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General Antenna Question


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#1 roadrunnernm

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Posted 16 January 2015 - 07:35 PM

Hello All, 

 

General Antenna Question. 

 

Vertical mounted antennas, such as on a vehicle. 

 

I know there's a up side to a bigger antenna, 

 

Is there a down side     (other than the awkward physical size)     of using a full wave length antenna versus using a quarter wave length antenna.

 

Thanks in advance for your help and patience.



#2 JohnE

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Posted 16 January 2015 - 09:25 PM

IME no. other that theft.

been running these for over 25 yrs

http://www.antenna.c...gi?id_num=10942


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#3 gdavis316

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Posted 16 January 2015 - 09:27 PM

I have found a roof mounted 1/4 wave works just a good as anything i have used.....bigger is not always better.  just my preference, and then no worries.



#4 zap

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Posted 17 January 2015 - 01:37 AM

Technically a full wave isn't naturally resonant (needs a matching coil).

 

I've found very little difference between 1/4, 5/8 wave, and 5/8 over 5/8. Very little difference between 25 and 45 Watts too.

 

I run a PCTEL wideband knob (it's almost a 1/4 wave long) as I use my radio on 3 different services.



#5 DoctorZ

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Posted 17 January 2015 - 03:02 AM

Hello All, 

 

General Antenna Question. 

 

Vertical mounted antennas, such as on a vehicle. 

 

I know there's a up side to a bigger antenna, 

 

Is there a down side     (other than the awkward physical size)     of using a full wave length antenna versus using a quarter wave length antenna.

 

Thanks in advance for your help and patience.

A general question deserves a good general answer.....

 

In a nut shell, the antenna is only 1/2 of the system; the other half is the ground plane.  In the case of your vehicle, it acts as the ground plane.  Placement of your antenna on your vehicle will also affect your transmit and receive.  If you mount the antenna on the rear of the vehicle, you will have a directional (or beam) system.  Your transmit and receive will work better in the direction of the most metal, or in your vehicle's case, towards the front if mounted on the rear.

 

For the best all around performance, mount the antenna in the middle of the vehicle and make sure it has a good contact with the metal of your vehicle.  Fiberglass bodies, vinyl tops, etc., will greatly reduce your antenna's performance not to mention possibly damage your transmitter.  If you have a fiberglass body, you will need to drill through it to the frame or roll cage and attach it to there.

 

As for wavelengths, they say the best is a 5/8 wave, or a .64 wave if you can find or make one.  5/8 = .625.  A full wave antenna tends to radiate most of it's signal up in the sky.  At 5/8 you have the "flatest" take off angle of your signal, which in theory helps it cover the distance the best.

 

As for power, for every doubling of power you will gain 3dbs.  Each number on your radio's S-Meter (if it has one) is 6dbs.  So if you are giving someone 3 S-units on his radio's meter at 25W of transmit power, you will need to increase your transmit power to 100W to give him 4 S-units on his meter, given you don't change anything else.

 

With all that said, my best advice is for you to buy an antenna that is designed for the frequency band(s) you wish to talk on (450-470) for GMRS, and mount it as close to the center of your vehicle as you can, making sure it has good contact with the sheet metal of your vehicle's body.  A good ground and matched SWR on your antenna system will do more for you than anything else.



#6 zap

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Posted 17 January 2015 - 11:56 AM

A general question deserves a good general answer.....

 

In a nut shell, the antenna is only 1/2 of the system; the other half is the ground plane.  In the case of your vehicle, it acts as the ground plane.  Placement of your antenna on your vehicle will also affect your transmit and receive.  If you mount the antenna on the rear of the vehicle, you will have a directional (or beam) system.  Your transmit and receive will work better in the direction of the most metal, or in your vehicle's case, towards the front if mounted on the rear.

 

For the best all around performance, mount the antenna in the middle of the vehicle and make sure it has a good contact with the metal of your vehicle.  Fiberglass bodies, vinyl tops, etc., will greatly reduce your antenna's performance not to mention possibly damage your transmitter.  If you have a fiberglass body, you will need to drill through it to the frame or roll cage and attach it to there.

 

As for wavelengths, they say the best is a 5/8 wave, or a .64 wave if you can find or make one.  5/8 = .625.  A full wave antenna tends to radiate most of it's signal up in the sky.  At 5/8 you have the "flatest" take off angle of your signal, which in theory helps it cover the distance the best.

 

As for power, for every doubling of power you will gain 3dbs.  Each number on your radio's S-Meter (if it has one) is 6dbs.  So if you are giving someone 3 S-units on his radio's meter at 25W of transmit power, you will need to increase your transmit power to 100W to give him 4 S-units on his meter, given you don't change anything else.

 

With all that said, my best advice is for you to buy an antenna that is designed for the frequency band(s) you wish to talk on (450-470) for GMRS, and mount it as close to the center of your vehicle as you can, making sure it has good contact with the sheet metal of your vehicle's body.  A good ground and matched SWR on your antenna system will do more for you than anything else.

