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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 01-14-2007, 2:16 PM
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Question 1/4 wave, 1/2 wave, 5/8 wave, full wave?

From what I understand a 1/2 wave antenna will perform better than a 1/4 wave, and a 5/8 wave will perform better than a 1/2 wave. So is it safe to assume a full wave will perform best of all, or is that not always the case?

If size is not a restriction, is it always best to use the longest design for a given frequency? Other than being smaller, is there any advantages to 1/4, 1/2, or 5/8 wave designs? Does any of this affect the band width of the antenna?

I'm only interested in "receive" only, not transmit, if it makes any difference.

Thanks!
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Old 01-14-2007, 2:21 PM
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Yes. Use the longest you can get away with. That's why the big 102" steel whips will roast on CB and VHF Lo because they're 1/4 wave. A full wave CB antenna would be 36'. Not too practical!
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Old 01-14-2007, 2:28 PM
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For recieve, the length isn't as critical as it is on transmit. When in transmit, if the antenna length is not right, the antenna will not resonate at the frequency you want it to, and will in turn radiate poorly, causing a high SWR and poor performance, and will also transmit "off" frequency, because the antenna is trying to resonate at it's own resonant frequency.

For optimal recpetion, it would be best to tune the antenna for recieve. This can be done by using an antenna length calculator. When an antenna is tuned for transmit, on say, 155.310, and the SWR becomes zero, a perfect match is created. Then the antenna will also be resonant for recieve at this frequency.

Kicker is, we can't always transmit through scanner antennae. So, I use the calculator to determine length for recieve. Confused yet? LOL..
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Old 01-14-2007, 2:40 PM
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Yes I understand all of that. I'm just wondering if say for instance an antenna of a shorter wavelength design would bring less noise, or interference into the radio, and would it have a wider bandwidth? I'm asking this because I often see systems using less than a full wavelength antenna that are not restricted from doing so. So I was assuming there may be other factors I'm not aware of for them choosing that design.
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Old 01-14-2007, 4:04 PM
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A 1/2 wave dipole is slightly better than a 1/4 wave ground plane and 5/8 or .64 wavelength vertical over a ground plane is slightly better still because they compress more signal at the horizon instead of wasting it up in the air. A full wavelength vertical antenna will have a big null at the horizon with lobes pointing up and down and a wet noodle will probably work better unless your talking to the space shuttle. That’s’ why I’m a little skeptical of the Off Center Fed Dipole project in the Wiki. It probably works fine on VHF low and hi, but UHF and 800 will have lots of lobes high in the sky and into the dirt with nulls at the horizon where you need the performance. On HF where hams string up horizontal antennas, the lobes on full wavelength and larger antennas (and OCFDs) can be put to good use with gain in some directions depending on antenna height. Keep in mind that antennas are reciprocal from transmit to receive and whatever is bad for transmit is the same and/or bad for receive.
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Old 01-14-2007, 4:21 PM
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In free space (or WAY above the ground), the antenna patterns look like this:
(this is from a web page for boaters, hence the water reference)



Simply put, as you go from 1/4, to 1/2, to 5/8 wave verticals, more energy is aimed at the horizon (better for longer distances in general).

As usual, more does not always equal better. A full wavelength antenna starts getting extra lobes:



(ignore the text on that pic)

Getting more complex as the antenna gets longer.

John K9RZZ

Last edited by k9rzz; 01-14-2007 at 4:28 PM..
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Old 01-14-2007, 4:27 PM
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Thanks everyone! Appreciate the info.
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Old 01-14-2007, 5:03 PM
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Quote:
Originally Posted by KG4LJF
Yes. Use the longest you can get away with. That's why the big 102" steel whips will roast on CB and VHF Lo because they're 1/4 wave. A full wave CB antenna would be 36'. Not too practical!
LOL, more like "Use the longest PROPERLY TUNED you can get away with"!
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Old 01-14-2007, 5:04 PM
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Quote:
Originally Posted by KG4LJF
. . . .and will also transmit "off" frequency, because the antenna is trying to resonate at it's own resonant frequency. . . .
Hu??????
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Old 01-14-2007, 6:20 PM
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Quote:
Originally Posted by prcguy
A 1/2 wave dipole is slightly better than a 1/4 wave ground plane and 5/8 or .64 wavelength vertical over a ground plane is slightly better still because they compress more signal at the horizon instead of wasting it up in the air. A full wavelength vertical antenna will have a big null at the horizon with lobes pointing up and down and a wet noodle will probably work better unless your talking to the space shuttle.
This statement, while 100% true, is worded in such a way that it's making me lol want to try making a wet noodle resonant on 2m or 70cm.
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Old 01-14-2007, 6:42 PM
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There is an odd facet of the hobby where people load up tubes of liquid and see how far they can transmit. I forget what they call it but there is probably a yahoo group set up for those folks.
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Quote:
Originally Posted by w4rez
This statement, while 100% true, is worded in such a way that it's making me lol want to try making a wet noodle resonant on 2m or 70cm.
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Old 01-14-2007, 7:04 PM
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There is no such thing as a full wave verticle antenna for any frequency. The design will not work electrically. The only full wave antenna design that I know of is a Loop.

