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Author Topic: 2m mobile dipole?  (Read 5181 times)
KG4OLW
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Posts: 168




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« on: June 07, 2013, 11:58:43 AM »

I have a Jeep wrangler, I have a high sierra 1800 mounted on the rear drivers side in line with the fender. I welded a bracket that bolted to the frame where the tow hook connects. However I have been looking for a place to put a 2 meter APRS antenna. The top is fiberglass and does not leave a lot of good spots to mount a vhf antenna.

I could mount it to the tire carrier, and deal with poor grounding issues, and poorer ground plane, which would cause directionality towards the front of the vehicle, but very week signal out the sides and rear.

I could mount it to the hood with a diamond k400c trunk mount, but again any of the verticals will have directionality along the longest length of metal.


My last thought was to create a 2m dipole and mount that to the tire carrier so that the entire 40" dipole is above the fiberglass roof (vertically oriented).



I searched the forum but could not find any relevant posts by anyone who has created a vertical 2m mobile antenna.

Any thoughts or suggestions?
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KJ4OBR
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Posts: 104




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« Reply #1 on: June 07, 2013, 12:54:30 PM »

I'd just get a 5/8ths wave antenna like a Larson NMO-50 and put it on the tire carrier. What you are trying to gain is going to be offset by problems in the long run.

73

Dave

I have a Jeep wrangler, I have a high sierra 1800 mounted on the rear drivers side in line with the fender. I welded a bracket that bolted to the frame where the tow hook connects. However I have been looking for a place to put a 2 meter APRS antenna. The top is fiberglass and does not leave a lot of good spots to mount a vhf antenna.

I could mount it to the tire carrier, and deal with poor grounding issues, and poorer ground plane, which would cause directionality towards the front of the vehicle, but very week signal out the sides and rear.

I could mount it to the hood with a diamond k400c trunk mount, but again any of the verticals will have directionality along the longest length of metal.


My last thought was to create a 2m dipole and mount that to the tire carrier so that the entire 40" dipole is above the fiberglass roof (vertically oriented).



I searched the forum but could not find any relevant posts by anyone who has created a vertical 2m mobile antenna.

Any thoughts or suggestions?
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KG4OLW
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Posts: 168




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« Reply #2 on: June 07, 2013, 06:03:22 PM »

Seems like it would not be too much longer than a 5/8 wave, and in small diameter pvc should be fairly small. I am going to attempt it, whats the fun without a little experimentation!
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KA4NMA
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Posts: 357




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« Reply #3 on: June 07, 2013, 08:50:13 PM »

Read Alan, K0BG.com website.  Check out your jeep.  Reread Alan's website.  Read the section on grounding and bonding.  Check out your Jeep. Reread Alan's website.  With proper bonding and grounding, the tire carrier could be useful.

Many years ago, I was running a mobile 10m radio to hatchback mounted .25 cent cb antenna from the local flea market.  I heard that car parts were painted, then assembled and that car parts need to be bonded together.  This was pre internet days.  I checked and no continuity between the hatchback and car body. No knowing better, I ran a 12 gauge wire from the hatchback to the car body, with plenty of length to open and close the hatchback (another mistake, it should be as short as possible).  I do not remember if I did anything else, but I did have continuity between the hatchback and car body.  My dad (a former CB'er when you had to have a license in the early 1960's) was so amazed that my first contact was Germany with 20 watts running into a converted CB whip.  I learned that you can improve a mobile ground plane. 

Randy ka4nma

Randy ka4nma
Randy ka4nma
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WX7G
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« Reply #4 on: June 08, 2013, 05:04:46 AM »

The 2 meter vertical dipole will work. It has the same current distribution as a 1/2 wavelength ground plane and will work the same.
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K0BG
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« Reply #5 on: June 08, 2013, 06:35:11 AM »

There are two problems when trying to use a vertical dipole, mobile or otherwise.

