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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 08-28-2011, 4:50 PM
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Default 1/4, 1/2, 5/8 wave??

hey all, i know what the different wavelenghts of antennas are, my question is what are the differences in them besides length. i.e. performance differences, pros/cons of each...

thanks
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Old 08-28-2011, 6:38 PM
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1/4 wave is unity gain, short (compared to the others at least) and generally works well. It has a fairly uniform pattern both vertically and horizontally (meaning its patters looks like a circle as far as range and half a doughnut as far as radiation angle goes) so it works well in urban areas where the sites are on high buildings. It generally requires a ground plane to work as designed.

1/2 wave is quite similar to the quarter wave antenna where its main advantage is it doesn't require a ground plane to work as designed. There are versions that provide a bit of gain, but those also are somewhat directional (directionality increases with gain).

5/8 wave antennas generally gives about 3 dB of gain, is longer than the others and requires a loading coil to work correctly. It's omnidirectional, but has a high angle wave (so it works well when the site is not nearby and on a tall tower or mountain top, but not so well when the site is nearby and atop a tall building). It requires a ground plane to work as designed.
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Old 08-28-2011, 7:33 PM
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thank you for clearing that up for me. i always wondered about that. i live in a very hilly terrain, so would it be best for me to use a 1/4 wave antenna? espescially for mobile.. i am working on getting my tech ticket and am just getting a few pointers out of the way.
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Old 08-29-2011, 2:00 AM
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What band(s) are you looking to use? The selection is probably better based on band coverage than antenna design. If you're using the lower frequencies 1/4 wave is probably the largest you can use mobile (often they're not even that, but a loaded smaller antenna just to keep things manageable). Antennas for the higher frequencies (2 meters and up) are generally smaller so you have a selection of antenna designs to choose from. When you move from a single-band to dual-band (or more), the choice of designs are fewer and are often more of a "which factor to compromize on" than "which wavelength should I use".

Normally I have several antennas that I change out depending on what I'm doing. Making sure that all of the antennas use a standard (and common) mount is important. If you standardize on the NMO style mount you'll find a great selection of antennas that will work on it from VHF-Low (30 MHz or so) on up through UHF or higher. They even make some CB antennas that use it as well as some in the GHz bands. Since the NMO mount is pretty much the standard for commercial two-way antennas they're quite common and easy to find.

Around town when I'm VHF Only, I often just have a 1/4 wave due to the size (about 19") so it's a better fit for parking garages, but change to a 5/8 wave for cross-country trips for the gain (and lower radiation angle) to reach further between towns. Most often though I simply stick on my dual-band VHF/UHF for good coverage on both bands (as long as the 3 foot or so height isn't going to be troublesome). I'll probably get the short version of the dual-bander (it's only about 19" tall) someday to get the best of both words and trade it out as necessary too.

Which antenna works best for you is often a trial-and-error process. Actually "error" is probably not the correct word since each antenna probably has a use as long it is resonant on the band you're using. Having a selection is good since you can easily replace one that gets damaged (or stolen) and get back on the air until your desired replacement arrives. You can often find used antennas at a swap meet or hamfest to help build your selection at minimal cost. Just make sure that they're in good shape and cut for the ham bands and not the commercial frequencies (which often would require a whip replacement and eliminate any savings). For scanner use though the commercial cut ones would be fine.
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Old 08-29-2011, 8:57 AM
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What 'size' antenna would be best for your situation? Take a look around, see what others are using. Not just hams, but the various commercial/public service agencies. I'd be willing to bet that a 5/8 wave is very common in your area (it is in most). I think you'll also notice that the vehicles with longer ranges have 'longer' antennas than those only for 'in town' use, that's another common difference in the 'size' of antenna used.
Aesthetics, how it 'looks', is a characteristic that seems to be showing up more lately. That's another one of those 'compromise' areas. Wha-da-ya want, 'looks' or performance? Or a compromise that will sort of do both? Your choice.
And while it can get expensive, having a selection of various sizes of antennas for a particular band does have a use (as described above). And then there's the idea of keeping the "significant other" happy. You are on your own there! I don't care what I say, if that "significant other" isn't happy with it, I did NOT say it!
- 'Doc
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Old 08-29-2011, 9:24 AM
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The problem with going by "size" is that factor is quite dependent on what frequency is being used. Comparing antennas from agency "A" that's on VHF-Low's antenna with agency "B" that's on the 800 MHz band would appear that agency "A" uses a very long antenna that must be many wavelengths long (while it's probably at best a 1/4 wave long) while agency the "B" antenna is so much shorter it must only be 1/4 wave (while it's most likely is actually a 5/8 wave over 1/4 wave design).

