In his first regular column on aerials for RadioUser, Keith Rawlings provides in-depth guidance to standing wave ratios and advises on the various feeders, lines and cables available for aerial construction.  

 

Welcome back to Aerials Now! In my pilot-article for this new column last month (RadioUser, June 2018: 8-11), I discussed some basic terms such as impedance and matching. This time around, I wish to look a little more at issues such as standing wave ratios and losses on feeder cables.

If the impedance of an aerial does not match that of the feeder – or of the radio it is connected to – this will lead to a mismatch. In the case of reception, power from the aerial will not fully reach the input of a receiver.

The term for this situation is standing waves and it is measured by a parameter called Voltage Standing Wave Ratio (VSWR). Put very simply, if we have an VSWR of 1.5:1 on our system, then 4% of the power is reflected. At a ratio of 2:1, just over 11% will be reflected; at a ratio of 3:1, some 25% will be reflected.

 

Feeders, Lines and Cables

That part of any system that transfers the signal from the aerial to the radio is termed the feeder. Commonly-found examples are (but not limited to) coaxial cable, open wire line, ribbon feeder and twin line.

The photograph in Fig. 1 illustrates some examples of feeder alternatives.

From left to right, you can see the following:

 

• A 75Ω twin line
• Two types of 300Ω ribbon feeder
• A 300Ω slotted ribbon cable
• A 450Ω slotted ribbon cable
• A cheap RG58 coaxial cable
• RG95 75Ω coaxial cable
• 75Ω PTFE TV coax with very thin braid
• 75Ω double-screened TV coax

 

The open wire line, ribbon feeder and twin line feeders tend to be used at HF frequencies, while coaxial cable finds uses from HF to UHF (and sometimes higher).

The more common types of coaxial cable encountered by RadioUser readers can be found with impedances of 50 and 75Ω but be aware other values can be found, such as 95 and 125Ω.

While coaxial cable is very convenient to use, it carries a penalty and that can be signal loss. For a given cable, the longer the run, and the higher the frequency, the greater the loss.

Looking at specifications, if cable such as RG58 is used for a 1090MHz Mode-S aerial, with a run of 50ft, then you can expect to lose around some 11 to 12dB (about two S-points).

At 10MHz, this same run of cable would typically exhibit a mere 0.7dB loss.

If using RG213 at 1090MHz, you can expect to lose approximately 4dB.

This is a great improvement but still more than half the signal.

You should avoid cheap coaxial cable, as this can be a false economy. This type of cable can be lossy. Very often, the screening is of poor quality, which can lead to signal leakage and interference.

It is tempting to use 75Ω TV cable and what I should be saying to you is that – even though it is relatively cheap and performs well – it is really not the thing to do.

However, as I have two aerials being fed by 75Ω Sat cable I am in no position to do so!

Just bear in mind that on a 50Ω system, there will be a mismatch when using 75Ω coaxial cable, and this will add to your losses.

You can go some way to minimise this mismatch on any aerials you make yourself, by tuning your aerial to match to coaxial cable.

Ribbon feeder and open wire line, on the other hand, are very different from coaxial cable. Ideally, they do not like to be poked through holes in a wall or be left laying against something. In fact, open wire line can be delicate. In some cases, it is not too happy to get wet. It certainly does not like to be brought down the side of a metal mast.

However, if you do use the correct type for the job and give it a straight run with no sharp bends, it is quite happy and will return this happiness to you by exhibiting extremely low loss.

Usually, radio hobbyists use the open wire line and ribbon feeder variants on HF. In terms of impedance, however, this is far from the 50Ω ideal many are looking for.

Cheap plain ribbon feeder is 300Ω and hates being wet.

The slotted variety is a great improvement and can also be found with both 300 and 450Ω impedance. Its construction can make it stronger than open wire line.

The impedance of open wire line depends on its wire spacing (and wire diameter) but is often quoted at 600Ω.

If you use any of this type of feeder on your systems, some form of matching network will usually be required.

 

An End-fed HF Aerial

A random wire end-fed (from here on just plain ‘end-fed’) really is the simplest of HF aerials. It is just that – a non-resonant, random, bit of wire. It is a good aerial to start off with and involves very little expense.

The impedance of a random length of wire will be very different to 50Ω. It is usually very much higher than even a 500Ω high-impedance input, found on some HF receivers.

To get around this, an Aerial/ Antenna Matching Unit (AMU, Fig. 2) is needed for best results. For this type of aerial, an L-Match network configuration, consisting of just a variable capacitance and adjustable inductance will work well. If you remember from last month, I explained the phenomenon of reactance.

I mentioned - jX (for capacitive reactance) and + jX (for inductive reactance).