 

Not entirely accurate. You don't have attach an antenna on a fiberglass body to the metal chassis, especially with above 136 MHz communications. There are options such as no ground plane antennas (half-wave, matched antenna) which offer Unity gain without a ground plane as well as ground plane kits for running ground plane dependent antenna.

 

Antenna placement on the roof of a vehicle isn't as important as most people think, as long as there is a quarter wave of ground plane (sheet metal) in all directions and you can get it as high as practically possible, you're golden. 



#7 PastorGary

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Posted 18 January 2015 - 05:52 AM

I am using a 1/2 wave EM Wave  # EMFLX-M10003-GPI antenna on one of our Chaplain Responder vehicles and we can't tell the performance difference between that one from EM Wave and a 5/8 PCTel Mosaic 5 db gain style.  No ground plane required.



#8 gdavis316

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Posted 18 January 2015 - 09:17 AM

pastor Gary, 

 

Great looking antenna, thanks for the info.  

 

Glenn



#9 zap

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Posted 18 January 2015 - 10:12 AM

That's a nice looking antenna pastor. I have a friend who may want a handful of those for his tractors since some of them don't have the OEM business band option.

#10 PastorGary

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Posted 18 January 2015 - 10:35 AM

I can recommend it for certain. There is a cutting chart with it and just make certain to use a quality NMO mount and cable. I have mine cut for 465 mhz - midway between the 467 T and 462 R frequencies and the  SWR   doesn't even register on the meter.

 

My source was my local Kenwood dealer but they are also available from >>>

 

http://shopwiscomm.c...z-P2106563.aspx



#11 JohnE

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Posted 18 January 2015 - 01:10 PM

I am using a 1/2 wave EM Wave  # EMFLX-M10003-GPI antenna on one of our Chaplain Responder vehicles and we can't tell the difference between that one from EM Wave and a 5/8 PCTel Mosaic 5 db gain style.  No ground plane required and this one is mounted at the side of the rear spoiler on a Ford Edge.

 

EM Wave Link >>> http://www.emwaveinc...003-GPI_2dB.pdf

I'll have to get one of those just to field test for S&G's.

what I can tell you is that in horizon and weak signal areas for a unity there is a vast difference when using the taller 5db antennas in both TX & RX.

at some point I will post pics of the antennas I have used over the yrs, but as I've said before I keep coming back to the AS mosaic.

also w/regard to the using on a non metallic surface you will need to make some kind of ground plane. 1/4 wave at minimum but "if" you have the room to make it as big as the antenna is long that would be better.


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#12 PastorGary

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Posted 18 January 2015 - 01:49 PM

Thanks, John -  I have seen the Mosaic antennas used on fiberglass roofs by mounting a 12 inch circle of thin aluminum sheet stock under the NMO mount in the inside of the vehicle... works great.  Just be aware that some fiberglass roofs are thinner than others, so FLEX may be present with the 31 inch center loaded Mosaics in some applications.



#13 quarterwave

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Posted 18 January 2015 - 10:27 PM

When I started in radio an older tech said the only difference in a 1/4 wave and gain antennae was the price. 

 

I have never gone wrong with a good old 1/4 wave on any of my UHF equipment. I used a 1/4 wave Vhf as a scanner antenna for years.

 

The only time I had any better service out of a gain antenna was when I had a BC780 scanner in my work van. It had a ladder rack on it, and a UHF Motorola Spectrum (about 20 year old vintage) out shined the 1/4 VHF for both VHF and UHF reception, and 800 was about the same. 



#14 DoctorZ

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Posted 19 January 2015 - 02:19 AM

Not entirely accurate. You don't have attach an antenna on a fiberglass body to the metal chassis, especially with above 136 MHz communications. There are options such as no ground plane antennas (half-wave, matched antenna) which offer Unity gain without a ground plane as well as ground plane kits for running ground plane dependent antenna.

 

Antenna placement on the roof of a vehicle isn't as important as most people think, as long as there is a quarter wave of ground plane (sheet metal) in all directions and you can get it as high as practically possible, you're golden. 

From the OP's post I gathered that he/she wanted the maximum gain out of a mobile antenna.  No antenna that isn't grounded to the vehicle's metal is going to meet that spec.



#15 DoctorZ

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Posted 19 January 2015 - 02:40 AM

I am using a 1/2 wave EM Wave  # EMFLX-M10003-GPI antenna on one of our Chaplain Responder vehicles and we can't tell the difference between that one from EM Wave and a 5/8 PCTel Mosaic 5 db gain style.  No ground plane required and this one is mounted at the side of the rear spoiler on a Ford Edge.

