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eHam Forums => Mobile Ham => Topic started by: KF5QJV on October 14, 2013, 10:03:26 AM



Title: "Best" Antenna Length for New Mobile Installation
Post by: KF5QJV on October 14, 2013, 10:03:26 AM
I'm struggling with antenna selection for a new 2M/70 installation in a 2008 Toyota Tacoma.  My uncertainty is largely around 1/4 wave vs. 1/2 wave vs. 5/8 wave.  I've heard/read comments to the effect of "longer is not necessarily better," depending upon geographic location, but I always thought more gain was better; and, it seems more length generally means more gain.

I live in the Houston area but approximately 20 miles from downtown, so I'm neither in the "city" nor way out in the "country."  I'm inclined to consider this predominantly suburban location as my primary area of operation, but the reality is that I'll likely be operating within the city and in more rural areas (e.g., to/from Austin/San Antonio) as well.

Assuming a mounting point at the center of the roof of the cab would be significantly more performant than any other location, this is where I intend to mount the antenna via a permanent NMO mount (if similar performance may be realized using a lip mount on the hood, please advise).  I've been leaning toward the Comet-NCG CA-2X4SRNMO antenna for its high gain (and good reviews), but how much would I really be compromising if I were to go with something shorter like their B-10 or SBB-2?  I like the practicality of a shorter antenna but not at any appreciable expense in performance.

For what it's worth (and without necessarily inciting a radio debate as part of this thread), I'm considering either the IC-2820H or FT-8800R radios.

Any thoughts are greatly appreciated!
Rob
KF5QJV


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: K6LCS on October 14, 2013, 10:30:14 AM
>> ... but I always thought more gain was better; and, it seems more length generally means more gain.

I just recently went from a unity gain antenna to one with a little gain. Stupidest thing I have done in a while. I live in a valley (as do many Southern Californians), surrounded by hilltop repeater systems. All was well with little or no gain antenna ... but add some gain, and I consistently hear a station on the same freq as a local machine - from MEXICO.

SO .. longer and more gain for a mobile rig is not necessarily the best way to go. Only you can answer what is "best" for you in your environment.

>> ... Assuming a mounting point at the center of the roof of the cab would be significantly more performant than any other location ...

For ME in MT region, that is NOT a significant point. And, put a long antenna on your roof, and you'll never drive through Jack in the Box again without pain, nor drive into a parking structure.

Do you have the 4-door model? If so, I'd use a quality lip mount there ... like a K400.

And if you do not have a camper shell and the read windows are not metallic glass. consider the Larsen KG through-the-glass systems. Unlike lesser-priced knock-offs, they DO work very well.

Clint K6LCS



Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: N6AJR on October 14, 2013, 11:41:14 AM
If you are just talking to repeaters, then a 10 inch tall mounted on a trunk lip mount will be just fine.  if you can stand outside the car with an HT then a small antenna will work.


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: M6GOM on October 14, 2013, 12:15:20 PM
I'm struggling with antenna selection for a new 2M/70 installation in a 2008 Toyota Tacoma.  My uncertainty is largely around 1/4 wave vs. 1/2 wave vs. 5/8 wave.  I've heard/read comments to the effect of "longer is not necessarily better," depending upon geographic location, but I always thought more gain was better; and, it seems more length generally means more gain.

Any thoughts are greatly appreciated!
Rob
KF5QJV

The difference in gain between a 1/4 wave and a 5/8 wave is negligible, barely a couple of dB...certainly not enough to make it a make or break decision on gain alone.

The 5/8 wave will put the maximum gain out at a lower angle than the 1/4 wave which may or may not be beneficial depending on the topography and heights of repeater above where you are that you're likely to encounter the most.

I used a 5/8 wave 2m antenna and interchanged between a 1/4 wave. Static I noticed little difference even to a repeater 50 miles away. On the move with the 5/8 wave, the antenna was swaying about due to the flexibility of the whip which meant the RX signal was forever rising and falling in strength - I believe its called picket fencing? The shorter 1/4 wave whip was rigid and didn't suffer from it.


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: K5LXP on October 14, 2013, 01:03:01 PM
Quarter waves are not as likely to get ripped off in the car wash.  

Carrying the thought a step further, try the quarter wave first.  They're all of what, $10?  Run it for a while and see what you think.  With an NMO mount you can always swap something else in later if you want to.

Mark K5LXP
Albuquerque, NM


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: AC4RD on October 14, 2013, 04:52:20 PM
With an $8 dual-band 3/8x24 2m/440 whip, quarterwave for both bands, I can hit a repeater 25 miles away pretty easily with 20 watts.  The repeater site is up high, and the terrain is mostly flat.  But a quarterwave is (as K5LXP hinted) a good way to start.  And inexpensive besides!  73 GL!  --ken


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: KF5QJV on October 15, 2013, 10:45:31 AM
Thanks for all the feedback.  I'll likely do the 1/4 wave now.  I will mostly be using repeaters, and this makes a lot of sense.  I guess I'm used to the relatively low performance of my HT and just have little/no experience with a real mobile rig, so I was thinking I needed something much bigger.  Despite having replaced my Yaesu VX-8's stock ducky with a pretty decent Diamond SRH320A, I've been a little surprised by the reception.

Rob KF5QJV


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: K6LCS on October 15, 2013, 11:56:07 AM
The concept of "line of sight" is an amazing one. I remember one of the first reception reports from a newly launched satellite a couple years ago was from a gentleman using his VX-8R and the STOCK DUCK! Yes - 250-500mW from 500+ miles away, and the stock duck on a ham HT ...