The longest fractional wave verticle that does work is a 3/4 wave but you don't see alot of them offered commercially but they are around. The hamstick people make a 3/4 for 2 meters but there is not enough difference to be gained really over a 5/8 wave.....which is the most popular design. To get longer and see added gain from there you start getting into colinear 5/8 designs which are two 5/8 sections, one atop the other. Hustler makes a popular one for 2 meters and this model # is CG - 144
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Old 01-15-2007, 10:27 AM
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You can make a vertical antenna any length you want. 1, 3, 5, 7 wavelengths? If you've got a use for it in your mind, and think it will work, go for it!

John K9RZZ
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Old 01-15-2007, 2:41 PM
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"You can make a vertical antenna any length you want"

Yes....yes you can. You can also dress a pig up in a gown and take it to the prom but that doesn't mean you'll arrive wearing arm candy.
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Old 01-15-2007, 2:45 PM
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Quote:
Originally Posted by prcguy
A 1/2 wave dipole is slightly better than a 1/4 wave ground plane and 5/8 or .64 wavelength vertical over a ground plane is slightly better still because they compress more signal at the horizon instead of wasting it up in the air. A full wavelength vertical antenna will have a big null at the horizon with lobes pointing up and down and a wet noodle will probably work better unless your talking to the space shuttle. Thatísí why Iím a little skeptical of the Off Center Fed Dipole project in the Wiki. It probably works fine on VHF low and hi, but UHF and 800 will have lots of lobes high in the sky and into the dirt with nulls at the horizon where you need the performance. On HF where hams string up horizontal antennas, the lobes on full wavelength and larger antennas (and OCFDs) can be put to good use with gain in some directions depending on antenna height. Keep in mind that antennas are reciprocal from transmit to receive and whatever is bad for transmit is the same and/or bad for receive.
prcguy
I read this post and others which were of interest. So much so, that I would like to do some experimenting with 1/2 , 5/8, and 3/4 wave antennas, and did some googling.

As any of 'my'antennas would be for receiving only, is a matching device necessary? If so, without sophisticated equipment, how would one determine the impedance (high) in order to make such a device.

Also, is the matching device in effect, a baluun connected inline?

AM
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Old 01-15-2007, 3:21 PM
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Quote:
Originally Posted by ampulman
I read this post and others which were of interest. So much so, that I would like to do some experimenting with 1/2 , 5/8, and 3/4 wave antennas, and did some googling.

As any of 'my'antennas would be for receiving only, is a matching device necessary? If so, without sophisticated equipment, how would one determine the impedance (high) in order to make such a device.

Also, is the matching device in effect, a baluun connected inline?

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Old 01-15-2007, 3:44 PM
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Quote:
Thatísí why Iím a little skeptical of the Off Center Fed Dipole project in the Wiki. It probably works fine on VHF low and hi, but UHF and 800 will have lots of lobes high in the sky and into the dirt with nulls at the horizon where you need the performance.
Confirmed, i've modeled it over and over

performance is fine at the design frequency

2x, 4x, it's all skyward.

Even the venerable J pole spits a lot of 440 up in the sky, though not so bad.
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Old 01-16-2007, 5:05 AM
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Quote:
Originally Posted by ampulman
I read this post and others which were of interest. So much so, that I would like to do some experimenting with 1/2 , 5/8, and 3/4 wave antennas, and did some googling.

As any of 'my'antennas would be for receiving only, is a matching device necessary? If so, without sophisticated equipment, how would one determine the impedance (high) in order to make such a device.

Also, is the matching device in effect, a baluun connected inline?

AM
You certainly would benefit from matching the antenna. Depending how you feed your antenna, it could be a balun (balanced/unbalanced), an unun (unbalanced/unbalanced), or something entirely different.

To save grief, I'd take a look at some antenna books - buy, borrow, or check the library. If you're willing to go 3/4 wave, you might try a J pole, Super J pole, co-linear array, or ??

John K9RZZ
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Old 01-16-2007, 6:35 AM
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One thing to remember is that when you look at the transmist patterns for an antenna, it will have the same gains and losses in it receive side.

So if your interests include aircraft, a simple 1/4w is a better deal than a high gain ommni. If you are on top of a mountain, that 1/4 will give you better reception of the valley belwo, while that 5/8 will give you better long range performance.
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Old 01-16-2007, 7:45 AM
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Two comments:

When looking at verticals, the best source is an AM broadcast engineering text. There is nothing new in that field, and much of it can be translated into VHF performance.

The other thing is the wet noodle concept; QST recently had a photo of a guy that used 2 watermelons in a "dipole" arrangement on a picnic table on the 2 meter band. I also remember an article many years ago about a guy that made a few contacts on 10 meters using a lightbulb mounted on a fence.

So experiment and have fun, find what works best for you. Invest in a book or two.
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