Looks by themselves are problematic. A base loaded VHF-Low band antenna can look quite similar to a VHF-Hi 5/8 wave antenna. They both can have a small loading coil at the base and a whip that's about 50" long (for example this VHF-Low antenna http://www.comtelcoantennas.com/PDF%...eets/A1801.pdf and this VHF-Hi antenna http://www.comtelcoantennas.com/PDF%...eets/A1813.pdf).
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Old 08-29-2011, 10:04 AM
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The best description I can give on differences electrically is the way we were taught in USAF school. The unity gain antenna vertically polarized has a donut over it. For gain as in 5/8 wave antenna, which is longer in length causes the donut to get squashed down, giving gain out over the horizontal plane while making the thickness of the donut less.
For mobile applications, I have never really seen any major difference in signal strength, only when operating in fringe areas. Base applications especially repeaters, high gain antenna's work the best, as long as you don't over shoot you operating area, by too high a gain at too high an elevation.
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Old 08-29-2011, 9:05 PM
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thank you guys for the explanation. i am a vol. firefighter and some of our trucks have 1/4 wave antennas while others have 5/8 wave. and the difference in the two are nada, as far as im concerned, but we do have dead spots with both trucks in our area as the highest powered radio is 45 watts. i have a motorola syntor x9000 in my jeep that is a 100 watt version with a 1/4 wave on the roof and i can talk to the dispatch on t/a from 30 miles away.just wondered ifthere was any difference in them. thank you very much guys. you are the type of people that want a man to become a ham. i look forward to talking to you on the radio soon. we have a testing oct. 1.


73s guys.
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Old 08-29-2011, 9:11 PM
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btw, i dont care how it looks, nor does my wife. i guess i got one of those...lol she is actually wanting to get her license to. so looks doesnt matter to us. just what works best. thanks again for clearing that up guys. i guess i have some "trying" to do.
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Old 08-30-2011, 4:37 AM
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She want's a license too? Oh are you in trouble!
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Old 08-30-2011, 4:27 PM
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Quote:
Originally Posted by n5ims View Post
1/4 wave is unity gain, short (compared to the others at least) and generally works well. It has a fairly uniform pattern both vertically and horizontally (meaning its patters looks like a circle as far as range and half a doughnut as far as radiation angle goes) so it works well in urban areas where the sites are on high buildings. It generally requires a ground plane to work as designed.

1/2 wave is quite similar to the quarter wave antenna where its main advantage is it doesn't require a ground plane to work as designed. There are versions that provide a bit of gain, but those also are somewhat directional (directionality increases with gain).

5/8 wave antennas generally gives about 3 dB of gain, is longer than the others and requires a loading coil to work correctly. It's omnidirectional, but has a high angle wave (so it works well when the site is not nearby and on a tall tower or mountain top, but not so well when the site is nearby and atop a tall building). It requires a ground plane to work as designed.
A few minor nits, OK, maybe not so minor. I have seen a couple of these repeated on these forums a few times, but they are incorrect.

A ¼ wave monopole is not unity gain, unless the “unity” is in reference to another ¼ wave monopole antenna, and ¼ wave monopole antennas are not normally used as reference. There are two prevalent references for antenna gain, the isotropic radiator and the ½ wave dipole. In reference to the isotropic the ¼ wave has -0.85 dB of gain (specified “dBi”, the “i” is what tells you isotropic based). In reference to the ½ wave dipole the ¼ wave has -3.0 dB of gain (specified as “dBd”, the “d” is what tells you dipole based). So a ¼ wave antenna can be said to have either -0.85 dBi or -3.0 dBd of gain, both are negative values and less than unity.

The ¼ wave monopole antenna can be used as a reference (indeed, anything can be a reference, say a paperclip that has been straightened out), and when a ¼ wave monopole is used for reference the designation is “dBq”, the “q” tells you it is ¼ wave monopole based. This is NOT a widely used standard, but it is used by some manufacturers to make their numbers “bigger”. In such a case the ¼ antenna would be said to have 0 dBq of gain, or to be “unity”.

By the same token the ½ wave antenna is said to have +2.15 dBi or 0 dBd of gain. Using the none standard dBq it would be said to have +3 dBq. The half wave dipole has 3 dB of gain over the quarter wave because it has twice the aperture, or capture area.

And a 5/8 wave antenna is said to have about +3 dBi or +0.85 dBd of gain. This could be said to have about +3.85 dBq. You called the 5/8 wave antenna “about 3 dB of gain” and this would be correct if using dBi, but in such a case your 1/4 wave example definitely would not be unity gain, or 0 dB, on the same scale.

T!
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Old 08-30-2011, 9:14 PM
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With a proper counterpoise (also called a 'ground plane') a 1/4 wave vertical is a 1/2 wave dipole (one element being the antenna and the other element being the counterpoise). This is why they show to be unity (or 0 dBd) gain antennas. Without a proper counterpoise the vertical will not perform properly and with no counterpoise at all would indeed be half the gain as you indicate (but this generally can't happen outside of straight theory since the feed line will attempt to act as the counterpoise).

Your 5/8 wave numbers also assume the counterpoise doesn't exist. With a proper counterpoise they will provide "about 3 dB of gain" as indicated.