Clint K6LCS


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: KF5QJV on October 15, 2013, 06:04:57 PM
Clint, I just checked out your site, and I'm surprised I hadn't come across it before now.  I'm still relatively new to amateur radio, but working satellites has been one of my primary interests.  I've had an Arrow, just for this purpose, for some time, but I just haven't gotten around to using it.  It seems I may be a little late to the SAT game, from all I've read, but I hope I'm wrong!  I've bookmarked your site and will certainly be referring to it in the future.

Rob KF5QJV


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: K6LCS on October 15, 2013, 06:14:30 PM
Thanks for the kind words. (Don't tell anyone, but I still have AO-51 and AO-27 programmed in my sat HTs ... (grin).) We have AMSAT-Fox to look forward to ... the "finicky" SO-50 (best worked true full duplex) ... packet bursts from the ISS ... there's still plenty to do with minimal equipment and space objects!

Make dure you report your "hearings" (and reports of NOT hearing a sat when you thought you should!) at the Live OSCAR Sat Status Site at ...

http://oscar.dcarr.org/

Clint K6LCS
909-241-7666 - cell



Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: K5BBC on October 16, 2013, 05:34:55 PM
" I'm considering either the IC-2820H or FT-8800R radios."

I've been using 2820s for about three years, and love them. I've used the FT8800R, and while I'm sure it's a good radio, I think the 2820 is far superior in switch and menu function, and %100 better display.

I've done the 1/4 wave 5/8 wave thing too.  I saw a small difference in flatland remote areas. Biggest difference I got in reports from others was on the UHF side using a gain antenna.

73


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: KA4NMA on October 16, 2013, 07:32:43 PM
Check out k0bg.com. Alan has one of the best mobile sites around.

Randy ka4nma


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: K1CJS on October 17, 2013, 02:01:07 PM
Your decision to use the 1/4 wave is probably going to work out the best.  As most have said, gain antennas can work better--and they can work worse.  If most of the repeaters you want to work are in a twenty to thirty mile radius of you, there is little need for more than a simple 1/4 wave whip--AND you won't have to worry as much if you do use a roof mount.


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: KK4CRY on October 18, 2013, 01:29:25 AM
I placed a NMO mount from Breedlove in center of my roof and Larson 2/70b, now to figure out what to do about these Composite beds damn toyota


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: M6GOM on October 18, 2013, 09:09:28 AM
For VHF/UHF I don't think you need to worry about the truck bed. There's plenty of metal in the roof to get the job done. HF is a different matter entirely.


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: KD4LLA on October 20, 2013, 03:18:23 PM
VHF is primarily "line-of-sight".  I have found that a (center) roof mounted quarter-wave works as well, or better, than an ill-mounted 5/8 wave antenna.

Mike


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: AA4HA on October 20, 2013, 04:37:38 PM
A quarter wave antenna (simple whip) has a radiation pattern that is shaped like a doughnut so you have gain (unity) in places where  you don't necessarily need it. Depending upon the terrain you are moving through, the band and how you have the antenna mounted on your vehicle you can "squash" that doughnut radiation pattern and have a positive gain (above unity) when using a 5/8 type antenna.

That is all that gain is on an antenna. It is nothing magical, you do not get something for nothing on an antenna. You compress the radiation pattern from isotropic and try to put those lobes where they do you the most good. On VHF/UHF that is probably to the horizon, HF it is either as a "cloud burner" for NVIS or a low takeoff angle for DX.

Depending upon those conditions (mounting, the angle from your vehicle up or down to the other station) this can be beneficial or harmful.

I try to start out with all other things being equal; a good antenna mounting position (smack dab in the top center of the roof) and then to do a comparative analysis of the antenna on its own merit. You will see a few dB of gain, hopefully in the direction that does you the most good.

Many years ago I did a wholesale replacement of the quarter wave antennas on a fleet of emergency vehicles. It did make a difference on fringe radio performance (15-20 miles out). In no case did it make things worse. These were those black, conical 3 dB gain Antenna Specialists mobile antennas with a whip. This was all VHF and UHF voice communications and made more of a difference where a moving vehicle would not be dropping in and out of squelch.

Picket fencing is a completely different effect than the bending of an antenna while the vehicle is in motion. It is moving though bands of diffraction on marginal signal conditions where reflected and refracted signals become additive or subtractive as the vehicle moves through these bands (think of ripples on a lake at a distance).

It could be argued that the larger antenna of a 5/8 wave antenna also has a larger effective aperture (capture area) and has a slight efficiency advantage as well.

Sure, a bigger, longer antenna can get wiped off the top of a vehicle if you drive under a tree branch, under a low garage door or car wash. For car washes I take off an NMO mount antenna, it inconveniences me by 20 seconds to do that.

When we compare things lets try to be apples to apples or oranges to oranges and not mix things up with different radios, bands, mounting spots, etc.. look at an antenna on the merits of the antenna and nothing else.


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: K0BG on October 21, 2013, 06:07:12 AM
Danny Richardson, K6MHE, wrote this article for CQ some time ago. I have a copy on my web site, and here is the URL:

http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf (http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf)


Title: RE: "Best" Antenna Length for New Mobile Installation
Post by: KQ6Q on October 22, 2013, 08:27:56 PM
If you're working repeaters, you don't need gain. As has been mentioned, and HT ducky will work. the mobile equivalent is a Hood lip mount and a short dual band whip. You'll never have to worry about the clearance in a parking garage.
kq6q