Most questions apply to coax feeder and antennas, how does it work, which is a good antenna (Arial), where should your antenna be on the truck, what about the SWR

cb1Antennas, Transmission Lines (coax often), Propagation (the way the radio signal travels when it leaves the antenna or arrives at the antenna).

These three subjects can be complex, so full details are always required. There are, however, some basic rules, so I will try to be as clear as possible with answers. There are a lot of ‘Myths’ regarding antennas (aerials), for example, fancy names and prices don’t mean better antennas. Base station antennas included.

As a help in choosing a rig and antenna the following should be considered:

  • Get a rig with an swr meter as an internal feature of the rig.
  • Put a little Vaseline (or white lithium grease 3oz size electrical tube) around the pl259 connecter threads that connect to the rig and sometimes antenna.
  • Buy a soldered co-ax feed with the best quality screening, which is normally a quantity of mesh screen.
  • Solid di-electric (that’s the bit between the centre conductor and screening) is essential for mobile use because it cannot get squashed.
  • If you have air spaced co-ax di-electric it must be inspected often to be sure it is not bent around a corner and made into an oval shape, doing this in enough places or over a short length will change the impedance and can (can) blow the output transistor of the rig.
  • Where the co-ax enters the antenna outside in the rain and all weathers, it must be sealed 100%
  • Electricians tape is not good.
  • Self-amalgamating tape is first class. Ask the man at the truck stop or CB repair shop for a few inches of this tape for 25/50p and ask how to use it. 100% clean and dry surfaces, stretch the tape around the connections. Also do the internal rig connection, trust me on this, you can also use silicone from a tube dispensed in a gun of the builders type. You should not use this really, but my experience has show it does not have a bad effect on the cable or connections except perhaps after 10 years or so. It will protect your system very well so use it. Remember if you don’t protect your cable and connections as I have described you are wasting your time. If you have used it unprotected throw the cable away and get new cable and do it again.

Don’t forget to set your SWR; you will need two people to do this well and you should keep the door of the cab shut when you set it. The person adjusting the adjuster on the antenna should stand back a few feet for each test.

Keep the antenna away from all metal bodywork as much as possible. If you locate the antenna on a mirror bracket, put it on the outer side away from the vehicle as much as possible. Mail me if you want information on a different situation or further explanation.

The best antenna would be one with an adjuster on the top of the antenna, like a cheap DV27 whip antenna. The reason for this is maximum efficiency of transfer of power from the rig to the antenna, equally max transfer of power from the antenna to the rig on receive. These two points are terribly important. In practise you will not beat what I have said here with any fancy antenna or fancy name. It will perform as well (vehicle limitations taken into account) and better than most, in all conditions.

Technically a top loaded antenna is better than a bottom loaded one, for ground wave communication, so a helical whip with tighter windings at the top is better than an expensive bottom loaded antenna (no names mentioned) given fair parity of cable, in truth the chubby or long helical usually has most adequate cable (inner wire that makes the antenna). All this said the cheap thin black, DV27 top adjuster type, about 3/4/5 ft long, will do as well and, in practice, better than most, even the most expensive ones for your mobile use. I myself would not waste a penny more.

Some companies in their literature advocate locating roof top antennas close to the roof, for reasons of the short distance to a possible ground system on the roof. Such advice can be misleading; any close bodies of any kind will interact with the radiation pattern, apart from changing the feed impedance. A low resistance connection to the antenna is what is required from an arbitrary ground system, like your truck or car. All bodies, metal, or otherwise, that are to the side or not essentially flat below the antenna, should be as remote as possible, in order that they do not form a relationship with the antenna of serious consideration. If they are below the antenna, which should be a balanced antenna in this case, it would not require the roof to operate well, but could utilise it to the advantage of the radiation pattern. Having the antenna in the clear would be better with its designed pattern, this also removes any local buildings and wiring from the strong radiation of the close by antenna, especially computers. (For example the cab of an HGV can be close to the vertical metal box of the trailer or rigid, the antenna will radiate sideways and the metal box will reflect back the radiation to the antenna PDQ. This is not good news, even for a low power CB antenna the impedance will change and the antenna will see itself as being too long. The answer would be to shorten the antenna physically because it would see this metal body as being part of itself (forming a strong relationship). This would be critical, and the whole system would become unstable, so I don’t advocate doing this, but I offer the words as an explanation: having metal below the antenna to which the antenna is attached is essentially good but it’s an arbitrary bit of metal regards dimension, and the antenna or coax screen needs to be attached to that ground with a low resistance connection, large magmounts can and generally will capacitively couple to the metal body but direct connection is better.)

cb2Vertical mobile antennas are unbalanced antennas. They need a connection to ground from the screen, this connection is important, however if you ever see a commercial mobile antenna for sale with small radials which themselves are like a CB rubber duck antenna, this could be an excellent. The base station equivalent is a one-quarter-wave antenna, with sloping radials, often commercially produced with three radials sloping at about 45 degrees. This is in practice efficient, especially if above the ground/buildings and/or reasonably away from other structures.

