A conversation regarding the advantages of various gain antennas ..... worth a read !!

I have a friend ( yes even I have a friend ). He and I worked for a company that specialised in turnkey communications solutions for all sorts of companies ( I won't name them  ). He was a project manager that has carried out work all over the world.

I raised the question with him regarding the differences between 3dB, 6dB and 9dB antennas.

This is how the conversation went:-


"I have a technical question about antennas ...
Youve seen those antenna radiation patterns ... the simple ones that demonstrate the difference between 3db - 6db and 9 db antennas ?

My question is, that although they show the shape of the radiation pattern .... what sort of distances do these patterns cover at lets say 5 watts.

Hundreds of metres, or many kilometres ?

I ask because another Ulysses rider and I fitted up UHF's with external Ground independent antennas to the bikes ( brilliant coverage by the way ) and I wanted to know if there was any real practical difference between a 3 - 6 and 9 db antenna and what distances these differences would cover ... if any."

The Stigs Answer:-

Re: I have a technical question about antennas ...
The diagrams on that brochure give you a good idea of what to expect from the pattern. To get gain the antenna the radiation pattern will be more focused, and the more focussed the pattern the more sensitive it will be to your position, the terrain, etc.

So to compare the 3 dB to the 9 dB, the 9 dB pattern is a very flat donut around the bike, so if you were on top of a range the power would tend to shoot out at the same level as the range, with little power getting into the valley below you. In that scenario the 3 dB would work better into the range below you, but a 9 dB would work better with someone 30 km out on the plains in the distance. It's simply a case of where the other dude is angle wise from perpendicular to the antenna, relative to the angular width of that radiation lobe.

I think they are a bit creative with their gains, I doubt the 9dB antenna has 9 dB of gain. But ignoring that, the relative performance characteristics are what you are interested in.

Assuming there are no terrain considerations - assuming you were in space, and assuming the other guy was in your optimum radiation (in the middle of the lobe), then :- if one antenna had 5 dB gain it is producing a field strength at the other station twice that of a 2 dB gain antenna - i.e. 3 dB improvement is twice the power into the receiver. But as the radiated power actually drops by square law, i.e. power drops to a quarter for a doubling of distance (so 6 dB drop for double the distance) you would get about 50% more range for the 5 dB compared to the 2 dB antenna (in space).

But that is in space. On the ground at UHF CB frequencies the terrain plays the biggest part. The curvature of the earth comes into play even on flat ground, and add a hill between you and the other guy and the range is totally screwed. The range then depends on the size of the hill, how close you and he are to the base of the hill, if there are any other hills or towers about that might reflect some signal, acting like a passive repeater for the signal, etc etc. Then you add in the factor of the beamwidth, i.e. how narrow is that lobe, and if it's narrow it will be even more effected by the angle of the bike (i.e. if you go up a hill your forward beam is aiming into space, if you are going down a hill your forward beam is aiming at the base of the hill and not out into the plains beyong where your mate is).

The other factors on vehicles is the ground plane of the vehicle and other obstructions, in your case your body, unless you have stuck that dipole way up high. (typical "ground independant" antennas are end-fed dipoles). In a recent test of a backpack mounted ground independant dipole I found the performance was 1 dB worse on the backpack (the base of the antenna would be at shoulder level, so the main obstruction was my head) compared to un-obstructed. So if you have your body between the antenna and the other guy, you could expect about 3 dB of reduced gain, depending on how close it is to you. This is typically bad luck..not a lot you can do about that (like a mobile phone would work a lot better if you didn't hold it up to your head). You could improve the performance significantly by raising the antenna a couple of metres, but, well, it would get a bit of attention.

So distance is really a lottery, dependant on all of these factors. With this power and frequency I have had conversations over a genuine 102 km distance - with a UHF handheld on a job in the Middle East, from the top of a 5000 ft mountain across the desert to the coast. We had an antenna I made from some earth cable at one end, and just the hand-held alone at the other. My earth wire antenna might have had about 6 dB gain. But you won't get that on the flat, and certainly nothing like it in hilly terrain, but you might do OK when both bikes are on the tops of hills for e.g. So, as strange as it might seem 2 bikes, each with the 4.5 dB antennas, could have a range of 1 km or 100 km, depending on terrain.

Basically, I wouldn't use anything other than a single dipole ground independant for your purpose (this is probably what you have - the 4.5 dB one on the brochure). I wouldn't go to the high gain antenna unless I was out on the nullabor or something, where the gain would be useful and would work, as the terrain is dead flat...the hiils and stuff not coming into the equation so much. If you used a high gain antenna in the normal terrain we ride in I'm sure the performance on average would be worse, better to try and lift what you have as high as you can live with - lifting it a metre is probably worth 3 dB of level.

Oh, and the longer the cable between the radio and the antenna the greater the loss, it's about 0.25 dB per metre, so it would take 12 metres to reduce the range by 25%, heh, I doubt you have that much cable!! but keep it to a minimum. Terrain is the biggest factor.

I hope this answers your question.

Antennas, range
Heh, I couln't help myself and ran some calcs this morning........

Using an Icom IC41S (5 watt) specs (typical non-toy unit)

If you were in space:

Hand-held to hand-held = 2,600 km range
ground independant to ground independant = 7,300 km range
9 dB to 9 dB at both ends = 20,600 km range
(both of the extra antenna models i used 3 metres of coax)

So the performance of the radio and antennas is not a limiting factor (bear in mind the hand-held to hand held model assumes no body losses).

Now, assuming we are on earth, but we are on a flat plain...no hills.

The distance between the two bikes with ground independant antennas at 1.2 metres base height is 9 km when the horizon starts to come into play, after 9 km the earth starts to get in the way and the signal will start to drop off.

If you raised the antenna by one metre the distance stretches out to 12 km. Another metre and it's over 14 km.

For the situation we had in the middle east (5000 ft mountain to the coast) we should have been able to get out to 160 km from that mountain with those antennas, according to theory, so the 102 km is well within that.

So if you are working into a repeater of course you need to take into account the height of the repeater, so the horizon is not such a limiting factor. But bike-to bike the range is limited by the horizon AND NO ANTENNA GAIN WILL SAVE YOU!! So don't bother with the high gain ones.
.... Of course fine tuning your bike to bike Comms equipment means bugger all if the guy your talking to is running a piece of string and a tin can !!
So you want to fit a cb radio to your bike ?

Here are some things I have learned:-

1. Generally for any given power output, the higher the frequency then the shorter the range .... But better the quality of the signal.

2. Bigger is best, if your going to fit a UHF radio to your bike then use a radio that you can fit a ground independant antenna to. Expecting to be heard on a half watt radio running an inbuilt stubby antenna is like .... Well it's just not gonna happen fella !!

However if you were to take that same half watt radio and fit it with a ground independant antenna, then that half a watt would get you out there a lot further.

3. Get Real !! A $30.00 UHF radio from Supercheap Auto is not going to perform like a professional $400.00 radio from someone who specialises in radio. Spend the money and buy QUALITY !!

I buy all my comms stuff from these guys:-

Prestige Communications

BlueRim Motorcycle Accessories

4. Radios are a two way device, running all the right gear will certainly help you get a strong signal out to your riding buddy .... But it wont help you hearing him if he is running a $30.00 rig.
Signal Range