Radio Frequency (RF) & Wireless Design

Wire antennas are usually used for RF frequencies at High Frequency (HF) = 30 MHz and below, all the way down to AM radio 540 KHz. The use of wires is due to the very long wavelengths for 30 MHz (10 meters = 32.81 feet) down to 0.5 MHz or 500 KHz (600 meters = 1968.5 feet), which are massive ! So with cost and area constraints, wire antennas are the best approach.

Ladder lines as RF transmission lines have been used since the invention of radio over 100 years ago. The more recent lingo is called Twin-Lead and comes from a manufactured process of separating wires with a plastic; typically 300 ohms : https://en.wikipedia.org/wiki/Twin-lead#Ladder_line

So what is the difference between a ladder line and twin-lead ? Mostly construction and impedance. Ladder lines are usually just bare wires that are separated by an air-gap with insulators ; historically just wood. More modern versions of ladder lines are bare wires that are separated by plastic spacers (e.g. PVC, Delran, Plexiglas, Polycarbonate, ... etc).

A typical ladder line starts at a gap of about 1 inch (25.4 mm) and goes up to about 6 inches (152.4 mm). The impedance is calculated from elementary electromagnetic physics of two opposite current carrying air gap conductors.

https://hamwaves.com/zc.circular/en/

As the wires get closer together, the impedance goes down, but since the impedance is calculated from the square of the distance, it is exponential not linear and the impedance falls off sharply. Therefore holding an impedance with closely spaced wires is very difficult ! This is where twin lead fully insulated wire comes into play.

Twin wire historically has been used for older television sets to match the antenna dipole (rabbit ears) to the television as a receiver. The 300 ohm impedance is held very consistently by vinyl covered wires. However, the vinyl itself has dielectric losses and will cause some losses thru a property called the Velocity Factor :

https://en.wikipedia.org/wiki/Velocity_factor

The velocity factor is just the inductive and capacitive elements of the wire affected by the spacers and the spacing in how fast the RF signals react to the incoming wave front. A typical coax has ~ 30 pF per foot of capacitance and as this distributed capacitance delays the propagation of the RF wave moving thru the coax, there is a phase shift.

https://www.dj0ip.de/open-wire-fed-ant/openwire-info/

For a digital analogy think of it as the rise-time. The velocity factor will change the phase of the antenna as well as incur power losses. This is why coax suffers so much worse that twin-lead or ladder lines. So if you want more power to (or from) the antenna, its time to improve your transmission line. A ladder line does this with efficiencies > 95% compared to even high quality, high cost LMR coax at about 85%.

Another problem that a ladder line solves is a back EMF current on the outside of coax that causes the outer shield to be part of the antenna, since the coax that feeds directly to a dipole antenna is directly inside the Near Field of the radiation pattern.

https://en.wikipedia.org/wiki/Electromagnetic_radiation#Near_and_far_fields

Typically coax to an antenna suffers from this problem and coax chokes, baluns, and other methods are used to suppress this unwanted EMI. Ladder lines will not suffer this affect since both wires are affected but the common mode noise and will cancel the affect as long as there are no nearby metal objects near the ladder line.

A typical good antenna installation will use a ladder line as far as possible to a balun that will convert the impedance to the typical 50 ohm transceiver impedance. Better systems use an antenna tuner as the balun to match both transmission lines and SWR at the same time. These are usually remote controlled from the station to maximize and optimize power transmission to the antenna. http://www.arrl.org/random-length-multiband-dipoles

https://www.qsl.net/wb1gfh/antenna.html

https://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htm

This article explains how a poorly designed (but well intentioned) feed line where a balun is used before the antenna tuner :

https://owenduffy.net/blog/?p=9915

For an introduction and the spacing effects, I recommend reading this web article on the subject : https://www.qsl.net/co8tw/openline.htm

Note that larger impedances at 600 ohms will suffer lower losses, but 450 ohms is still very good ! Typically 600 ohms ladder line is constructed with spacers, but 450 ohms ladder line may be purchased pre-made or plastic insulator formed with window cut-outs. The lesser the insulation connecting the wires, the lower the losses, but the harder to control the transmission line impedance, so trade-offs will be made. Here is preformed 450 ohm ladder line : https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1993/12/page70/index.html

Note that anyone can create a low cost, highly optimized antenna with simple wire and ladder line spreaders within just an hour ! So it is not necessary to spend a lot of money to get good performance from an antenna, but it is necessary to spend time working on the details ! Here's some MS-DOS days 'READ ME FIRST' guidelines :

https://www.trueladderline.com/technical-info/

Here are some commercial ladder line spreaders :

http://www.dtsohio.com/73cnc/laddersnap.html