To elaborate a little more with regard to the height above the ground for this base antenna, there will be a practical height above the ground that would be complimentary to its efficient operation and radiation pattern. This assumes the practical considerations that we can’t all just hoist an antenna as high as theory would like us to do. Above 5.5 metres is reasonably good for CB 27 MHz. Having no local structures around it that are higher is also the aim. Tuning the antenna is very important; for all situations failure to tune the antenna after installation can mean you have wasted your time. This type of antenna should be very good in that its length will be near as per the book correct, given it is away from structures at its side by some wavelengths, so your swr will/should/can read good without adjustment.

As a little help for the house owner on an estate, putting an antenna in-between the buildings of your home and the neighbour’s home can cause interference problems, especially in old domestic equipment. The higher up over the roof line the better it will be for everyone in reducing and eliminating TVI or any other possible interference. Also, having a ground plane as a part of the antenna itself is a very good plus, and it just happens to be a very efficient antenna. Remember you can’t win everything unless you are a military operation.

Mounting antennas in field situations would be best served by being at a (or better said not at a bad height) correct height for the frequency being used, but practicalities come into effect of course, but be aware that in depth the matter is quite complex. As an example even a train passing at some two miles distance, effects the radiation pattern of your house antenna, so again be aware of the vagaries of the truck mounted antenna. Every single truck in the world will technically have a different radiation pattern, given the same antenna, even the same truck…

Ground plane antennas themselves, however, are planned and are good antennas because they are in practice efficient, being part of the antenna design, necessary or casual other bodies/structures are not planned or considered in the design, so they can never be desirable. Your mag mount vertical antenna will capacitively couple to a steel roof and may not need an earth connection, but if you do supply one then you may well have to retune your antenna. I have never seen a mobile electrical quarter wave antenna for CB which has its own quarter wave rubber duck type (or helical) ground planes as a planned part of the antenna. If there were such an antenna manufactured it would be very desirable.

An SWR meter is a used to make sure you get the power transferred from your CB set to the antenna. It is vital that it is used and is correct; the inbuilt swr meter will always read accurately, given it’s not faulty of course. swr meters placed in the coax cable can have essentially a 1 in 180 chance of being really correct, unless the antenna is pretuned, and then due to the presence of the vehicle and other bodies it will need to be de-tuned to tune it… if you see the funny side of it. If a seperate swr meter is close to the rig (1 foot) you can consider that it will read near enough correctly. .

Mounting your antenna on the cab with a metal box container behind spells doom, your swr will be crazy, even if it’s reasonable at chan 19, the bandwidth will be so narrow, meaning you cannot operate on chan 1 or 40 if it’s to close relative to output power.

If you can mount your antenna high and be legal it’s better with all obstacles below its base.

Your co-ax cable feed will only radiate if the match to the antenna is incorrect.

Mounting an antenna on a truck is always problematic because there will be metal alongside and near to the antenna – keep away from it as much as possible. The antenna will and does couple with any other body near to it, especially metal, this makes the antenna seem longer than it really is, electrically.

Having long lengths of co-ax for your antenna are not needed, they will if employed seek to mask a mismatch problem that should not be there in the first place. Having a length of co-ax cable wound over your cab to get a low swr is not the way to go – it will just hide sins committed by bad or poor installation.

I do understand that trucks are difficult for roof antennas because of rear metalwork. Vans are good as they have no metal wall behind them. Using burners in mobile situations is not for wimps, you have to like exposing yourself to high power RF fields, given that the normal truck installation cannot be perfect, you will get heaps of RF in your cab. I remember listening to one chap who told his mate that when he transmitted he could see what was like a cobweb structure in his eyes and he kept transmitting. Antennas and transmission lines are quite an involved issue and I will always be happy to engage you on this subject at any level for your unique situation. Drive with care all of you out there, get home safely every time. Remember someone loves you back home, and they want you in one piece.

Send in your questions!