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Author Topic: vhf/uhf ground plane question  (Read 9546 times)
W5ARP
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Posts: 53




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« on: December 19, 2013, 12:33:58 PM »

I have a (hopefully) easy ground plane question that I can't find a simple answer to.

I'd like to mount a dual band vhf/uhf antenna on my truck (Lexus LX 450):

(stock photo, not mine)


The rear bumper has a built-in antenna mount but that seems to be a bad idea since the body of the truck will get in the way.  The roof is out for various reasons.  So it seems the next best option is the driver side fender (the broadcast antenna is on the passenger side).

The antenna will be mounted to the fender using one of these:

http://www.gamiviti.com/html/products_antennamounts.html



So the antenna will bolt to a mount on the fender, not the hood, and the hood will not really be electrically connected to the antenna. (maybe there will be a ground path through the chassis)  Does it still work as a ground plane? In this situation, am I better with a 2m 1/2 wave or a 2m 1/4 wave?  How will 70cm fare?

thanks,

Geof
KF5YKA
« Last Edit: December 19, 2013, 12:41:02 PM by KF5YKA » Logged
KJ4OBR
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Posts: 103




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« Reply #1 on: December 19, 2013, 12:59:49 PM »

I do the same thing and it works fine. I think the issues start to show up more when you move into the HF bands. Give it a shot, you can always add grounding braids if you need them later.

73
Dave
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M6GOM
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Posts: 874




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« Reply #2 on: December 19, 2013, 01:22:08 PM »

It'll be directional to an extent but it'll work OK.
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W5ARP
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Posts: 53




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« Reply #3 on: December 19, 2013, 03:16:48 PM »

thanks.  Yeah, I figured that with that position, if the hood was a ground plane, efficiency wouldn't be uniform in all directions.  Does that mean I'd be better off with a half wave like the Comet SSB-5 or Diamond NR73B?  The comet is 38" and the diamond is 33" which wouldn't be unreasonable on a truck that size (plus there is little vegetation off road here to catch and damage it.)  I know the 19" quarter wave antennas are nice a short, but I'm not too concerned about that.

Geof
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AA4HA
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Posts: 1377




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« Reply #4 on: December 19, 2013, 04:53:29 PM »

Do you have the luggage rack on top? Could you mount the antenna to a cross piece from one side to the other?

This would give you a big, flat ground plane below the antenna, even if you are not directly attached to it. The pattern would me more omni-directional.

Because you would not be mounted flat to the roof the efficiency would be down and the antenna would require some trimming because the RF length of the base of the antenna would be different.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KB7FSC
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Posts: 37




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« Reply #5 on: December 19, 2013, 05:58:40 PM »

Hi Geof,

In two of my vehicles, I have a 1/4 wave antennas with permanent NMO mounts in the roof.  In my third vehicle, I have a fender mounted NMO and a comet ssb-5 1/2 wave.  I find comparable performance of the fender mount to the roof mount.  In theory, you would be better off with a roof mount, but since that doesn't sound like an option, go with the fender mount and don't look back.  It will work good for you.  Have fun!

Wane - KB7FSC
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W4KVW
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Posts: 472




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« Reply #6 on: December 19, 2013, 09:23:27 PM »

Check the specs because many of today's mobile antennas do not require a ground plane & they work great.I believe the LARSEN 2/70 is just one of them & an outstanding choice for both 2 meters & 440 mhz.  Grin

Clayton
W4KVW
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KB4QAA
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Posts: 2259




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« Reply #7 on: December 23, 2013, 09:09:29 AM »

.I would choose the hood bracket mount over a luggage rack.

Even though the hood is not directly connected to the bracket, it would pay dividends in performance and reduced noise to bond the hood to the body.   See Alan's website for info.  www.k0bg.com

.....
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W5ARP
Member

Posts: 53




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« Reply #8 on: December 23, 2013, 01:01:11 PM »

I may consider the roof rack antenna in the future, but right now I don't have a roof rack.

The truck is being outfitted for a very long vacation up to the Yukon, Northwest Territories, and Alaska.  From my research, Yukon and Alaska, have a fair amount of 2m repeaters.  NWT has about 2.  Anyway, there is a lot of customizing that will happen to this truck, and a proper roof rack is on the list.  Most likely something like this:



That said, based on my budget, the roof rack is at least a year away, and I have a dual band radio on the way.  The fender mount is cheap, and any antenna under $100 is cheap enough that if I decide to change, I'm not going to cry over the cost.  So I'm thinking the fender mount above, a Larsen 2/70 NMO, a six foot cable through the firewall to the radio.  And we'll see how well it works for a year before I have to decide if it needs improvement.

thanks for everyone's input.

Geof
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M6GOM
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Posts: 874




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« Reply #9 on: December 24, 2013, 04:18:37 AM »

$100 on a VHF/UHF mobile antenna? Don't think I've ever paid more than the UK equivalent of $30.  Grin

I compared my £10 VHF jobbie to a friends all singing top of the range Diamond and noticed no performance difference to my favourite test repeater 55 miles away. That upset him somewhat.
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W5ARP
Member

Posts: 53




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« Reply #10 on: December 24, 2013, 07:12:21 AM »

That's kind of my point.  I'm not buying a $100 antenna, just saying that under $100, I won't cry if I have to replace it in a year.

Geof
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HURRICAINE
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Posts: 0




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« Reply #11 on: January 01, 2014, 07:40:52 PM »

You won't get a Larsen NMO 2/70 antenna new for less then $100.00

If you were going on vacation, there would be no reason why you couldn't put it on the roof with a mag mount.
That is where it belongs in the first place.

The fender mount will cause an image which will make the signal a little better in the direction with the most mass and poor in the opposing direction.  Hence any signal towards the drivers side fender and left rear would be ok, and signal to the right would be diminished.

The mount would probably cause some issues with engine noise, which might couple with the transmit.  I don't know a whole lot about the Lexus or which radio you plan to buy.
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W5ARP
Member

Posts: 53




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« Reply #12 on: January 02, 2014, 11:33:31 AM »

Actually I can get a 2/70NMO from aes for $80.

The roof, while I understand that it is ideal from a radio standpoint, is potentially problematic for other reasons.  Yes I don't have a roof rack yet, but I will eventually, and once the roof rack is on, the only accessible flat area of the roof will be taken up by the glass sunroof.

I get that for a 1/4 wave antenna, the ground plane is vital and mounting on the fender gives a GP in some directions and not others, so should be more efficient in some directions, but if I use a 1/2 wave, non GP antenna like the Larsen, will the asymmetry of the GP at that location impact the output?
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KA4NMA
Member

Posts: 317




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« Reply #13 on: January 02, 2014, 09:28:35 PM »

Before you decide, check out k0bg.com.  Alan is very well known among mobile hams and his website is a treasure trove of information.  Be sure to check the antenna section, mounts, grounding and power issue's

I used to have a Ford Explorer and mounted a NMO fender mount on the body at the top of the lid.  I still could open the back lid.  I had a comet and went thru 2 of them.  One was broken and the other was lost when I wrecked the Ford.  The Comet Antenna was very stiff.  I replaced the Explorer with a mini van and I roof mounted a NMO mount with a 2/70.  It was a great system.

The Larson 2/70 is one of the better antenna's.

Randy ka4nma
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