This is the second part of a two-part article.
If you arrived first on this page, you might wish to first read Part 1 – Choosing an Antenna – before then reading this second part.
After you’ve selected the best antenna for your needs (using the information in the first part of this article series to help) you now need to match the antenna to your radio, and to ‘tune’ it for best efficiency.
Please keep reading for information on how to do these two essential things.
Fitting Your Radio To An External Antenna
This sounds like a simple process, but – alas – it is not. While the process itself is indeed simple – you connect your antenna to your radio – the complexity arises because different radios and different antennas use different sizes and shapes and genders of connectors.
Murphy’s Law being what it is, there’s every good chance that the radio you have will have a different type of connector on its antenna output than your antenna will have at the end of its lead.
There are a number of reasons why this mess of different connector types exists these days. There’s no ‘ideal’ connector size for our purposes, and you probably shouldn’t give too much thought as to the connector type used by specific radios or antennas – especially for mobile use. You have two choices after selecting your radio and antenna(s) – either get an adapter to connect from one style of connector (on the radio) to a different style of connector (on the antenna), or alternatively, if you are able to conveniently do so, cut the connector off the antenna lead and wire the lead up to the correct type of connector to work with the radio you have.
This latter choice is slightly better, but is usually not practical for portable radio antennas, only for mobile and base station antennas. You want to have as strong as possible a mechanical connection between the antenna and the radio on a handheld portable unit, and if you start adding extra connectors, then you are weakening the connection, and the connectors are generally not designed to be load bearing devices.
It is therefore highly recommended that you limit your antenna choice, for an HT, to only those antennas that have the correct connector built-in to their base.
When evaluating your connector needs for mobile and base station antennas, the most common types of connectors you are likely to encounter include :
|PL-259/SO-239 (‘UHF’ connectors)||These connectors are sometimes referred to as ‘UHF’ connectors, although in strange contradiction to their name, they are not very good at UHF frequencies.
This is because when they were given that name, the term ‘UHF’ referred to much lower frequencies than the term now refers to.
They are however relatively sturdy and straightforward to specify and use. Male plugs (PL-259) are usually on antenna leads and female sockets (SO-239) are usually on radio outputs and SWR meters.
|SMA/RP-SMA||These tiny connectors are clearly much better suited for the miniature size of today’s handheld/portable radios. Unfortunately, there is a confusing mess of different SMA and RP-SMA (reverse polarity) connectors and so you need to focus very carefully on two things.
The first variable is whether you want a connector that screws into/over the other connector it is pairing with, or whether it has a threaded exterior that the other connector will screw into/over.
The second variable is whether, in the middle/center of the connector, there is a hole for the lead from the other connector to be inserted into, or whether there is a sticking out ‘prong’ that will go into the other connector’s hole.
Originally, connectors with external threads had internal holes and were called female SMA and those with internal prongs and external bolt type things that screwed into the other connector were called male. And that is sort of intuitively obvious, isn’t it.
But now there are the RP series which combine a ‘female’ exterior thread concept with a male interior prong/hole concept.
You also need to understand, when a device is described as using a certain type of SMA connector, does the description mean it needs this type of connector on the other piece of equipment, or has this type of connector on itself.
You can guess at some of this (particularly the external threading) by looking at enlarged pictures of devices, but few pictures clearly show if there is a prong or hole in the center of the connector.
|BNC||BNC connectors are back to simple and easily identified and understood territory again. There are two very obviously different types of connector, one fits over and around the other, and connects via a bayonet type twist/lock.|
|Others||Oh yes, there are lots of other types of connectors too. N connectors. TMC. Mini-UHF. And that’s just the start of the list!
These less common types of connectors are not deal breakers, but it might take some more effort to find (or make) an adapter.
The situation can become even more complex still when buying a mobile antenna. Some mobile antennas are nothing more than the antenna mast itself, with a screw thread on the bottom of the mast. If that is all you get, you then need a compatible mount to screw the antenna into.
Unfortunately, there’s not one universal thread type or mast diameter, so you’ll need to carefully ensure that the diameters and threads are compatible between the actual antenna mast and the base it needs to be affixed to.
But wait, there’s more. Maybe (or maybe not) the base mount device has a coaxial cable pre-wired into it, to enable the signal to be run between the radio and antenna.
