A simple aerial question

Asked by a boater, answered with a broom stick.
snow
My TV has gone fuzzy do I need a new aerial?
No, you’ve moved your boat and it can’t see the TV transmitter mast so well.
Prove it!
TV aerial gets taped to broom stick and lifted aloft.
dancing8
OK you win, I’ll get a bigger pole.

So is that it then, TV poor reception is always improved by raising the aerial?
And pointing it in the right direction helps, you will usually see an improvement.

Transmitters and in particular CB sets!

Warning, this bit is long winded.
Get a hot drink and snacks before you start reading!
teabiscuits

The positioning of a transmitting aerial is also important, especially on a vehicle.
There are a few “simple rules and facts” behind that.

  • The higher the better. Low is bad.
  • Center of your roof is best on metal.
  • If the roof is not metal, use a no-ground plane type.
  • Magmounts work, sometimes quite well, but direct contact with metal is better
  • An aerial mounted on one corner will be directional in operation.
  • If the aerial has a coil, mount that coil ABOVE the roof line.
    Coax:-
  • Don’t skimp on coax, the better the quality the further you can hear or speak.
  • Run your coax into the vehicle in such a way to prevent physical damage.
  • Use connectors NOT taped joints when extending cables.
  • Every extra connector in line losses power.
  • Always do a neat permanent job.
    Tuning:-
  • Always tune the aerial with everything in its place, windows shut, and your usual body count in the car.
  • When tuning a wound aerial, only cut little bits off at a time.
  • Long aerials will always find something to hit.
  • The shorter the aerial, the less you will hear and be able to speak too.
  • Use a SWR meter (that works) and a known good patch lead.

Tuning the debate:-
Old time CB’ers used to cut their coax in 9 feet lengths as standard BUT a lot of ex-spurts will say that this is not necessary. Fine, I bow to their superior knowledge.
However it works for me on my CB sets and has for years working 11 meters.
IF HOWEVER you find that you cannot adequately tune your aerial to a safe level i.e. an SWR of 1.5:1 or less, a number of things may be happening.

  1. Are you certain everything is connected properly?
  2. Are you sure your patch lead and SWR meter is working properly and fitted correctly?
  3. The ground plane may be insufficient. A HUGE one to beat on some vehicles.
    Reposition the aerial to a more “metal area”.
    At worst you may need to fit a non ground plane (Think marine type) aerial.
  4. With a mag mount the ground plane is meant to be achieved “capacitively” (sorry geek speak) using the closeness of the magnet to the metal roof. Which works ‘sort of‘ at CB frequencies but not very well unless you have a massively wide base.
    Personally I’d scrap the mag, fit a hole mount or some other metal to metal connector.
    As for security of the aerial, fit the aerial on a quick release fitting.
  5. If you are using a metal to metal connection (hole, gutter, bumper, or mirror mount, which includes using a metal strap), either there is a little bit of rust formation making a less than perfect contact OR the earth strap is too long. Inspect and repair.
  6. You made a mistake when assembling the aerial connectors (PL259 or TNC or whatever) or it wasn’t screwed on correctly, or is suffering from damp or corrosion.
    Inspect and repair. (It happens, even to radio Amateurs)
  7. The coax lead itself is damaged. Replace, NEVER SPLICE OR TRY TO REPAIR.
  8. If you still have a problem, try fitting a clip on ferrite ring AS CLOSE TO THE AERIAL AS YOU CAN GET IT”. This is a sort of “electronic radio frequency type fault” (sorry geek speak again) and the ferrite stops the coax contributing to the problem.
  9. A controversal one (and the ex-spurts will be lining up to batter me AGAIN), fit a longer style aerial. Generally speaking you’ll get less problems with longer steel!
    This does not mean just weld a long bit of wire onto the aerial! Go and buy a proper longer one suitable for your radio.
  10. Give up and ask someone with more knowledge than me.
    Maybe you should have done that first.

As for the tuning process itself, the SWRing  bit?
(although some do SWEAR a lot when doing it)

It’s not rocket science, it’s not hard, and you don’t need to be a radio Amateur to do it right. Here’s some old notes I did which may help you although if you have NEVER done this before, learn from someone who has. Practical beats books every time.

Tuning An Aerial
When you connect an aerial to a transmitter (be it Ham, CB, or PMR) you need to make sure that it is working correctly.

To do this you need to adjust it’s length to “tune it in” to your radio.
To do this you’ll need to connect a test meter in line called a SWR Meter.

What is a SWR meter used for?
Simply put, a SWR meter compares the power that can be delivered by the CB to what is actually leaving the aerial.
By adjusting the length of the aerial you “tune it” to your CB rig.
Get it right, and the SWR will be 1:1 thus all the power goes out into space and the aerial is “tuned” to pick out incoming transmissions.
So more power out and better reception.

If something is not quite right i.e. things aren’t correctly fitted, worked loose, not tuned, or damaged, some of the power will be “reflected” back into the RADIO.

Too much “reflected” power can destroy the transmitter let alone affect your reception. A badly tuned aerial is rather like a badly tuned guitar string. Grating on the ears, makes loads of noise, and in radio terms causes loads of interference that can affect your neighbours TV & stereo.
Not a good thing.

So what’s good and bad levels of SWR?

1.5:1 to 1:1 is brilliant. 96-100% of your power hits the airways.
2.0:1 to 1.75:1 isn’t great. 89-93% of your power goes out
10:1 to 3.0 is disastrous. 33-75% is all that gets out.

