2 meter mobile antenna help?

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BrettL

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I have a truck that I don't mind drilling holes in. I can mount a homebrew or manufacturer antenna on top of the cab or on the tool box in the center. I will be using a ball mount for the mounting applications.

I am currently using a Comet SSB-7. It seems to work but I want more/better. I don't want a mag mount. I want a antenna with good ground capabilities. I have a MFJ Antenna Analyzer and have been reading about Antenna Theory. If I read about Antenna Theory for ever how long God intends for me to live I will never understand all of it!

The Comet SSB-7 has the PL-259 fitting that connects to the SO-32 base. I though about using a SO-32 chassis mount on top my roof and screwing the antenna into it. This way it would be grounded instead of on a magnet. I know I will have to make a larger square plate underneath the chassis mount to add stability to the roof so it don't flex the roof causing too much stress.

I have not found a lot of info about 5/8, 3/4,7/8 wave antennas for mobile applications. I want better omin direction gain than the Comet SSB-7 if it is possible. I read where a guy built a j-pole with 300 ohm twin lead and mounted on his rear bumper. I liked the idea except it did not ground to the vehicle and was on the bumper. I DO NOT mind scraping the tops of things. If I have to park away from something and walk to it then that's fine. I need the exercise anways!!

I have multiple CB antennas I can destroy or I can go buy the materials new. I want to use shortest length of coax if I can.

Any ideas or where I can get some good info on a high Omni gain grounded mobile antenna?
 

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SCPD

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If you don't mind drilling holes... why not go with an NMO mount? It'll offer more options for you in the future should you decide to upgrade/need to replace an antenna. Depending on your location/geography, a 1/4 wave may work better for you than a 1/2 or 5/8 wave and it could be less expensive as well. The NMO could/should be easier to install on the roof top vs. installing the ball mount on a tool box, fender, etc. If you plan it right, the hole can be drilled and the NMO mount installed, and coax cable routed in less than an hour.
 

BrettL

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There is a NMO mount a top my truck. It has the name Larsen with a small antenna on it. Not sure if they make different kind of mounts. I tried to analyze the small antenna with my MFJ 259-B but at the last frequency setting of 174 Mhz the SWR and the Resonance started coming down some. Then I ran out of the analyzers capabilities. It don't go up to UHF frequencies. This antenna on the NMO is mounted near the rear of the roof. I can get it to the center so my antenna can get a better ground plane if it will help it. I would rather build my own 5/8 wave collinear to put on my roof. I would rather have the headache and trying to tune and make bridge or whatever it takes to do it than to spend the $50.00. If I get too aggravated after many attempts then I may consider purchasing a antenna.

I wanted to build one and try and get a better understanding with what I'm working with using materials I have readily availiable. I just need to join a Amateur Radio Club and make friends I guess. I'm still Lone Wolf at the moment. I get tired of reading for hours. I don't understand most of it so I'm just going to spend a lot of time trying to figure it out.
 

kayn1n32008

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Go with a NMO mount centre of your roof. Put a mono band(if only using it on VHF ) 5/8 wave antenna on it. Larsen, Maxrad, PCTel, Sinclabs all make 5/8 wave coils. Cut it just above the centre of the 2m band and trim for best VSWR. Unless you need a wide band antenna, or need an antenna that will work with out a ground plane stay away from 1/2 wave antennas. Myself, I stay away from the hammy companies like comet, NCG and MFJ.

I currently have an Larsen NMO-2/70(dual band) for my TM-V71a and a Larsen VHF-WB for my work radio, it is cut for 162MHz, and it works ok on our local ARES simplex in the 145.XXX range.


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Seven-Delta-FortyOne

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I'll second (or third) the NMO mount suggestion. That's what I have on the truck, with a Larsen 5/8 wave.

There have been studies that suggest that there is no significant gain from a 7/8 over a 5/8 vertical. You really need to go to a collinear for more gain than what you get with a 5/8 wave.

In stead of using the analyzer, you could figure the length mathematically. Length = 234 divided by Frequency in MHZ , for a 1/2 wave vertical. Take the product, and divide by 4, and multiply that number by 5, to find length of a 5/8 wave. This will be in feet, and hundredths of a foot. You'll have to convert to get inches.

If you have a Larsen NMO mount, there is probably a allen screw that holds the whip. that should enable you to remove whatever is in there, and put in a 5/8 wave whip.