Depending on the length of a piece of wire, it may be either ‘short’ or ‘long’, in regard to wavelength – capacitive if it is short, and inductive if it is long. The matching network can ‘electrically lengthen’ or ‘shorten’ the system by adding either inductance or capacitance.

Another problem with bringing a bit of wire right back to your operating position is that it will bring it within a short distance of interference-generating devices, possibly leading to more noise than wanted signals.

A solution is to use a run of coaxial cable from the feed point back to the AMU, preferably after your wire has been connected through a transformer (often called a BALUN or Magnetic Long Wire Balun).

A ratio of 9:1 is popular.

If this is done it is not essential to use an L-Match configured AMU. In fact, you may possibly be able to dispense with the AMU altogether, although the resulting match will be a compromise.

 

Earth, Ground Radials and Counterpoise

The above is only one half of the aerial. You will really need a good earth, ground radials or counterpoise to complete the system.

An ‘earth’ can be a copper stake driven into the ground. However, there are some safety issues here and I will leave this for the time being. You can find some more pertinent information on this here:

https://tinyurl.com/ycxvp9vn

A radial system is a series of wires that extend out from the feed point in all directions, either buried or laying on the ground. This adds some complexity.

A counterpoise is a wire – or a series of wires – placed below the main aerial as a substitute for an earth or ground connection. It/they can be laid on the ground or suspended above-ground by a few inches. Preferably, they are connected at the feed point.

Effective and simple to implement I would suggest a counterpoise to start off with.

A counterpoise ought to be at least a quarter-wave long. Multiple wires of different lengths can be used for different frequencies and can be left permanently connected. If only a single wire is used – and if this is made to the same length of the main element – it should work.

Incidentally, it is possible to tune a single counterpoise to length with an L-Match, just as you would the main aerial.

If you bring the aerial wire directly into your station, a counterpoise may be connected to the ground-point on an AMU and laid out where convenient.

So far, I have not suggested a length of ‘random’ wire. In general, the longer (within reason) and straighter it is, the better it will be. However, with gardens as they often are, it may be necessary to ‘fit’ the aerial to the space available.

I have found that 66ft works well but shorter lengths will still work. There may be wire lengths where it is not possible to get a good match with a particular AMU on a particular frequency. In this case, some experimentation with the wire length may be needed.

Just about any type of copper wire will do. You should avoid copper-plated steel. Ideally, hard drawn copper wire of around 14 to 16 (British) Standard Wire Gauge will be best. I have used thin, stranded, hook-up wire and, since it is flexible, this has stood up well on short runs.

If you want to splash out, you can try Flex Weave or some of the Kevlar types.

You may also wish to look out for surplus twin wire, as used by BT on their telephone lines. This is hard drawn and very strong.

You ought to try and keep your wire as one single piece with no joins if possible and bear in mind the weight, these aerials can get quite heavy, especially those with long runs.

For receiving purposes, insulators can be just about anything non-conductive. I have a dipole up at the moment, with cable ties looped around a plastic gutter at one end and loosely-looped around the branch of a plum tree at the other end.

Now please refer to Fig. 3. Ideally, Section A needs to be as high (and in the clear) as possible.

Section B can be coaxial cable connected to a 9:1 transformer up at the top of the mast, with some coaxial cable brought to the operating position.

In an Inverted L configuration – as depicted here – it could be an extension of the main wire brought down to ground level, where it is fed into the transformer/BALUN (in truth, an UNUN).

This, in turn, is connected to the main run of coaxial cable.

The first method keeps the wire up and relatively clear of household interference; the second way can be more prone to picking up noise. However, it has the advantage that the feed point can be closer to the ground, adding an amount of vertical polarisation to the system.

The illustration (Fig. 3) also shows the feeder running to an upstairs shack. It can just as easily be run into a ground floor position, of course.

Moreover, in this example, a single counterpoise runs under the aerial, a few inches above the ground.

This is just one possible configuration.

The arrangement can be reversed, with the feed point at the base of the mast and away from the house and the coaxial cable running back to the operating position, even to the point of being buried by a few inches.

I have used plastic overflow pipe to protect the cable in situations like this.

Each and every installation will be different; therefore, please free to experiment.

Readers may have noticed that I have not used the term ‘long wire’. Technically, a long wire needs to be one wavelength or more in length. So, whether yours is an end fed or a long wire depends on how long it is and what frequency you are using it on!

Next month I will follow on with the end-fed aerials theme, by undertaking a mini-review of a cheap eBay BALUN. I will also discuss a home-made alternative. Finally, I aim to include points raised by readers, which came in too late to be included this month.

Until then: Good Listening!

 

This article was featured in the July 2018 issue of Radio User