Newbie Question but.......

I am trying to get my head around atternas, so can somebody clear this one up ?

As I understand it a 1/2 wave on 27Mhz CB should be 1/2 of 11m or 5.5m, & a 1/4 wave will be half of that ie 2.75m.

However I notice most mobile attenas for use on vehicles, have a coiled section in them like a spring, so there actual height can be more like 1.5-2m.
So my question If say I had 1/2 wave antenna whose overall height is 2m, if I could pull out the coiled part so it was totally straight would it then be 5.5m ?

Most mobile CB antennas are designed as a 1/4 wave consisting of a whip and coil , the general consensus is the springer type antennas don’t work very well

Get yourself a Sirio HP 4000 and all well be well


Craig

That’s what I’ve always thought, it makes the length easier for having on a car.

You are right about the lengths, i.e 5.5m being half wave on 11m

Secretelephant wrote:

So my question If say I had 1/2 wave antenna whose overall height is 2m, if I could pull out the coiled part so it was totally straight would it then be 5.5m ?

Nope. Might be another 15-20cm at a push.

There's no such thing as a half wave antenna for CB which is 2 metres long unless it's fitted with the mother of all capacity hats at the top to give it the electrical length of a halfwave. However there are plenty of 2 metre antennas which have a matching circuit at the base to present a 50 ohm impedance your radio wants to see. If it's not around 5-5.5m it's not a halfwave, 5/8 wave or whatever the manufacturer claims no matter how much wire they put in the coils.

So basically once you move either side of 1/4 wave (102"/2.6m) or odd multiple of that, i.e 3/4 wave, you need to use a matching circuit to bring the impedance to 50 ohms. The coils at the bottom of mobile antennas are matching circuits, a tuning circuit, and they do not add any additional length to the antenna.

Ok, however what I am asking is this. I have a PNI S75 antenna, which I assume is a 1/4 wave. In theory then it should be 2.75m long but it is nothing like. Nearer to around maybe 1.5m. So
how is that possible ? Or am I wrong and it would be classed as an 1/8 wave ?
By the same token how can walkie talkies have their tiny antennas. Or are they classed as 1/20 wave etc ?

What you have is a "loaded" aerial.
There is a coil that brings the impedance to 50 ohms.
If there wasn't, the short aerial would have a very poor match to coax, in other words the SWR would be in the next county

If an antenna is quoted as 1/4 wave, 1/2 wave, etc, that’s the actual physical length of the antenna.
To me, that means just the length of the rod or wire, not including any coil or matching unit at the base.
You could say an antenna is an 1/8 wave, or any fraction if you wanted to, but the only really meaningful lengths in RF terms, are the common ones such as 1/4, 1/2, 5/8 and sometimes you see antennas that are physically 7/8 wave in length, and maybe one or two other ‘rare’ sizes.

Walkie Talkies the ones I think you’re talking about, operate at VHF and UHF frequencies.
To calculate the wavelength (in metres) from a frequency (in MHz), you use the formula:
300 / Frequency (MHz).
The 300 is that light travels at 300 million metres per second (approx.)
So, a PMR Walkie Talkie operating on say 446.00625 MHz (Channel 1), a full 1 wave would be:
300 / 446.00625 = 67 cm (approx.)
This full 1 wave ‘could’ be used, with some extra matching circuitry inside the rig, but it’s more usual and easier matching wise, to take the 1/4 wave length of:
67 cm / 4 = 17 cm approx.
You could ‘then’ make this smaller again if you wish (some manufacturers do), by adding that extra matching circuit, BUT you couldn’t really call it a 1/4 wave any more, AND it’s performance will be reduced.

To round up, the higher in frequency you go, the smaller will be the wavelength, and hence any fraction of it.

When it comes to antennas and length, there are two lengths that are important.  Physical length, and electrical length.

An antenna's physical length is, well, how long the antenna physically is.  An antenna's electrical length is how long the antenna appears to be at the antenna's feed point.

All mobile CB antennas are 1/4 wavelength antennas electrically (even those that claim to be 5/8 wavelength and such), yet most of them are shorter than a physical 1/4 wavelength in length.  They do this by adding an inductor.  An inductor will add electrical length to a shorter antenna, thus making it appear longer electrically.  This allows you to still have low SWR/resonance at the feed point making it tune-able with the radio and coax.

This is actually a two way street.  Although far far less common, it is also possible to take a longer antenna and make it appear to be electrically shorter.  The Imax antenna is an example of this.  They use a capacitor part way up the radiator to make the antenna appear shorter electrically than it physically is.  This allows the longer antenna to use the same matching network as the shorter a99, among other things.


The DB

Ahah thanks. So there is more to it than just a piece of stainless rod.

I am just trying to get my head around antennas, and how they work.

I have a PNI S75 Magmounted antenna, which I removed from the Mag mount and fitted to a home made bracket on a motorbike. However the SWR is hopeless.
However if I re-attach back in the magmount & place it on some sheet steel the SWR is next to nothing.

Am I correct in assuming that the antenna will be matched to the magmount.
What I dont understand is antennas that use mirror mounts etc that dont have any ground plane like a metal car roof etc.

To quote a friend of mine "White man's magic"

Secretelephant wrote:

Ahah thanks. So there is more to it than just a piece of stainless rod.

I am just trying to get my head around antennas, and how they work.

I have a PNI S75 Magmounted antenna, which I removed from the Mag mount and fitted to a home made bracket on a motorbike. However the SWR is hopeless.
However if I re-attach back in the magmount & place it on some sheet steel the SWR is next to nothing.

Am I correct in assuming that the antenna will be matched to the magmount.
What I dont understand is antennas that use mirror mounts etc that dont have any ground plane like a metal car roof etc.

An antenna is more than just the whip. The vehicle the antenna is attached to makes up the, as many people call it the "other half" of the antenna. This "other half" is just as important to the antenna as the length of the whip. If you take the antenna off of one car and put it on another, or even a different part of the same car, you often have to adjust its length to compensate.

A motorbike will be no where near the same as a car as far as the "other half" of the antenna is concerned, so at minimum would require a much larger adjustment to the length of the whip to compensate, if it is even possible.

Antenna's can be finicky beasts...


The DB

Thought about using an antenna matcher, which might work, BUT while Googling for something else, came across a website that sells 'No Ground Plane' antennas, suitable for motorbikes amongst other things.

The seller was based in the USA, but you might be able to find someone closer.
Might be worth a look.

Now where's my bike  
73's Gary

Secretelephant wrote:

Ahah thanks. So there is more to it than just a piece of stainless rod.

I am just trying to get my head around antennas, and how they work.

I have a PNI S75 Magmounted antenna, which I removed from the Mag mount and fitted to a home made bracket on a motorbike. However the SWR is hopeless.
However if I re-attach back in the magmount & place it on some sheet steel the SWR is next to nothing.

Am I correct in assuming that the antenna will be matched to the magmount.
What I dont understand is antennas that use mirror mounts etc that dont have any ground plane like a metal car roof etc.

Have a look at this site......everything you ever wanted to know about antennas.
http://www.k0bg.com/myths.html