hi M Rose, and
@El-Dracho can i ask what you think of all the different areials and radios please, also what watts or where to put them please. i have read above and found it very informative. i have heard its best to keep on roof by some people and then on the front, side or rear of the vehicle by others etc. little bit confusing. i have also heard people saying that you can have 5,25,50 watt machines but i am now confused. was wondering if people can help me please.
many thanks scrappy wolf.
Hi Scrappy Wolf,
TLDR: More transmit power is nice, but it's better to have a high quality antenna (2-3dBi gain for hill country, 5-6dBi gain in the flat lands) and to do a good job installing and matching it to your radio. The difference between antenna mount points is a compromise is only one part of the balance - fender vs roof isn't the end of the world.
The LONG version:
Comparisons like this can be a little complicated, so let me first introduce a relative unit of measurement for the power of a signal (also known as "gain"): decibels (isotropic) or "dBi" - we use this to measure relative radiated power, nominally versus an "isotropic" antenna (an isotropic antenna is one that if you put it in the center of a spherical shell, it would radiate its signal evenly across the inner surface of that shell), but dBi can also be used to compare either full systems (radio, antenna cable, antenna, antenna mount) or components of the system. dBi is measured on a logarithmic scale:
dBi difference | signal power ratio |
+20 dBi | 100x |
+10 dBi | 10x |
+3 dBi | 2x |
0 dBi | 1x |
-3 dBi | 0.5x |
-10 dBi | 0.1x |
-20 dBi | 0.01x |
So, when we compare a radio with three different transmit power settings of 5W, 25W, and 50W, we can think of these +0dBi, +3.3dBi, +10dBi settings. A 3 dBi difference in signal power is enough to be able to notice the difference, 0-2 dBi differences aren't usually enough to notice.
Next, we look at antennas. A 0dBi (aka isotropic or omni) antenna spreads the signal evenly across a sphere enclosing it. Antennas with "higher gain" basically "squish" this spherical pattern so that less signal is radiated up or down, and more signal is radiated toward the horizon (the equator of an enclosing sphere instead of the poles). For mobile use, if the gain is very high, the narrowly squished signal might "miss" a receiver on higher or lower ground. High gain (5-6 dBi) antennas are usually most useful on level terrain, whereas moderate gain (2-3 dBi) antennas are generally better on hilly terrain.
All other things being equal, the 3 dBi difference in gain equates to about a 2x (or 0.5x) difference in signal power and this represents a transmit/receive range difference of between 1.2-1.4x, if there are no ground clutter obstructions to the signal path. In practice, VHF/UHF radios are more generally limited by "line of sight" (the signals don't bounce off the upper atmosphere like HF signals). Ground clutter and line of sight (radio horizon) will usually limit radio range before power. (This is why repeaters are located on tall mountains, towers, buildings - to improve line of sight - a repeater antenna on a tall mountain can been "seen" 50-100 miles away.)
The coax cable connecting the antenna to the radio will also have some "loss" associated with it as will each connector. It varies based on the type of coax (there are dozens) but usually in the range of a few dBi per 100ft - don't bother chopping 5ft off your coax antenna cable, you won't notice the difference, but avoid adding extra connectors if you can - one piece, long enough, one connector at each end.
And finally there's the mount point - the "best" antenna position and mount is specific to each antenna. Of the two antennas I carry (one 6 dBi and one 3 dBi), one "likes" center of roof and one "likes" corner of roof. Finding the "perfect" place to mount an antenna on a particular vehicle usually requires a fair amount of testing, but in general consider the following diagram as a rule of thumb for comparison. For an antenna that "likes" the center of the roof, the largest difference in mounting position will only be ~3dBi, reflecting a difference in the radio range of ~1.2-1.4x.
Combining the radio power setting with antenna placement in this example to compare 5W w/roof-center antenna versus 25W (+3.3dBi vs 5W) w/mag-mount on the trunk corner (-3.4 dBi vs roof-center) tells us that these are about the same (3.3-3.4=-0.1 dBi).
This works for comparing antenna gains as well: what about a short 3 dBi gain antenna on my roof (which *will* fit in the garage) vs a longer 6 dBi antenna mounted on the front fender? The front fender antenna loses about 2.4 dBi, so 6-2.4=3.6dBi vs 3dBi on the roof - a difference of 0.6dBi, meaning the 6dBi fender antenna is slightly better, though not noticeably so (on flat terrain - remember the signal pattern differences).
How important can all this be? Consider a few combinations of 5W or 50W transmitter with omni (0dBi gain) or moderate gain (3dBi) antennas, mounted on the rear fender (-3.4 dBi) or the roof (0dBi) with a poor SWR (-2dBi) or a good SWR (-0.1dBi).
5W, omni on rear fender, poor SWR
0+0-3.4-2dBi = -5.4dBi
5W, omni, roof, good SWR
0+0+0-0.1 = -0.1dBi
5W, mod-gain, roof, good SWR
0+3+0-0.1 = 2.9dBi
50W, omni on rear fender, poor SWR
10+0-3.4-2dBi = 4.6dBi
50W, mod-gain, roof, good SWR
10+3+0-0.1 = 12.9dBi
The difference, worst to best is 18.3dBi which is about 67x difference in signal strength, about half (10dBi) due to the 10x power difference and about half (8.3dBi) due to the better antenna/install - while this won't change the line of sight, it might help the signal "punch through" some ground clutter (trees, buildings, rolling hills). Comparing "more antenna" head to head aginst "more power", the difference is only 1.7 dBi - not quite noticeable.
So why not just purchase the most wattage you can? More radio wattage ONLY allows you to transmit further, it doesn't help you *receive* from further - this is the magic of antenna efficiency - it improves both transmit AND receive. The HF guys who talk and listen around the world are typically doing so with radios running in the 5-50W range, but they are using extremely efficient antennas (along with a wave length that bounces off the upper atmosphere). My former Baofeng handheld (it died) with 5W and a rooftop antenna could use a mountaintop repeater ~50 miles away (with nice clear line of sight, clear dry day). My FTM-400XDR does the same on 5W, but if the line of sight is heavily obstructed, even 50W won't help. The difference? At 5W, neither quite managed full FM quieting and the Baofeng signal wasn't very "clean". At 25W the FTM-400XDR manages full quieting with a solid strong signal, 50W is unnecessary for this 50mile link, but I could likely still reach it from 70+ miles at 50W if the line of sight was good enough, while at 100miles, I can often still *hear* a repeater (they're often 100+W), but my signal isn't strong enough to to transmit clearly through it.
So, I tend to worry less about the radio's transmit power (the difference between 25W and and 50W is just noticeable) and I think more about the antenna (moderate 2-3dBi gain for hilly terrain, higher 5-6dBi gain for flat areas), antenna mount type (appropriate to the antenna), antenna mounting position (experiment), and antenna matching (SWR), since a good antenna system can easily "gain" you more dBi than additional transmit power.
