various PCB layouts, including the changes in track width in the input line, the quarterwave sections, and the output lines.
A possible design using coax, above. If not using the resistor, you can use an N, BNC, or UHF Tee-Connector to join the cables. This allows them to immediately run towards the two folded dipoles. Some 50 ohm cable is likely needed. With a CP yagi this may not be needed. Waterproofing mastic tape, black electrical tape, and grey electrical tape is required if using on a tower. The last layer might look like the galvanised tower legs to cockatoos, so they leave it alone.
Yagi and similar antennas often have the driven element split into two sections at the point it crosses the boom, and plastic used to mount the elements, with the disadvantages that it is weaker, and the plastic eventually degrades. Several other arrangements, which use a continuous driven element, and also allow them to be grounded (that, is, directly connected to the boom. Some are named for the Greek character which they look most like.
The Delta match looks like the upper-case delta Δ. The feedline is spread to connect some distance either side of the centre-point of the driven element.
The Gamma match looks like an upper-case Γ. Most often this consists of a coaxial connection, with the shield connected to the centre point. The inner is connected via a capacitor to a tube running parallel to the element, and connected some distance from the centre point. While a discreet capacitor may be included, often the connection from the coax is to a thick wire running within, and insulated from, the tube, forming a capacitor.
The stub match uses an additional short section of coaxial cable. It may be in a U shape for convenience.
The Beta or hairpin match typically consists of a hairpin shaped loop or stub of aluminium wire. It is also called the Beta matching system, or the hairpin match, both used below. The driven element must be split, and insulated from the boom. The centre of the loop may be grounded. I am unsure of the reason for the term beta, as I can't see how it looks like Β or β. To cancel the inductance of the hairpin the driven elements must be made shorter than a halfwave, so they are capacitive.
These are actual questions from the NCVEC Extra exam pool.
E9E01
Which matching system for Yagi antennas requires the driven element to be insulated from the boom?
A. Gamma
B. Beta or hairpin
C. Shunt-fed
D. T-match
This is the beta match, also termed the hairpin match for its shape, answer B.
E9E02
What antenna matching system matches coaxial cable to an antenna by connecting the shield to the center of the antenna and the conductor a fraction of a wavelength to one side?
A. Gamma match
B. Delta match
C. T-match
D. Stub match
The is the Gamma match, Γ, answer A.
E9E03
What matching system uses a short length of transmission line connected in parallel with the feed line at or near the feed point?
A. Gamma match
B. Delta match
C. T-match
D. Stub match
The match which uses a section or stub of transmission line is the stub match, answer D.
E9E04
What is the purpose of the series capacitor in a gamma match?
A. To provide DC isolation between the feed line and the antenna
B. To cancel unwanted inductive reactance
C. To provide a rejection notch that prevents the radiation of harmonics
D. To transform the antenna impedance to a higher value
The capacitor cancels the unwanted inductance of the matching network, answer B.
E9E05
What Yagi driven element feed point impedance is required to use a beta or hairpin matching system?
A. Capacitive (driven element electrically shorter than 1/2 wavelength)
B. Inductive (driven element electrically longer than 1/2 wavelength)
C. Purely resistive
D. Purely reactive
The element mast be made shorter that a halfwave, and thus capacitive to counter the inductive nature of the hairpin match, answer A.
E9E06
Which of these transmission line impedances would be suitable for constructing a quarter-wave Q-section for matching a 100-ohm feed point impedance to a 50-ohm transmission line?
A. 50 ohms
B. 62 ohms
C. 75 ohms
D. 90 ohms
Half-way between 100 and 50 is 75 ohms, answer C.
These are great for matching a delta loop to a 50 ohm cable.
E9E07
What parameter describes the interactions of a load and transmission line?
A. Characteristic impedance
B. Reflection coefficient
C. Velocity factor
D. Dielectric constant
Mismatch generates reflections, so it is reflection coefficient, answer B.
E9E08
What is a use for a Wilkinson divider?
A. To divide the operating frequency of a transmitter signal so it can be used on a lower frequency band
B. To feed high-impedance antennas from a low-impedance source
C. To divide power equally between two 50-ohm loads while maintaining 50-ohm input impedance
D. To divide the frequency of the input to a counter to increase its frequency range
The Wilkinson divides power equally between two outputs, while preserving impedance. Answer C.
E9E09
Which of the following is used to shunt feed a grounded tower at its base?
A. Double-bazooka match
B. Beta or hairpin match
C. Gamma match
D. All of these choices are correct
A Gamma match can also be used to feed a grounded tower, answer C.
E9E10 has been deleted.
E9E11
What is the purpose of using multiple driven elements connected through phasing lines?
A. To control the antenna's radiation pattern
B. To prevent harmonic radiation from the transmitter
C. To allow single-band antennas to operate on other bands
D. To create a low-angle radiation pattern
These control the radiation pattern of the antenna, answer A.
The phasing lines ensure that each of the driven elements is driven with the correct phase, so they work in concert. While this applies to things like HF towers, if you see a phased array VHF TV antenna, popular in rural Australia, or a bow-tie UHF one, you will see the central feedpoint, with parallel tubes of wires running to the two centre-most driven elements, then these lines crossing over to feed the two outer-most (or the top and bottome ones in Horizontal areas).
E9F01
What is the velocity factor of a transmission line?
A. The ratio of its characteristic impedance to its termination impedance
B. The ratio of its termination impedance to its characteristic impedance
C. The velocity of the wave in the transmission line multiplied by the velocity of light in a vacuum
D. The velocity of the wave in the transmission line divided by the velocity of light in a vacuum
This is the velocity at which a radio wave travels in the cable divided by the speed of light, answer D.