Delta
 

prcguy

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A 5/8 wave whip and especially a 1/2 wave whip is nowhere near 50 ohms, so you can't simply replace a whip on a particular coil or mount and have a working antenna. There is stuff in the base coil that helps match a 5/8 wave, 1/2 wave or other whip lengths that are not inherently 50 ohms.
prcguy

I'll second (or third) the NMO mount suggestion. That's what I have on the truck, with a Larsen 5/8 wave.

There have been studies that suggest that there is no significant gain from a 7/8 over a 5/8 vertical. You really need to go to a collinear for more gain than what you get with a 5/8 wave.

In stead of using the analyzer, you could figure the length mathematically. Length = 234 divided by Frequency in MHZ , for a 1/2 wave vertical. Take the product, and divide by 4, and multiply that number by 5, to find length of a 5/8 wave. This will be in feet, and hundredths of a foot. You'll have to convert to get inches.

If you have a Larsen NMO mount, there is probably a allen screw that holds the whip. that should enable you to remove whatever is in there, and put in a 5/8 wave whip.




Delta
 

Project25_MASTR

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Quick basic antenna theory. Antennas are only naturally resonant at odd quarter waves. Otherwise they will need to be matched. Think of a 5/8 wave antenna as an antenna that is physically 5/8 of a wave long but electrically (as seen by the radio) 3/4 wave long.

You'll probably spend a few hours building and antenna and matching it properly. You've already got an NMO mount on the truck. Find the coax, spend the $30 on a NMO antenna and the 10 minutes swapping it and running it. The antenna will probably outlast the truck...what's your time worth to yourself?
 

BrettL

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In stead of using the analyzer, you could figure the length mathematically. Length = 234 divided by Frequency in MHZ , for a 1/2 wave vertical. Take the product, and divide by 4, and multiply that number by 5, to find length of a 5/8 wave. This will be in feet, and hundredths of a foot. You'll have to convert to get inches.

Delta

2.010 is what I got and converted into inches is a little over 24 inches. Now this Is for a 5/8 wave mobile antenna and a piece of stainless steel I was planning on using.

But this is NOT the 6 or 7 foot the manufacturers are getting. So that should mean that they are collinear. So how would I get the antenna a collinear ? Would I stack 2 more elements on top of one another to achieve the gain results I want and that would get it higher to the 6-7 foot ?
 

prcguy

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The formula for a 1/2 wave is 468/frequency so a 2m 5/8 wave would be about 4ft. Usually a collinear will have a 1/4 wave (or 90deg phase shift) coil between the 5/8 or other length elements to place all elements in phase so their patterns combine and provide gain. A 5/8 over 5/8 collinear would then be about 8ft tall and there are other combinations like 5/8 over 1/2 wave which would come out shorter.
prcguy

2.010 is what I got and converted into inches is a little over 24 inches. Now this Is for a 5/8 wave mobile antenna and a piece of stainless steel I was planning on using.

But this is NOT the 6 or 7 foot the manufacturers are getting. So that should mean that they are collinear. So how would I get the antenna a collinear ? Would I stack 2 more elements on top of one another to achieve the gain results I want and that would get it higher to the 6-7 foot ?
 

tech020

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You're missing something here. At 146Mhz, a 1/4 wave is about 19". 5/8 wave would be 2 1/2 times 19" or about 46.5". You would also need a matching coil to make it properly tune. A 5/8 on the roof of your truck is about as good as it gets. With a NMO mount, you can swap out for a 19" whip if you want a lower profile.
 

kayn1n32008

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You're missing something here. At 146Mhz, a 1/4 wave is about 19". 5/8 wave would be 2 1/2 times 19" or about 46.5". You would also need a matching coil to make it properly tune. A 5/8 on the roof of your truck is about as good as it gets. With a NMO mount, you can swap out for a 19" whip if you want a lower profile.


What he said... Yes you can buy a collinear 5/8 over 5/8 but you WILL bang it off of low trees, overpasses, and anything else low to the ground... A good old Larsen NMO-150 will serve you just fine.
 

BrettL

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Ok, I guess I will just buy one from a manufacturer. I don't care how long it will be. I was still going to put it on top of my truck cab and let it hit tree limbs. I was reading where an added coil presents a loss, but if it has to be added to be tuned to make the 5/8 collinear then I can somewhat understand.

