MHz and Bandwidths tuning for UHF antenna

Robin, what leads you to the conclusion that maximum range doesn't occur when an aerial is resonant ?

Hi Footloose

I know what your getting at by using the term "resonant" , but I
believe you mean when the aerial is tuned.

There are 3 seperate terms involved, maximum field strength
, resonance and maximum power transfered into aerial (radiated).
Each is a different parameter.

An aerial can be resonant without be tuned for max power out.

Commonly people tune an aerial for a low VSWR which basically
causes the most power to flow down the aerial cable and be sent
out into space.

However , this power may not be in the desired direction , and
so maximum range is not achieved.

For example, in my car I use a 1/4 whip(not quite) mounted in
the centre of the cars roof.

It can be tuned for lowest VSWR and all 5 watts goes out into space.

However the highest field strength is radiated out from the car at
an angle of roughly 35 degrees.

Whereas what we want is for the power to head to the horizon instead
of up at an angle.

In the case of my aerial, by lengthening it to approx 1/3rd of wavelength
then much more power heads out to the horizon giving me longer range,
but the VSWR is not low but around 3:1 , and the aerial is not resonant.

Robin Miller

Hi Robyn, I guess that we are trading terms here. I think that you're saying that just because the aerial is resonant at a particular frequency (at which point maximum current will flow in the aerial), that doesn't mean that all of the radiated power will go in the required direction.
I totally agree. Different fractions of a wavelength at that frequency will have different take off angles. That's why, of course, your 1/4 wave is better under some conditions than a gain aerial where the gain is achieved at the expense of other facors.
However, the performance of a tranceiver which has a feedline properly matched to an aerial at its resonant frequency will always be better than an unmatched aerial.
If you change the length of your aerial and get a high swr, would this not negate any gain in performance in the desired direction ? And in your case, wouldn't you be better off with a 5/8 ?
All mobile aerials are a compromise. Some times we need a higher angle of radiation and sometimes a lower one.
But for the average 4wder, whatever looks good on the bar is going to get their $$.

Hi Footloose

Yep is so important to get the terms right because the answers do change
with the definitions.

An aerial can be resonant and a lousy performer still because while all the
power goes down the cable it could be dissipated into heating up matching
components etc before being radiated.

Where people can afford the best components, losses cause by high Vswr are
minimal, and some satellite systems run 5:1 Vswr standard specifically to deliver
a desired radiation pattern.

Where you feel that matched transceiver will perform better than an unmatched
, It again depends on your definition of performance.

I define performance in terms of range in a direction, usually horizontal
as I don't play with HF much. And an adjustment that upsets the
matching but causes more field strength is a winner.

This is how I tune my aerials.

Good to mention 5/8, what a lovely aerial, but for some strange
reason this is just about the only type not made for UHF CB !

Robin Miller

Robin, interesting comments.
Just thinking back to my days of ant modelling ranges, I can't ever remember accepting a higher swr for any reason. If there was a mis match for the sake of pattern then it was fixed. Of course the ant might end up being used at much higher powers than one might see in the 4wd world. Much, much higher in some cases.
I'm also surprised to hear that sat systems run that system. (No wonder they are so unreliable -)
Oh and it wasn't HF actually. My original brief was DC to daylight :)
But the main thing is that it works for you. And that's all that counts.
Yep, discovered the 5/8 when playing mobile on 2m around 1966. Great aerial for some things but horses for courses I guess. Surprised that at least one aerial mob don't make them for UHF. After all, their founder spent some time expounding the virtues of the 5/8 to me one very cold winters night before he made his first $ from aerials :)))

Hi Guys,

1st up Antenna or Aerial??

I'm following , I think, what you are talking about.

VSWR keeps on cropping up, ?? Could you guys translate that into simpletons terms. The max field strength is pretty simple to visualise.

For my application I found the 9db was good for 30k's back to our base for river condition reports, as long as the car, (bullbar mounted aerial), was parked away from the base. Also, it was good on H'way because I usually go past someone I know and can then chat for a bit longer b4 they zoom on out of range.

I'm in a different location now but still in the desert, so flat as, hence 4.5db compromise.

I'll check the notes, I've found on the net Robin, , but from memory 2cm off should take it to 470-490MHz?

I take your point that for the avge user it's much of a muchance but what does changing the aerials MHz do??

Thanks,
Slammmin

Antennae = aerial. Interchangeable term to most. From memory, aerial is an old English term while antennae was the American term.
VSWR = Voltage Standing Wave Ratio. Product of mismatching a transmission line to it's load (usually an aerial). Optimum is 1:1, (but in practise usually unattainable, 1.1 to 1 is about the best you'll see).
Being simplistic, one can think of it as what's going to the aerial and whats coming back because the aerial isn't matched properly.
Changing the length of the aerial will change the angle of takeoff and it's feed impedance so you will get a higher VSWR.
Robin rekkons it works for him in flat country...try it and see. (sorry, no flat country around here)

Hi Guys

Yell out if I'm making this to hard Slammin.

I see you picked up on antenna aerial , I was just being pedantic
by sticking with aerial , I think I use them interchangeably
normally , my wife says I use aerial because engineers are always
looking for efficentcy and aerial is one letter shorter.

2cm is safe bet and changes the aerial "MHZ" to somewhere around 470.

All it means is that the aerial is resonant at that mhz, which means
that when looking into the aerial that it has only resistance and
no inductance or capacitance.

If the resistance is about the same as the cable (usually 50ohms)
then power will flow down the cable to the aerial and into space.

If the aerial resistance is different then as per Footloose some
bounces back to the radio and is loss (effectivily).

Often getting an aerial completely right is a two step process
1st you make it resonant , and then you convert whatever resistance
it has to the same value as the radio and cable using (in effect only)
a transformer.

A good aerial is designed so that when it is resonant then its resistance
is 50ohm anyway and so you don't have to do the next step. This is much
more so/easier for UHF than HF.

With your 9db aerial Slammin, it only has to be bent 5 degrees from vertical
to make its performance worst than the 4.5db one. So while good for
flat country it needs to be mounted such that when driving at 100kph
then it does not bend. Where I drive (mostly Vic bush) these aerials
can get knocked by bush , and if made flexible to withstand this then
they bend at 100kph.

Footloose - The satellite aerial was only relevent in very early days
when they were basically a fold out rod , probably all dishes now, don't know.

Would love to knock up a 5/8 for UHF cb and its on my very long
rainy day list of projects, unfortunately there are less rainy days these
days. I guess they use those 4.5db stainless whips instead (3/4 lamba)
probably because its 1 quick twist and cut on a machine.

Robin Miller