If a signal travels at 200,000,000 metres per second, this over 300,000,000 is 0.667, the VF of typical coax with a solid PE (polyethylene) dielectric.
E9F02
Which of the following has the biggest effect on the velocity factor of a transmission line?
A. The characteristic impedance
B. The transmission line length
C. The insulating dielectric material
D. The center conductor resistivity
This is the nature of the dielectric material, including whether is is air, solid or foamed, answer C.
Maybe they have tried to make the answer more obvious but dielectric and insulating means pretty much the same thing.
E9F03
Why is the electrical length of a coaxial cable longer than its physical length?
A. Skin effect is less pronounced in the coaxial cable
B. Skin effect is more pronounced in the coaxial cable
C. Electromagnetic waves move faster in coaxial cable than in air
D. Electromagnetic waves move more slowly in a coaxial cable than in air
Signals travel more slowly in coax than in air, answer D.
6.6 metres of 0.66 VF cable would be electrically a wavelength at 30 MHz, which has a free space wavelength of 10 metres. It would also be a quarterwave at 7.5 MHz.
E9F04
What impedance does a 1/2-wavelength transmission line present to an RF generator when the line is shorted at the far end?
A. Very high impedance
B. Very low impedance
C. The same as the characteristic impedance of the line
D. The same as the output impedance of the RF generator
A half-wave of coax transfers the impedance, so it looks like a short (very low impedance) in this case, answer B.
E9F05
What is microstrip?
A. Special shielding material designed for microwave frequencies
B. Miniature coax used for low power applications
C. Short lengths of coax mounted on printed circuit boards to minimize time delay between microwave circuits
D. Precision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies
This are printed circuit board traces of precise width, above a groundplane, with the thickness of the board an important factor, answer D.
E9F06
What is the approximate physical length of an air-insulated, parallel conductor transmission line that is electrically 1/2 wavelength long at 14.10 MHz?
A. 7.0 meters
B. 8.5 meters
C. 10.6 meters
D. 13.3 meters
This is the 20 metre band, so half-wave is around 10 metres. Calculating it, 299.7 / 14.1 = 21.255 m. 21.255 / 2 = 10.6275 metres, answer C.
You will note they have not factored in a VF, as this is fairly high (close to 1) with this cable type.
E9F07
How does parallel conductor transmission line compare to coaxial cable with a plastic dielectric?
A. Lower loss
B. Higher SWR
C. Smaller reflection coefficient
D. Lower velocity factor
The only valid answer is that is has lower loss, answer A.
E9F08
Which of the following is a significant difference between foam dielectric coaxial cable and solid dielectric coaxial cable, assuming all other parameters are the same?
A. Foam dielectric coaxial cable has lower safe maximum operating voltage
B. Foam dielectric coaxial cable has lower loss per unit of length
C. Foam dielectric coaxial cable has higher velocity factor
D. All of these choices are correct
All factors are fairly logical, and are correct, answer D.
E9F09
What impedance does a 1/4-wavelength transmission line present to an RF generator when the line is shorted at the far end?
A. Very high impedance
B. Very low impedance
C. The same as the characteristic impedance of the transmission line
D. The same as the generator output impedance
A quarterwave reverses the impedance, so a very high impedance (essentially an open circuit) is seen, answer A.
E9F10
What impedance does a 1/8-wavelength transmission line present to an RF generator when the line is shorted at the far end?
A. A capacitive reactance
B. The same as the characteristic impedance of the line
C. An inductive reactance
D. Zero
Like most closed loops, this is an inductive reactance, answer C.
E9F11
What impedance does a 1/8 wavelength transmission line present to a generator when the line is open at the far end?
A. The same as the characteristic impedance of the line
B. An inductive reactance
C. A capacitive reactance
D. Infinite
This presents a capacitive reactance, answer C.
Like two insulated conductors in parallel, this behaves like a regular capacitor.
E9F12
What impedance does a 1/4 wavelength transmission line present to an RF generator when the line is open at the far end?
A. The same as the characteristic impedance of the line
B. The same as the input impedance to the generator
C. Very high impedance
D. Very low impedance
A quarter-wave section reverses the impedance, so it appears like a near short (low impedance), answer D.
Two deleted questions relied on knowing the VF of cable types. The first of questions asked the approximate physical length of a solid polyethylene dielectric coaxial transmission line that is electrically 1/4 wavelength long at 14.1 MHz?
The wavelength is 299.7 / 14.1 = 21.255 metre. Thus it is 21.255 / 4 x 0.66 = 3.507 metres. A check is to remember it is 20 metre band, so the a quarter-wave is space is about 5 metres. Solid polyethylene has a VF of 0.66, so this is about 3.3 metres.
The other was wanted the approximate physical length of a foam polyethylene dielectric coaxial transmission line that is electrically 1/4 wavelength long at 7.2 MHz? 299.7 / 7.2 x 0.25 x 0.80 = 10.40625 x 0.80 = 8.325 metres. To check: This is the 40 metre band, so a quarterwave is 10 metres in free space, multiply by the VF of 0.8, and we get about 8 metres.
Loop antennas consisting of a wavelength of wire, such as the Delta have a 102 ohm impedance. A 75 ohm coax of a quarter wavelength transforms this to 50 ohms, or close enough that the SWR is close to 1:1.
On to: Antennas 4 - Smith Charts; Loops and Beverage Antennas
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, February 2026.
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