If you simply run the numbers based on straight theory you'll get results that translate to real world use like the isotropic dipole translates to a real world dipole, the numbers just don't line up. The problem with the theory is that many real-world variables are simply left out (basically because they vary with the environment). This is how real-world factors like the counterpoise often turns up to be missing when running the formulas.
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Old 08-31-2011, 11:23 AM
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Quote:
Originally Posted by n5ims View Post
With a proper counterpoise (also called a 'ground plane') a 1/4 wave vertical is a 1/2 wave dipole (one element being the antenna and the other element being the counterpoise). This is why they show to be unity (or 0 dBd) gain antennas. Without a proper counterpoise the vertical will not perform properly and with no counterpoise at all would indeed be half the gain as you indicate (but this generally can't happen outside of straight theory since the feed line will attempt to act as the counterpoise).

Your 5/8 wave numbers also assume the counterpoise doesn't exist. With a proper counterpoise they will provide "about 3 dB of gain" as indicated.

If you simply run the numbers based on straight theory you'll get results that translate to real world use like the isotropic dipole translates to a real world dipole, the numbers just don't line up. The problem with the theory is that many real-world variables are simply left out (basically because they vary with the environment). This is how real-world factors like the counterpoise often turns up to be missing when running the formulas.
In several aspects I have no real argument with what your are saying, however there is a need to level the field, and that was what I was doing by reverting to the theoretical.

Any apparent gain of a ¼ wave in reference to either an isotropic or a dipole is dependent on the specifics of the counterpoise. Indeed, a ¼ wave monopole can be said to have 5.16 dBi (Kraus, 1950) under certain specific conditions, far greater than unity (you might also look at Wolf, 1966, defining the ¼ wave monopole as having twice the affective area of a dipole, although I have always taken exception to that paper). It is also possible to define conditions where both ½ and 5/8 wave antennas have greater gain than their theoretical configurations. And we have not touched on the ½ wave monopole issue, an antenna that certainly has more gain than the ¼ wave monopole.

The “real-world” application can be extremely variable.

For example, a 39 foot long 18 element wide spaced Yagi on 2 meters could be said to have as much as 27 dBd of gain…but unless you specify the exact height above ground, the ground conditions, and the departure angle, it would not exhibit such a number. So better manufacturers go with the “free space” (a condition impossible in most applications) gain value instead of over ground, and in such a case the value is more like 15 dBd. And this is a bit of an oddity, because many Yagi manufacturers keep to the theoretical while many mobile manufacturers do not. It is not uncommon to see ¼ wave mobile advertised as “0 dB” without specifying either the efficiency or the mechanical configuration of the ground plane or counterpoise.

As you said, my 5/8 numbers are correct without counterpoise, the gain is even greater in many real World applications.

To the OP, and related to his question. As you can see, there are many variables. As a general rule of thumb, for a specific application, the physically larger an antenna is for a given frequency the better its potential to perform is. A full sized ¼ wave antenna will sometimes outperform, in the real world, a reduced size 5/8 wave antenna, despite the fact that the 5/8 wave has a theoretical advantage.

Mobile antennas typically are ¼ wave or 5/8 wave, or some multiple or combination thereof (multiples and combinations used in collinear arrays). ½ wave antennas traditionally have not been used in mobile application, but certainly can be, either as monopoles or as dipoles...and there has been a large trend recently (last 10 years or a bit more) to use ½ waves in mobile application. ½ wave antennas in the form of dipoles are frequently used in fixed location applications (call it base station application, if you want).

For practical applications in a mobile world the ¼ wave vertical is often all that is needed. Unless you are in a fringe area or are looking for just a little extra range the added gain of a 5/8 might not matter. A high gain collinear mobile antenna can add up to 6 or even 9 dB to your signal, but again, unless you are in a fringe area you might never notice the difference. If you want the best possible performance decide what is the tallest antenna you can get away with, keeping in mind your parking garage/overhang limitations, and find the largest antenna (in actual size, height in inches) designed for your frequency range that you can fit in that space. As a general starting point this is hard to beat, real capture area on the antenna is an important factor.

With either of my off-road SUVs on HF (160 to 10 meters) I use a ¼ wave, depending on the band it might be a loaded ¼ or not. On 6M I use a 5/8 wave. On 2M it depends on the vehicle, either a 5/8 wave or a collinear antenna. On UHF and 1.2 GHz they are all collinears of some kind. However, these are all fairly tall antennas, and I must keep them in mind at times when driving under things. My wifes SUV has a simple dual band antenna (Diamond NR770B), ½ wave on 2M and collinear 5/8 wave on 70cm, and still mounted on a motorized mount so she can lower it in parking garages. I use the same antenna on my convertible to keep everything a bit more low key.

There is a near infinite selection of antenna options out there, at some point you just pull the trigger and jump in.

T!

Last edited by Token; 08-31-2011 at 11:28 AM..
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Old 08-31-2011, 12:10 PM
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Your descriptions are quite intense for this crowd, while I agree that some manufacturers will pad their results by quoting obscure antenna references, for the most part at least in commercial 2-way radio the 1/4 is considered unity over a theoritical antenna (isotropic) and a 5/8 is considered to be 3db gain.
Getting into antenna theory is more for those that want to build their own, as I have on many occasions. Mainly in the 70 mhz band, since they are so few made and are expensive.
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