 

EM Wave Link >>> http://www.emwaveinc...003-GPI_2dB.pdf

Well since we're showing off our antennas of choice, here's mine:

 

http://www.hamradio....m?pid=H0-001424



#16 Guest_spd641_*

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Posted 19 January 2015 - 09:27 AM

Well since we're showing off our antennas of choice, here's mine:

 

http://www.hamradio....m?pid=H0-001424

I am curious about the performance of that antenna would you please enlighten us with your experience using this antenna...William



#17 zap

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Posted 19 January 2015 - 10:16 AM

The 

 

I am curious about the performance of that antenna would you please enlighten us with your experience using this antenna...William

 

I have one on my Jeep and had one on my Trailblazer. They work as advertised as far as bandwidth goes but that's really the only good thing I have to say about them. I've never noticed one outperforming a 1/4 wave antenna and they aren't built to what I would call commercial standards. In fact for a while I actually switched to a Austin 500C (highly recommend that antenna but UHF is only about 10 MHz wide...after consulting with Richard he didn't think he could build one with enough UHF bandwidth to cover both 70 cm and GMRS) an noticed no change, especially on the VHF bandwidth.

 

I don't see much of a point in DX'ing UHF FM. 10 miles simplex is more than adequate for me, especially for any simplex family road trip GMRS activities one might be doing. The only difference I've noticed between a common 5/8 over 5/8 and my little PCTEL knob is about 1/2 mile less repeater usage over flat terrain.

 

I just run a regular Larsen NMOQ for VHF on my DD. For UHF I run this.

http://www.hol4g.com...=164550&sc=3566

 

My XJ currently has a CA2X4SR...it's okay. Working on moving over to the PCTEL and a NMO150WB where the Comet is. I may move the APRS tracker over to the 150 and then just run the Sti-Co quarter wave being used for ARPS for my VX-4000.

 

Most current pics I have of the Jeep (has a Larsen NMO-Q with spring instead of the Sti-Co).

536357ED-81D3-4249-BA0A-D791BABD1506_zps

 

3eb1317a-26f5-4207-befd-ae5ed1640f7f_zps



#18 DoctorZ

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Posted 20 January 2015 - 12:20 AM

I am curious about the performance of that antenna would you please enlighten us with your experience using this antenna...William

First off, let me state that no wide banded antenna will perform as well as one cut for a specific frequency range.  However, I find the performance of my CA2x4SR to be excellent!  It even tunes up great on my favorite VHF band, 220Hhz, and it's not even rated for that band!



#19 DoctorZ

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Posted 20 January 2015 - 12:31 AM

The 

 

 

I have one on my Jeep and had one on my Trailblazer. They work as advertised as far as bandwidth goes but that's really the only good thing I have to say about them. I've never noticed one outperforming a 1/4 wave antenna and they aren't built to what I would call commercial standards. In fact for a while I actually switched to a Austin 500C (highly recommend that antenna but UHF is only about 10 MHz wide...after consulting with Richard he didn't think he could build one with enough UHF bandwidth to cover both 70 cm and GMRS) an noticed no change, especially on the VHF bandwidth.

 

I don't see much of a point in DX'ing UHF FM. 10 miles simplex is more than adequate for me, especially for any simplex family road trip GMRS activities one might be doing. The only difference I've noticed between a common 5/8 over 5/8 and my little PCTEL knob is about 1/2 mile less repeater usage over flat terrain.

 

I just run a regular Larsen NMOQ for VHF on my DD. For UHF I run this.

http://www.hol4g.com...=164550&sc=3566

 

My XJ currently has a CA2X4SR...it's okay. Working on moving over to the PCTEL and a NMO150WB where the Comet is. I may move the APRS tracker over to the 150 and then just run the Sti-Co quarter wave being used for ARPS for my VX-4000.

 

Most current pics I have of the Jeep (has a Larsen NMO-Q with spring instead of the Sti-Co).

536357ED-81D3-4249-BA0A-D791BABD1506_zps

 

3eb1317a-26f5-4207-befd-ae5ed1640f7f_zps

 

I guess antenna preference is a matter of personal taste.  I have tried Larsen monoband antennas because others swear by them, and they have been the WORST pieces of junk I've ever tried.  No matter if you like them, that's great.  I'm glad you're happy with them.  I do question your testing techniques though, considering after close examination of your pictures, the Comet CA2x4SR seems to be buried down about half way below your roof line, while your precious 1/4 wave is mounted at the prime location top-center of your vehicle.  Do you think it's a fair test to compare those two antennas this way?

 

Assuming the SWR is still within acceptable range being the radiating element is so close to all that metal, I'd say about 50% of your signal is getting absorbed by your vehicle's metal roof, not to mention the strange take-off angles your transmissions must make from the Comet.



#20 DoctorZ

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Posted 20 January 2015 - 01:07 AM

Here is a little diagram taken from a very old book I have called, "The Big Dummy's Guide To C.B. Radio."  I realize that 11-meters is not the same as UHF 400Mhz, but the same principles still apply:

 


Attached File  MobileCBAntenna1.jpg   217.79KB   5 downloads






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