Above 10, (needle flies to the right and stays there) you’ve got VERY serious problems. Either a lead or aerial is broken or you’ve got an electrical short.

Tuning Your Aerial To Your Rig

Read the little paper that came with the SWR meter to familiarize yourself with the switches (Forward / Reverse or FWD /REV), the scale, the control knob (CAL) to control the needle, and the connections on the back.

A Typical Set Up

Install everything into it’s correct place.
Once you have finished your installation, don’t move things round. SWR may change if you do.

About costs.
A cheapy little CB only SWR meter will set you back £9 – £15.
You’ll need a little patch lead which plugs into the back of the CB sets aerial socket and into the “IN” plug of the SWR meter. That costs £4 – £8.

Note: The aerial lead then goes into the “OUT” plug of the SWR meter.

Switch on the set and select LOW POWER until you have SWR’d the aerial.
Put the switch to “FWD” and transmit. Turn the CAL knob so that the needle swings over to the far right of the scale and align the needle with the set (or Cal) mark.

Still transmitting, switch over to “REV” (Reflected) and the needle should drop down. Note that reading.

CAUTION! If the needle hardly drops when you switch over, slams into the right hand side and stays there, or does not come out of the red portion of the scale STOP TRANSMITTING.Something is VERY WRONG.

Check the aerial plug is connected properly, and your coax and aerial for damage, or breaks.
Assuming that it did drop, you can happily carry on.

For UK Only Sets (40 channels Only)
Take readings on Ch. 1 and 40 and make a note of them.

On each test re-tune the needle to the set mark on FWD each time you change channel.
DON’T FORGET, EVERY TIME!!!

If the SWR is higher on Ch.1 than Ch. 40 Lengthen the aerial
If the SWR is Lower on Ch.1 than Ch. 40 Shorten the aerial
In this case you would slacken the grub screw in the aerial and SHORTEN the aerial.

Typically the readings shown above will balance round 1.3 to 1.5 at both ends.

Now for sets that use the whole 80 plus channels 26 to 28 Mhz.
Make sure you have an aerial that can cope with the wide range of frequencies.

Your low channel is CEPT Channel 01 …. 26.965 MHz

The mid channel is whatever you are going to use the most i.e.
US/ CEPT would be Channel 40 27.405 MHz
UK FM (spits on the ground) Channel 1 27.60125 MHz
If you use a Freeband CB set, then choose 27.000 MHz

The high channel is UKFM  Channel. 40 …. 27.99125 MHz

The same rules apply.

And finally
Some leave the SWR meter in line after tuning to monitor their set up as the more ‘clever’ ones show the transmitted power.
You can if you want BUT you can also take it out and then it’s one less thing to worry about. I take mine out as every connector you add in the line between the CB set and the aerial loses a little bit of signal. (And once again the ex-spurts will probably say nonsense).

Ho hum. Isn’t life grand!

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6 Responses to A simple aerial question

  1. equippedcat says:

    A longer antenna is not necessarily better. The best antenna is one which is a useful fraction of the wavelength you are transmitting on (which is inversely proportional to the frequency). 1/4 wavelength antennas are common; I hear that 5/8 wavelength is better.

    The higher the frequency, the shorter the wavelength, and the antenna.

    • Practical CB’ers would argue that case and put it rather poeticly.
      “The longer the twig, the further you speak” and “To hear needs length”.

      Both are right, both achieved easily, leaving the proper base stations to put up the 18 foot poles or centre fed wire dipolees.

      My main vehicle aerial is a 5/8 1.5m wound pole. On the boat, it’s a straight forward 9 foot (trimmed a tad) ex-forces ‘tank’ whip. Old school, just like me.

      A lot of theory is expounded by a lot of ‘experts’, usually radio hams.
      Mainly you’ll find the older CB’ers keep things basic and LONG.
      And as there are more CB’ers than hams working long distance on low power SSB (4 to 8 watts) in the 26-28 Mhz (11m) band. One of us has got it right.

      • equippedcat says:

        Well yes, as long as it is a useful fraction of the wavelength, longer is better. Why a 1/4 wavelength is ok and 5/8 noticeably better. At the lower frequencies, the longer antennas can get ungainly; on the “11m” band, the wavelength is about 11m long, so a full wavelength antenna would have 11m of antenna element. Of course, some of it could be wound so the physical length would be more manageable.

        However, a longer length is not guaranteed to give better performance. Let us take the 5/8 wavelength CB antenna, which is effectively 6.875m long. 7m or 8m would be longer, but probably noticeably less effective.

      • Practical radio experence my friend. That’s what cuts in the end.

        Maths and real life has too many variables.
        For example the velocity factor through different coax, connectors, and ground plane effects.
        Let alone the differences between transmitters i.e. some like a bit of capacitance, others love a raised or lowered inductance. That’s why we tune.

        I once worked for an radar designer and he was one scary, brainy (if a bit eccentric) guy.

        He had sussed it straight away when he described the aerial design as a bit of a black art.

        The maths on one side,
        The theory on the other,
        Then what happens in real life.

        His first build test engineer would look at his drawings on the rolls of lining paper and mark the errors with little red crosses.
        Then he’d build it “as designed”, pass it to test, and just smile quietly when the amendments (as marked) came through to make it work.

        In four years that process never changed. Design, comment, build anyway, test, redesign, rebuild, and then it worked.

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