About the only good thing that can be said of a mount with no included cable is that you are then free to build a cable of the exact length you need, of the best type of coax cable, and with the correct connector on it. Other than that, it is a hassle.
Tuning Your Antenna and Coax to Your Radio
If you are fitting an antenna directly to your HT, there’s not much more you need to consider. But if you are adding a mobile or fixed antenna to your radio (HT, mobile or base) then there’s one more important step, and that is making sure your antenna matches the output of your radio as closely as possible – that is, your antenna is ‘tuned’ to your radio and its location.
We’ll avoid the electrical theory and simply say that not all antennas are exactly matched with all radio transmitters. But – here’s the good news – it is usually possible to adjust (ie tune) the antenna to improve the match between it and the transmitter. This can make a big difference to the effective power transmitted from the antenna.
The degree of efficiency of your antenna compared to your transmitter is termed its standing wave ratio, always abbreviated as its SWR. An ideal transmission system has an SWR of 1:1, meaning no power is reflected back from the antenna. A SWR value of 1.5:1 (usually just referred to as 1.5) means that there is a 4% loss of power, a SWR ratio of 2 means 11.1% loss, 2.5 is an 18.4% loss, and at 3.0, the loss has grown to 25%.
This power is not just ‘lost’ – it is reflected back to the transmitting circuits and can damage them. It is hard to say at exactly what point a high SWR value changes from not just being a regrettable loss of power to becoming a dangerous amount of power being reflected back into the transmitter, and it probably varies depending on the design of the transmitter that is experiencing the reflected power. It is uncommon to see values lower/better than about 1.2, and if your SWR is above 3 or so, then that is perhaps becoming cause for concern.
So for all reasons, you want to have an antenna with as low an SWR as possible.
Although there two things to consider with SWR. The first is how to measure it, and the second is how to change it.
To measure SWR, you need a special device – an SWR meter. These are readily available and not too expensive.
SWR meters only work on some frequencies and are also power limited too, so you need to ensure that the SWR meter you get is designed to work with the frequencies you’ll be transmitting and can handle the power output of your transmitter. An SWR meter also needs to be of the same impedance as the feed line and antenna you are using – in our case, that is almost always 50 ohm, and you can assume that your meter is also 50 ohm unless it states to the contrary.
Some SWR meters display their value by a needle moving on a scaled analog meter, others have two needles moving and you read the value from where the two needles intersect. It is unclear if the dual needle system is actually appreciably more accurate than a single needle system, especially for our type of ‘close enough is good enough’ purposes (as opposed to scientifically calibrating and measuring exact laboratory values).
But dual needle systems do have one advantage. You don’t need to calibrate them every time before you take a reading. Calibration isn’t a difficult process to do with a single needle system, but the challenge comes when you forget to calibrate, meaning that perhaps you are getting an inaccurate reading and making the wrong assumptions.
Note also it is important to attach the transmitter and antenna leads to the SWR meter the correct way around. If you swap them, you’re unlikely to harm the meter, but you’ll get an inaccurate reading.
Fortunately, a basic SWR meter is not unduly expensive, and a unit such as this Workman 104 seems to represent as a good compromise between cost and functionality, and with a price of under $40 on Amazon, is good value too.
The Workman 104 can handle power outputs of up to 150W and covers frequencies from 120 MHz – 500 MHz. and so is probably more than sufficient for your needs.
If you wanted to treat yourself to a dual needle system, then this $60 unit on Amazon seems like the best choice currently.
When you measure the SWR of your system, you should have your antenna mounted the way you will use it. In other words, if mounting an antenna on your vehicle, have the antenna already mounted where it will be on the vehicle (the SWR value will change depending on where on the vehicle the antenna is located). Some sources suggest you should also close the vehicle’s doors to further ensure that the system being measured is the same as the one the antenna will actually be used in.
Once you have your SWR meter connected into your transmission line, you now want to take SWR readings across the band you want to transmit on. We suggest you get a piece of paper to write them down, because it can get confusing and you want to track what is happening when you make changes. Maybe have a set of columns for each different frequency you are taking readings at, plus another column where you can write what you did to change the setup from the previous set of measurements, and then one line for each set of measurements.
The first part of changing your antenna’s SWR performance is to understand what it is at present and how you want to change it.
