wave confusion! Mobile antenna related

[snakecharmer]

I have a handfull of antennas that I have collected and am trying to figure out the best use with my scanners. Basically I have 3 scanners in my truck and will be configuring each one for use in a particular band. One will monitor the 800mhz trunked stuff, one will do the 450mhz band and one will do the 150mhz band. In a couple of cases I have more than one antenna designed for a particular band and here is when the confusion sets in. Figuring which is most likely to give better performance and why.

I admit to being basically antenna stupid so please bare with me on this one. I have been googling all night and am probably more confused than I was to begin with. I am completely lost when it comes to wavelenth. I mean I understand that the relationship between any particular band and the physical length of the radio wave but I don't really understand how it translates to antennas. For instance....why is it that you only really see 1/4, 1/2 & 5/8 wave antennas? I understand that a full wave or even half wave antenna in a lower band would be prohibitively long but in something like 800mhz, why bother with a 1/4 wave antenna when a full wave antenna would be just over a foot long? And why stop there, why not use a monster 24" whip for a double wave super antenna?! I'm sure there is a good reason but this is where I get lost.

And when choosing for instance weather to use a 1/4 wave vs a 5/8 wave antenna, is it simply a matter of bigger is better and it comes down to aesthetics or is there a trade off somewhere like the longer wavelength has more range but in a narrower band or what? And to really turn my brain to mush, why are there single band antennas that are 5/8 over 1/4 or 5/8 over 5/8? I kind of understand in a dual band antenna like a 2m/440mhz but why in a single band? I mean a 1/4 over 1/4 wave antenna would be about the same size as a 1/2 wave antenna so what is the advantage?

Sorry for all the questions as Im sure this stuff has been beat to death around here but I cant find it.

 

[n5ims]

Here’s a dumb analogy (don’t you hate them?), while not 100% accurate it helps explain the differences without going through all the theory and stuff.

A simple wire antenna radiates similar to a doughnut with the antenna stuck into the hole. Most of the signal radiates evenly along the length of the wire and very little from the pointy end.

Assuming this is a vertically polarized antenna, like most mobile antennas are, like this doughnut, the signal is round and sweet to the horizon, but not much signal goes straight up to the sky. Different wavelength antennas simply squish the doughnut into different shapes, giving you the different patterns. (Yes, I know this is more like a balloon, but doughnuts sound tastier.)

A quarter wave antenna is like the classic unsquished doughnut. A fairly even radiation pattern so we call it a “no gain” antenna. You get good coverage both horizontally and at fairly good angles (roughly 45 degrees) vertically. This antenna is good in cities where you’re on the ground and receiving from high antennas as well as ground signals fairly close to you.

A 5/8 wave antenna will squish the top of the doughnut, which pushes the pattern further out, but not as tall. This gives it “gain” in the purely horizontal plain, but reduces the signal vertically. You can talk further away, ground to ground, but not as far ground to high antennas close in. Ground to high antennas far away is also generally good since the distance brings them closer to the horizon from where you are.

I hope this helps, I’m going to get a doughnut!

 

[snakecharmer]

Thanks for the donut analagy, that does help answer the core question. For instance....I have 2 antennas that are tuned for the 155ish mhz range. One is a simple 1/4 wave whip while one is a 2.4db gain 5/8 wave no ground plane antenna. The 155mhz stuff I monitor is farther away than the 800mhz stuff I monitor. So armed with your information I would think that the 5/8 wave antenna would be a better choice. But of course that brings up a new issue. The 5/8 wave is too tall to go on top of my truck especially since I already have a fairly long CB antenna up there. That would leave it to be mounted most likely on the side of my bed toolbox. Being a no ground plane antenna, this should work but if I understand what I was reading last night, you just loose the 2.4db gain that you would have if you used the antenna with a proper ground plane? So the main question is that for a decent distance, wold the 5/8 wave without a groundplane still outperform the 1/4 wave mounted on the roof, probably near one of the rear corners?

 

[zz0468]

Thanks for the donut analagy, that does help answer the core question. For instance....I have 2 antennas that are tuned for the 155ish mhz range. One is a simple 1/4 wave whip while one is a 2.4db gain 5/8 wave no ground plane antenna. The 155mhz stuff I monitor is farther away than the 800mhz stuff I monitor. So armed with your information I would think that the 5/8 wave antenna would be a better choice. But of course that brings up a new issue. The 5/8 wave is too tall to go on top of my truck especially since I already have a fairly long CB antenna up there. That would leave it to be mounted most likely on the side of my bed toolbox. Being a no ground plane antenna, this should work but if I understand what I was reading last night, you just loose the 2.4db gain that you would have if you used the antenna with a proper ground plane? So the main question is that for a decent distance, wold the 5/8 wave without a groundplane still outperform the 1/4 wave mounted on the roof, probably near one of the rear corners?

