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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 10-06-2011, 12:57 AM
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Default Does height of antenna really matter

Does height of the antenna on an 800MHz mobile antenna really matter? I see a lot of police cars with this antenna but then I also see some with taller types like this one. What gives? What's the difference between those 2 and how will they perform against one another? Thanks.
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Old 10-06-2011, 1:36 AM
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Height of ANY antenna matters greatly.

Your first link is to a simple, probably 1/4 wave, design. I would venture to say it is "unity" gain, or has very little gain.

The second link is a high-gain antenna.

Height, ground plane, and the length and quality of the coax are all taken into consideration with such high frequency applications.
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Old 10-06-2011, 5:13 AM
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That second link shows an antenna that's made up of more than just one 'element' (or antenna). sort of three antennas added together to make an antenna that's "three times" better. No, it really isn't "three times better", but it's definitely better than just "one" antenna. (Those 'elements' or "antennas" are stacked one on top of another.)
Antenna height. It's just like you can see further from a higher level than you can at a lower level. You can see further on top of your house than you can from the basement. Antennas at UHF are almost exactly the same way. The antennas on those cars may not be very high off the ground, but you can bet that the (base) antennas they are listening to are. That's one reason why the car's have a more limited range than the base radios.
- 'Doc

Last edited by LtDoc; 10-06-2011 at 5:17 AM..
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Old 10-06-2011, 5:29 AM
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Obviously the photos are of two different designs, one is clearly labelled a 5/8 wave with gain expressed in dbd and the other is a 1/4 wave. [dbd is gain over a dipole but dealers often use dbi [gain over an isotropic antenna]

It's not the "height" of the antenna that makes the difference in gain...
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Old 10-06-2011, 10:34 AM
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Ok, I'm having a hard time understanding gain. Why would I want gain on a scanner antenna? Also, why do those other antennas not have gain that are used on public safety vehicles??? Would this antenna be any good for a scanner?
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Old 10-06-2011, 3:32 PM
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Originally Posted by NewSDScanner View Post
Ok, I'm having a hard time understanding gain. Why would I want gain on a scanner antenna? Also, why do those other antennas not have gain that are used on public safety vehicles??? Would this antenna be any good for a scanner?
You can think of gain as being somewhat like a telescope; It effectively "amplifies" the amount of signal coming in to the receiver.

But, like a telescope, the more gain you have, the narrower the field of view. You get a highly magnified, but extremely restricted view of what you're looking at. In a public safety vehicle, you don't want that, you want something that receives equally well from all directions, regardless of the way the vehicle is pointed.
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Old 10-06-2011, 4:37 PM
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Quoting Dr. Ruth "size matters".
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Old 10-06-2011, 5:08 PM
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Not exactly, gain and directionality are two different factors. One way to achieve gain is to make it focus on one direction but that isn't the only way. If we are talking about scanners to receive public safety, more often than not, that is a vertically polarized and maybe omnidirectional signal. That's why so many police cars have omni vertical antennas. To get a meaningful answer to your question, you need to be more specific about the applicaiton you have in mind. BTW like with any antenna, the one you posted has many advocates, I don't have one on MY vehicle and wouldn't want one personally. I use a MOT 5/8 wave NMO mount 900 MHz 5 db gain antenna for my trunking system(s).
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Old 10-07-2011, 11:06 AM
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Question Height or Length

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Originally Posted by NewSDScanner View Post
Does height of the antenna on an 800MHz mobile antenna really matter? I see a lot of police cars with this antenna but then I also see some with taller types like this one. What gives? What's the difference between those 2 and how will they perform against one another? Thanks.
Don't confuse the LENGTH of the antenna, with the antenna's HEIGHT (above average terrain) - two separate issues.

LENGTH of antenna affects what frequencies it is tuned for. The longer antenna is typically better suited for lower frequencies. They are often designed with coils so as to have good coverage in several bands.

HEIGHT is significant. This is why "broadcasters" spend lots of money to build tall towers and/or put their transmitters on mountain tops. Elevation is key "seeing" to the curvature of the Earth and maximizing coverage area.
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Old 10-07-2011, 11:16 AM
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Question I am using a spider antenna and I have it sitting on top of a 20 foot PCV pipe. I cannot seem to pick up any better then with the Rubber duck antenna that came with the radio. most everyone in my area is trunking. Do I need to ground the antenna for better reception. Change the PCV to a medal pole of what antenna would you say to use? I can only pick up my own county trunking and not the next county over.
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Old 10-07-2011, 1:18 PM
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Question I am using a spider antenna and I have it sitting on top of a 20 foot PCV pipe. I cannot seem to pick up any better then with the Rubber duck antenna that came with the radio.
Antennas are not so much a "device" as they are a "system". I am not sure if the spider antenna you speak of is a base antenna or mobile. Base antennas typically have radials that act as a ground plane. Mobile antennas use the metal of the vehicle they are mounted on for a ground plane. If you're trying to raise a mobile antenna, it won't work (or work well anyway).

Quote:
most everyone in my area is trunking.
My translation of your statement is that these trunked systems are 800MHz. That makes me wonder what kind of coax you are using from the antenna to the scanner. If you're just using "any old coax" that could be your problem. A common problem is the length and type/quality of the coax between an antenna and receiver. Longer coax = more loss. To get the benefit out of raising your antenna you need coax that will not add more loss than you will be gaining from the increased height.

Quote:
Do I need to ground the antenna for better reception.
The typical reason for grounding is lightning protection. Otherwise you're just waving a metal device out in the sky, asking for it to be hit, and have the voltage travel directly to your radio. A ground plane is needed (see above).

Quote:
Change the PCV to a medal pole of what antenna would you say to use? I can only pick up my own county trunking and not the next county over.
More details would help. What is the antenna model, is it a base antenna? What frequencies are the systems on you want to receive? Next I'd check the RR DB for transmitting locations of those systems to determine their antenna height. Your antenna has to be high enough to "see" theirs over the curvature of the Earth for reliable reception.

A metal mast to support the antenna would be better than PVC, depending on where the bottom of that mast is. If it is into the ground, great. But if it is just mounted on the roof, not much difference. Either way a solid ground "wire" is best for lightning protection with no reliance on the mast for conductivity.
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Old 10-07-2011, 2:49 PM
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Quote:
Originally Posted by gmclam View Post
Don't confuse the LENGTH of the antenna, with the antenna's HEIGHT (above average terrain) - two separate issues.

LENGTH of antenna affects what frequencies it is tuned for. The longer antenna is typically better suited for lower frequencies. They are often designed with coils so as to have good coverage in several bands.

HEIGHT is significant. This is why "broadcasters" spend lots of money to build tall towers and/or put their transmitters on mountain tops. Elevation is key "seeing" to the curvature of the Earth and maximizing coverage area.
AHA! You brought up a very valid point. Thank you.

Thanks also to the people who better explained gain to me!
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Old 10-15-2011, 1:04 PM
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The first antenna shows unity gain (no gain over a dipole) and the second shows 5 DBd gain (5 DB over a dipole) 5 DB is quite significant over zero. Lets assume you were to transmit 10 watts. The first antenna would theoretically transmit an effective radiated power of 10 watts (not including cable loss, etc). The second antenna would give you an effective radiated power (with the same 10 watts) of about 37 watts. The same would hold true on received signals, ie. you would receive a signal 5 DB higher than the unity gain antenna.
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