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eHam Forums => Mobile Ham => Topic started by: KF7GTU on June 25, 2013, 03:31:39 PM



Title: Best antenna mount...
Post by: KF7GTU on June 25, 2013, 03:31:39 PM
Hello folks... :)

My wife and I are taking a trip, 3000+ miles in all. I have a Yaesu FT-2900 and a Diamond 5/8 mag mount currently. Works great, back country or freeway, all speeds.

One problem, it is mounted in the center of the roof (2007 Kia Sportage) and that position will interfere with the 2 mountain bikes that will be placed on top. SO... I am hoping to hear some advice.

I was looking at the Comet CP-5 series, either the UHF or NMO connector. Then, adding a short black dual band antenna, though I would prefer a mono VHF band antenna, I can't seem to find a black 1/4 wave or short VHF mobile antenna. A 5/8 wave is definitely out of the question anywhere but smack dab in the center of the roof area. It is simply too long and annoyingly in the way.

I was thinking that one of those MFJ-1738 units might be ideal for this situation, but our vehicle has factory tinted glass windows. Supposedly, this glass mount will not work in that case.

Has anyone had any personal experience with using a 2m mobile rig with a full roof rack? Any wisdom/guidance would be very appreciated.

Thanks!

Jason
KF7GTU


Title: RE: Best antenna mount...
Post by: K1CJS on June 25, 2013, 07:50:54 PM
Supposedly there is some room in front of the roof rack.  If possible, use the NMO mount and put it as close to 19 inches back from the front of the roof line as possible.  It should work fine--even if there are bikes on the roof.


Title: RE: Best antenna mount...
Post by: N6JSX on June 25, 2013, 09:00:51 PM
Your question only covers part of the issue with mobile antennas.

First, you need to understand the antenna and what it needs to give you optimal performance and a uniform omnidirectional radiation pattern.  As a rule of thumb any antenna that has a base loading coil wants to see a uniform and equidistant metal counterpoise (aka the center of the vehicle roof/trunk lid) to obtain the best radiation pattern with the lowest angle of radiation (for maximum flat-land/simplex distance).

If you (like me) want to put you antenna on the vehicle edge, roof rack, mirror you need to look into what the antenna needs for a proper counterpoise radiation pattern. I have selected and built my own CB-whip J antenna (2m/70cm) that requires no vehicle counterpoise - it affect it creates its own. My article: http://www.eham.net/articles/28805

If there is a drawback to this J antenna it is in its size, ~65" tall.

[and the AE5QB J is "not" that same as mine --- I have grave issues with his antenna and would like to see some real data and patterns he will not divulge!]

I mount mine on my SUV roof rack rail and get fabulous long distance results, due to using a J in the clear (by design the J exhibits one of the lowest angels of radiation that can be obtained on a mobile).  My article:  http://www.eham.net/articles/28807
Notice where I placed my mount, next to the center roof rail support to minimize wind loading affects and to minimize the coax run into the vehicle rear passenger door.

I could just as easily (if needed) turn my roof rail mount "out" on the passenger side and mount the J, still getting the same great results. Most all states allow a 6" (past mirror) over hang on the passenger side of the vehicle but must be flush with the drivers mirror side).

Here is an alternative to drilling holes in the roof. Can be removed leaving no marks it was ever there.

Do not use one of those glass mounts as you will LOOSE +50% of your TX power out and greatly reduce your RX sensitivity!


Title: RE: Best antenna mount...
Post by: KF7GTU on June 26, 2013, 12:59:13 AM
Thank you folks...

We'll keep all of this in mind and let you know what we decided to do and how well it works out. Our trip is in September, so I'll let you know then!

Thank you again, have a great day!

Jason


Title: RE: Best antenna mount...
Post by: WN2C on June 26, 2013, 01:30:00 AM
My suggestion is to get a different rack and put the bicycles on the back of the car.

Rick WN2C


Title: RE: Best antenna mount...
Post by: K5LXP on June 26, 2013, 07:21:34 AM
Diamond 5/8 mag mount currently. ...One problem, it is mounted in the center of the roof

It's a mag mount.  So move it.  About the only accommodation I'd be concerned with is that the whip doesn't contact the metal parts of the bike while underway.  You might exchange the 5/8 for a quarter wave for the trip, which would be more compact and have a smaller "reach" radius from the mount.

Technically the bikes in close proximity will affect the pattern.  Depending on what you expect of it, it may not matter.  On cross country trips you're not going to be working a lot of VHF and for what little you do, optimized range isn't going to be much of a factor. 