Mounting the shorter antenna on the roof will get you a better omni-directional pattern. The gain antenna on the toolbox may work better in some directions, and in other directions, not so good. And it may be enough to notice. The 2.4 db difference is BARELY noticeable, so, I would opt for the 1/4 wave on the roof, if it was my install.

And to really turn my brain to mush, why are there single band antennas that are 5/8 over 1/4 or 5/8 over 5/8? I kind of understand in a dual band antenna like a 2m/440mhz but why in a single band? I mean a 1/4 over 1/4 wave antenna would be about the same size as a 1/2 wave antenna so what is the advantage?

Throw out the 1/2 wave antennas for now. They don't belong in this part of the discussion. The 1/4 or 5/8 over 5/8 antennas are phased collinear antennas, and it's done simply for more gain. In the balloon (or donut) analogy from above, it's squishing the balloon even more. More gain at the horizon (5-6 db?) and less at the higher angles. Some of those antennas are over kill. High gain antennas don't do well in a mobile environment.

Back to the 1/2 antenna for a minute... the 1/4 over 1/4 antenna you mention, well, that's called a dipole, and it's fed in the middle, not the end. An end fed 1/2 antenna has a high feedpoint impedence, and would need a matching network where it connects to the coax.

 

[prcguy]

On the question about full wave and and other antennas, using a 1/4 wave ground plane with its doughnut pattern at the horizon as an example, every time you add a half wavelength to its length you will add a set of lobes. In the case of a 3/4 wave vertical ground plane there will be a null or loss of signal at the horizon and most of the signal will go at angles up and down where you don't need it. As the antenna gets longer there will be more lobes and gain in those lobes but they may not be at the horizon. To produce gain at the horizon you need to add elements in phase and a 1/4 wavelength delay section (usually in a coil form) is placed between 1/2 wave or 5/8 wave elements so their patterns combine in phase and reinforce the main lobe at the horizon. This is not quite as effective as a large base station antenna that has room to separate the active elements at the proper spacing to achieve the maximum gain, so it takes about twice as many collinear stacked elements as used in mobile antennas to get the same gain as an exposed dipole array base antenna. Another interesting point is a 3/4 wave antenna will match fairly close to 50ohms so its common to use a 1/4 wave antenna at VHF where it provides a reasonable match at UHF (3 X the frequency). Even though the pattern is not ideal at UHF it's a convenient way to get two bands with one antenna.
prcguy

 

[fineshot1]

On the question about full wave and and other antennas, using a 1/4 wave ground plane with its doughnut pattern at the horizon as an example, every time you add a half wavelength to its length you will add a set of lobes. In the case of a 3/4 wave vertical ground plane there will be a null or loss of signal at the horizon and most of the signal will go at angles up and down where you don't need it. As the antenna gets longer there will be more lobes and gain in those lobes but they may not be at the horizon. To produce gain at the horizon you need to add elements in phase and a 1/4 wavelength delay section (usually in a coil form) is placed between 1/2 wave or 5/8 wave elements so their patterns combine in phase and reinforce the main lobe at the horizon. This is not quite as effective as a large base station antenna that has room to separate the active elements at the proper spacing to achieve the maximum gain, so it takes about twice as many collinear stacked elements as used in mobile antennas to get the same gain as an exposed dipole array base antenna. Another interesting point is a 3/4 wave antenna will match fairly close to 50ohms so its common to use a 1/4 wave antenna at VHF where it provides a reasonable match at UHF (3 X the frequency). Even though the pattern is not ideal at UHF it's a convenient way to get two bands with one antenna.
prcguy

I have a Austin multi-band (2mtr/220/440) antenna (1/4, 1/4, 3/4) I think it was called the Metropolitan or something like that - anyway it is 3/4 wave on uhf and was useless as the radiation pattern was too high an angle and all the rf was going up & away from the horizon. I could not even reach reasonably local repeaters. Worked ok on 2mtr and 220 where it was 1/4 wave on each band.

 

[wannabescannist]

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Great thread you guys, lots of good info.

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