Let’s say you are tuning an antenna for the 70cm band. This ranges from a low of 420 MHz to a high of 450 MHz. We suggest, to start with, you get perhaps a set of seven readings, at 420, 425, 430, 435, 440, 445 and 450 MHz (remember, with single needle SWR meters, you need to recalibrate before each reading). These readings will show one of four possible situations
- The SWR value consistently decreases from 420 to 450 MHz – this means the ideal frequency for your antenna is higher than 450 MHz
- The SWR value decreases for a while then turns around and increases – the ideal frequency is where the SWR is at a minimum
- The SWR value consistently increases from 420 to 450 MHz – this means the ideal frequency is lower than 420 MHz
- The SWR value is more or less the same all the way across the band – discussed below
The optimum solution is to have the SWR at the lowest in the part of the frequency band you’ll be using the most; the part that is most important to you. If you wish to be able to use the band equally, then the SWR should be lowest in the middle of the band.
If by some happy chance, the SWR is already at a minimum at the key point of the band, then there’s nothing more you need to do. But if it is not, then you need to tune your antenna to move its ideal frequency from whatever it is to whatever you now wish it to be.
There are many different ways an antenna can be tuned, but in the case of a typical mobile antenna, the most practical way is by varying its length. Pretty much every mobile antenna you’ll come across will have a way of adjusting the length of the antenna – usually there’s a tiny set-screw somewhere and when you loosen it, you can then slide a section of antenna up or down (or in and out of whatever it is sliding to/from) to adjust the antenna length.
If you wish to raise the ideal frequency to a higher value, you need to shorten the antenna length. Do we need to add that if you wish to lower the ideal frequency, you need to increase the antenna length?
Small adjustments can have big effects. Try adjusting the antenna length by perhaps half an inch and then redo your readings (remembering to recalibrate each time, of course). You can then see on your sheet of paper what the impact of the half-inch adjustment was, giving you a feeling for if you need to make more of an adjustment, or if you made too much of an adjustment, and so on.
It will probably take a few adjustments to get things close to good, but it will be time well spent.
Special Tuning Situations 1 – Flat SWR and Narrow Band
It is relatively straightforward to tune an antenna for the 2m and 1.25m bands. This is because each of these bands is ‘narrow’ with not much difference between the upper and lower frequencies. The 2m band has only a 1.4% spread from its midpoint to its upper and lower limits, and the 1.25m band has an even tinier 0.7% spread.
This means that sometimes you won’t see any difference in SWR across the entire band, because whatever change there might be is less than that which is picked up by the SWR meter.
But that is not the same as having the antenna fully optimized. Maybe the SWR can still be improved – unless it is already very low (say less than 1.5) we’d recommend you test the antenna by increasing its length until you see the SWR change – does it go up or down? If the SWR is unchanged or reduces, keep adding to the antenna length until it is clear you’ve reached the minimum and the SWR starts to increase again.
If even a slight increase in length immediately causes the SWR to increase, you should instead start shortening the antenna length until such time as the SWR has decreased to the minimum.
Special Tuning Situations 2 – Broad Bands
Unlike the 2m and 1.25m bands, the 70cm band has a very broad range of frequencies (from 420 – 450 MHz), with a 3.5% spread above and below the central point of the band.
This can mean that there is a large range in SWR values between one part of the band and another part of the band. That can be a nuisance if you wish to be able to transmit and receive all the way across the band, but if your main usage will be restricted to only communicating with other known users, we suggest you agree among yourselves on which part of the band you’ll use and so tune your antenna for best performance in that part of the band.
An optimized antenna for your radio will give you more range for both sending and receiving than anything else you can do to your radio. Instead of spending many hundreds (or even thousands) of dollars on fancy radio transmitters and receivers (which will also use much more power – something that will be at a premium in any ‘grid-down’ type situation) you can instead spend a few tens of dollars on a better antenna and get the same end result – longer range and more resilient and reliable communication capabilities.
The information in this two-part series and our other radio communication articles can help you choose the best radio and antenna for your requirements.
Read More in Part One (and Other Articles Too)
This is the second part of a two-part article. If you have no already done so, you might wish to also read Part 1 – Choosing an Antenna – for additional information on getting the best antenna for your radio communication needs.
We have a lot of other helpful articles about radio (and regular) communications too – please see our listing of Communications topics.