In a past life I used to have a service van with a ladder rack and ladders clamped to it, and somewhere in the middle the end of my 2M quarter wave poked out.  If it made a difference I couldn't tell with the repeaters and simplex I worked with it.

If you wanted to make a science experiment out of it, run some A/B tests in the driveway with and without bikes, and the antenna placed in different spots. 


Mark K5LXP
Albuquerque, NM


Title: RE: Best antenna mount...
Post by: W5LZ on June 29, 2013, 05:57:33 PM
Why not use your present antenna and place it on the hood of your Kia?  There's no particular liability in doing that, typical radiation pattern will be very nearly the same as now.
 - Paul


Title: RE: Best antenna mount...
Post by: K1CJS on June 30, 2013, 05:30:59 AM
Why not use your present antenna and place it on the hood of your Kia?  There's no particular liability in doing that, typical radiation pattern will be very nearly the same as now.

There is the possible problem of having something in the line of vision that some motor vehicle authorities take a dim view of.  No matter that it isn't really blocking anything, just having it there may be problematic to some policemen/departments.


Title: RE: Best antenna mount...
Post by: KG4RUL on June 30, 2013, 06:36:45 AM
Why not use your present antenna and place it on the hood of your Kia?  There's no particular liability in doing that, typical radiation pattern will be very nearly the same as now.

There is the possible problem of having something in the line of vision that some motor vehicle authorities take a dim view of.  No matter that it isn't really blocking anything, just having it there may be problematic to some policemen/departments.

Providing the Kia body panels are not plastic, why not mount it, like they used to mount car radio antennas, on the front fender?


Title: RE: Best antenna mount...
Post by: W5LZ on July 01, 2013, 05:45:00 PM
If the hood or fender is typical metal there shouldn't be any problems.  The likelihood of a LEO citing anyone for an antenna on the front of a vehicle is slim to none.
 - Paul


Title: RE: Best antenna mount...
Post by: K1CJS on July 02, 2013, 05:28:29 AM
If the hood or fender is typical metal there shouldn't be any problems.  The likelihood of a LEO citing anyone for an antenna on the front of a vehicle is slim to none.

True enough if the antenna is mounted to the side and is out of the way--most LEOs wouldn't look twice.  

If the antenna were to be put in the middle of the hood, however, and the vehicle was being driven with it there, there would always be the odd LEO that would complain about it.  Not necessarily ticket the offender, but stop him and make him move or remove it, and under most state laws that LEO would be correct in doing so.


Title: RE: Best antenna mount...
Post by: N6AJR on July 02, 2013, 10:46:50 AM
SO check out the MFJ 310.  its a gizmo that you roll up in your rear  drivers side window and put your rubber duck on it. You can also put a modest replacement antenna on it too.  this gives you a small antenna  above the roof line of the  car. also great for rental cars and such. it has a 7 foot cord. cool way to do things,.


Title: RE: Best antenna mount...
Post by: KA4NMA on July 03, 2013, 07:58:59 PM
Check out Alan's website at k0bg.com.  He has a ton of good information.  I would recommend the Larson 2/70 with a nmo mount.

Randy ka4nma


Title: RE: Best antenna mount...
Post by: KA2ODP on July 14, 2013, 12:11:47 AM
Nobody has mentioned checking the SWR!  The bikes in close proximity will affect the radiation pattern and the SWR, with the SWR being the important factor.  The resulting SWR readings will tell you if your radio can live with the arrangement.  Just judging by the receive peformance is meaningless.  Just going by the fact you can still hit a repeater is meaningless if high SWR is causing the transmitter output section to "foldback", resulting in low output power.  If the VSWR is reasonable, (less than 2.0:1) then you have your answer as to whether the antenna location will work with the bikes nearby.  If you are not going to check the SWR, there is no point in asking if your arrangement is going to work.  The SWR reading will tell you if your transmitter will tolerate the nearby bikes.


Title: RE: Best antenna mount...
Post by: K1CJS on July 14, 2013, 08:44:47 AM
...If the VSWR is reasonable, (less than 2.0:1) then you have your answer...

Even a 3.0:1 VSWR can be reasonable.  You said it yourself--IF you can hit the repeater...  As long as you're not going to be having a marathon session with the transmitter of your transceiver, you won't have to worry too much about getting a 2.0:1 or better VSWR.  That little extra difference (2.0:1 to 3.0:1) isn't going to mean that much--to the signal or to the transceiver.

Get the mount in the place where it's going to be, then set up the rig and adjust the setup for best VSWR without the bikes on the vehicle.  Then put the bikes on the vehicle roof.  Measure the VSWR again.  If there is a vast difference between the readings, you may have cause to worry.  If not, you're good to go.