If I were to build it I could make it where it is long and take a beating. Is there a rule of thumb with the length of low loss coax to run from my transmitter to antenna to help tune the SWR a notch? I would prefer the shortest distance that gets me to the transmitter with a little slack. When I operated CB I ran 9 feet of coax instead of the 18 ft they claimed. I always had to make my antennas a bit longer to achieve a match. Truthfully I never got a SWR under 1.7-1.9 but focused more on having no reflect and it was close to 40 Ohms. That was 10 years ago when I did that stuff. I about forgot what little bit I knew and having to re-learn it.

When I get a little more educated I will make me one in the future. I just didn't want to waste a bunch of materials in the process and have hundreds in an antenna I can buy for $50.00. I just like the challenge and get results and maybe get a better understanding what happens when I do this or that to the antenna. Take notes and such so I can revert back to what happened.
 
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BrettL

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There are very few 2m antennas that use a ball mount or 3/8-24 thread but you are in luck, one of the highest gain 2m mobile antennas made fits your mount. Check out the Hustler CG-144, which is a dual 5/8 colinear and it stands 7ft tall. Here is one on Ebay: Hustler CG 144 5 8 Wave 2M 5 2DB VHF Mobile Radio Antenna 3 8 24 Mount | eBay
prcguy

So if I add the 3 inch ball mount it will be longer and I would have to cut or re-tune this antenna for a match?

YEah before I used that formula I was getting around 4 ft 1/4 inch using the antenna calculator.

Sorry didn't mean to double post.....I had two tabs open.
 

teufler

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A comet CSB770A is a 5/8 wave 2 meter and a 5/8 wave uhf, 51" tall. It is a afold over design for parking in garages . The Comet CSAB780A is the same but only 1/2 wave design. Anything taller, you will break at usually the top coil when you hit a tree or a bridge over pass. You reach a point of gain over function.
 

prcguy

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The Hustler CG-144 is made for ball mount use and its tunable. Just make sure the center conductor of the coax going to the ball mount is as short as possible. It's also a very rugged antenna and I've known people that have run them for 10+ years with no problems. One guy had his on the roof of a VW van and it was always hitting something but never seemed to break.

There is another "secret" antenna out there that might be better than the Hustler CG-144 and this one probably has more gain than any 2m mobile antenna out there. Its 8ft tall and uses a PL-259 mount but I have a PL to NMO adapter on one and it works fantastic.

This antenna has a base coil and fold over feature right above the coil and its also fairly broad band covering more than the 2m band with a great match. Best thing is its only $30 including shipping. The person selling them doesn't really know what they are and after I order another one I'll post a link to them, since there is a very limited supply.
prcguy

So if I add the 3 inch ball mount it will be longer and I would have to cut or re-tune this antenna for a match?

YEah before I used that formula I was getting around 4 ft 1/4 inch using the antenna calculator.

Sorry didn't mean to double post.....I had two tabs open.
 

Seven-Delta-FortyOne

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A 5/8 wave whip and especially a 1/2 wave whip is nowhere near 50 ohms, so you can't simply replace a whip on a particular coil or mount and have a working antenna. There is stuff in the base coil that helps match a 5/8 wave, 1/2 wave or other whip lengths that are not inherently 50 ohms.
prcguy

My NMO mount doesn't have a coil of any kind. It's merely a holder for a thin steel whip.

I tuned the whip at a full 5/8 wave at 147.000 MHZ, and never had a problem.



Delta
 

prcguy

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What's probably happened is you tuned the whip to a 3/4 wavelength which will match well to 50 ohms. Problem is a 3/4 wavelength whip puts most of the energy up and down at high angles with little at the horizon. A 5/8 wavelength whip at resonance is a poor match to 50 ohms, so if you got a good match its probably not 5/8 wavelength.
prcguy

My NMO mount doesn't have a coil of any kind. It's merely a holder for a thin steel whip.

I tuned the whip at a full 5/8 wave at 147.000 MHZ, and never had a problem.



Delta
 

Seven-Delta-FortyOne

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What's probably happened is you tuned the whip to a 3/4 wavelength which will match well to 50 ohms. Problem is a 3/4 wavelength whip puts most of the energy up and down at high angles with little at the horizon. A 5/8 wavelength whip at resonance is a poor match to 50 ohms, so if you got a good match its probably not 5/8 wavelength.
prcguy

I didn't know that.

I've never put an SWR meter on it, I just tuned it mathematically, and went with that. :eek:

Now I need to go re-check it.

I did the same with my GMRS whip. It's on an NMO mount, tuned mathematically to 5/8 wave at 462.500 MHZ.




Delta
 
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