The lower end is closer to conducting material, whether that be the earth, or the vehicle body. This makes tuning a bit difficult. The worse scenario is the requirement that the coax must be orientated perpendicular to the antenna.
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KG4OLW
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« Reply #6 on: June 11, 2013, 06:36:23 PM »

I went with the Larson 2m/7b nmo on a diamond k400cnmo mount, works fantastic.
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KE7FD
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« Reply #7 on: June 12, 2013, 08:27:37 AM »

I once had a Hustler CG-144 2m Colinear vertical that was simply the best antenna I've ever encountered for mobile use.  However and alas, it got wrapped up in my in-laws garage door and is no more.  The size of the beast is not for the faint of heart, but does it play well!  If your mounting, install and actual use of such an antenna is compatible, that's what I'd suggest using since some digipeaters and igates can be few and far between; reaching out and touching someone can be marginal in places.

Glen - KE7FD
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W5LZ
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Posts: 477




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« Reply #8 on: June 13, 2013, 06:25:21 AM »

Another method is that if the top of the Jeep isn't metal, then you add a little metal to make the 'other half' of a typical mobile antenna.  A 1/4 wave groundplane is what you'd end up with (or 5/8 wave groundplane if that 'size' of antenna grabs'ya).  That metal that you add doesn't have to be obtrusive, put it on the inside of that roof.  Or use metal tape and glue it to the top of the roof.  Or just mount it to the front fender or hood?  That 'directionality' with mobile antennas isn't always very directional at all.
 - 'Doc
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KE7FD
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« Reply #9 on: June 13, 2013, 08:03:27 AM »

I recall a friend of mine who did commercial installs yeas ago on fiberglass 4x4 cabs, used aluminum tape to provide the needed ground plane.

Glen - KE7FD
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N6AJR
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« Reply #10 on: June 16, 2013, 12:15:40 PM »

for aprs, you can probably use most any  mobile antenna mounted on the tire carrier, as apes will work on very low power signals.
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KE7FD
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« Reply #11 on: June 20, 2013, 10:34:28 AM »

You should do a site survey before you go too far down the road deciding on what you're going to do.  If the area where you'll be using APRS is covered well with digi's or igates then you can probably get away with using a minimum antenna and power.  If however, these are few and far between, you'll need to increase your power and beef up the antenna gain. The place to look at this is:
http://aprs.fi/#!lat=39.99680&lng=-75.14850

Slide the map around to look at the areas you will be operating and maybe even ask some of the APRS users where you live about coverage.  It'll be a real bummer if you get all set up and things fall short and you need to rethink things.  On the flip side you might be able to get by using a much smaller set up.  In my area there are some very good coverage with some very dark shadows where hills block coverage.  I sometimes get much better coverage from a far distant digi than a closer one (N8UVQ-4 as opposed to W3EXW).  I once had a D700A as still have a HT based APRS and either one could sometimes hit the digi in Ohio 30+ miles away than the digi 10 miles away all because of terrain. So, like I said do a site survey, talk to some locals and you'll be doing yourself a big favor.  You could just get the best antenna that will work on your Jeep and a power amp to just use brute force but it's better to listen to other traffic letting your client play nice with others.  I often drive to Columbus, OH and there is NO coverage in places along I70 east of Zanesville, Ohio.  In that case, a brute force client might be the only option. 

Again, do a site survey of the areas you plan to spend time in with the understanding you won't know everywhere you're going to be but try to figure out where you'll spend most of your time.

Glen - KE7FD
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K4LJA
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Posts: 22




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« Reply #12 on: June 21, 2013, 01:42:05 PM »

Also --- check http://www.jk-forum.com/jk-cb-ham-radios-128/ for other tips and ideas.  Good luck.  Love my JK Sport.

Randy in Monroe, Louisiana
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W4KVW
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Posts: 508




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« Reply #13 on: June 21, 2013, 07:06:53 PM »

He ALREADY posted that he went with a Larson 2/70 NMO which was my choice as well since that's what I run on my pickup truck.Very good antenna with good performance on both bands.Larson antennas are tough to beat for price,performance,& dependability. {:>)   Smiley   Grin

Clayton
W4KVW
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