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Author Topic: VHF Antenna Efficiency-Mag Mount vs Drilled Hole NMO  (Read 11608 times)
KF6DBZ
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Posts: 28




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« Reply #15 on: August 25, 2011, 11:46:22 PM »

I checked the Autozone website for a replacement battery and this is what they had.
Duralast
Part Number: 151R-DLG
Weight: 26.5 lbs
Notes: *** 370 cold cranking Amps (460 cranking Amps) *** 45 reserve minutes. -> 3 year free replacement <- Currently available through OE dealer only.


Don't sound like a whole lot of power.

This is the ratings for my 2005 Dodge Dakota. which i consider to ba a normal size and power battery.

Duralast, 850 cold cranking Amps (1000 cranking Amps) *** 150 reserve minutes. -> 3 year free replacement <- Heat shield may need to be removed or modified if not using O.E. battery.
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K0BG
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« Reply #16 on: August 26, 2011, 06:04:55 AM »

Again, the designated size of the battery is primarily based on the accessory load. Included in that accessory load is the starting current. Every single Honda, Toyota, Kia, Subaru, and about 20 others all use reduction gear starters. The inrush current is less than 1/3 that of a direct drive, like the older Dodges used. Small 4 cylinders obviously are easier to crank, and a large V8, especially diesels. However, those facts alone does not mean, or infer that the batteries in Hondas, or any other vehicle, are to small, or prone to fail.

If you want a rude awakening, go to the JD Powers web site. Look up your Dakota, and compare it to any Japanese or Korean vehicle you choose.
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K5LXP
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« Reply #17 on: August 26, 2011, 06:43:24 AM »

Reserve capacity is how many minutes the battery will source 25 amps down to 10.5V.   Consider how much power a radio will draw and at what duty cycle, and you can get a rough idea how long you can run the radio.  Two mitigating factors are the voltage at which the radio quits, and what remaining charge is required to start the vehicle, both of which you'd have to determine for the equipment you have.

If you operate a typical 100W HF transceiver conservatively, say at 50W you can easily run for several hours of typical duty cycle SSB or CW and still have plenty of capacity remaining to start the car, provided the battery is good to begin with.  Running the numbers for my Icom 706 at 50W TX power, 25% TX/75% RX I get just under 4Ah of required power.  With a 45 min RC battery (25*.75=18.75Ah) you could operate for over 2 hours down to 50%.

Drawing an SLI battery down to 50% SOC or less repeatedly isn't good for it, so depending on how often you want to do this will dictate whether you need a different SLI (like an Optima), run the car for 10 minutes every hour to bring it back up, or carry an separate deep cycle battery to run the radio from.

I will admit that at face value this does sound like a tiny car battery.  The one in my toy Ranger pickup is twice as heavy and has three times the reserve capacity.  I never think twice about running radios or other accessories from it but at a minimum I would come up with some kind of low voltage indicator so you don't get distracted and end up stranded.  Simply running the car for 10 minutes every hour or so would be plenty to keep you going indefinitely.


Mark K5LXP
Albuquerque, NM
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K7RBW
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« Reply #18 on: August 26, 2011, 07:13:19 AM »

RE: Mobile antennas (side tracking from batteries, for a minute), I stuck a Larsen 2m Kulglass antenna on my car yesterday and it seems to do as good or better than the mag mount it replaced.

I'd like to do a roof-mount but I can't find the time to take the interior apart to find where to mount it right (there are places where it'll work with the sunroof and places where it'll completely mess up the roof and I can only see that with the headliner dropped).

Before permanently mounting it, I taped it on to see how it tuned up (in case the window tinting or something might cause a problem. It had about a 4Mhz bandwidth @ < 2.0 SWR but centered on about 147-148 Mhz. That was close enough to continue (i.e. at least it worked).

After adhering the antenna but before dressing the cable, the center freq went down to about 144 w/ the same +/- 2MHz BW. The difference being because of removing all the plastic, etc. from the mounting surfaces. The coupling through the glass is very sensitive to the distance!

After dressing the cable along the A-pillar and tucking it in behind the dash, the center freq went up about 500 KHz. (Hmmm.) So I cut off about 2" and got it centered on 146 so that's where I left it.

Repeater QSOs, good. Simplex QSOs, not bad. The only thing about the antenna that surprised me was how tall it is! (almost 4') At least it's painted black so it's not too obvious, but it does stick up there! Road test coming this weekend.

Clearly not as ideal as a through-the-roof antenna, but a lot easier to mount and as good or better than the mag mount it replaced so it'll do for the time being.

Anyway, I just wanted to throw that into the mix as an option that falls somewhere between a mag mount and a roof mount.
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KD8DEY
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« Reply #19 on: August 26, 2011, 12:47:28 PM »

What is a "stock batter?"  Huh

My "stock" batter involves flour, my own secret blend of spices, and BEER.

I use it on shrimp or chicken Smiley
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KF6DBZ
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« Reply #20 on: August 26, 2011, 01:10:04 PM »

I was thinking of pancakes Grin
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KI4SDY
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« Reply #21 on: August 26, 2011, 09:33:24 PM »

I thought it had something to do fried chicken, being from the South! Maybe we could try some of KD8DEY's secret recipe on the chicken. I like beer!  Grin

My only question is; how do you get the chicken to quit running long enough to get the batter on him?  Undecided
« Last Edit: August 28, 2011, 09:35:21 AM by KI4SDY » Logged
AA4PB
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« Reply #22 on: August 27, 2011, 12:56:08 PM »

The car headlights (which includes tail lamps, side lamps, and parking lamps) probably draw around the same current as a 100W radio does on transmit with full continuous power output. In normal operation you spend most of the time in receive which draws a small fraction of the current that your headlights take. You can't judge the running time of the radio by the 30 minutes that your headlights will run off the battery.

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KD4LLA
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« Reply #23 on: August 28, 2011, 01:36:02 PM »

RE: Mobile antennas (side tracking from batteries, for a minute)...
Before permanently mounting it, I taped it on to see how it tuned up (in case the window tinting or something
Repeater QSOs, good. Simplex QSOs, not bad. The only thing about the antenna that surprised me was how tall it is! (almost 4') At least it's painted black so it's not too obvious, but it does stick up there! Road test coming this weekend.

Clearly not as ideal as a through-the-roof antenna, but a lot easier to mount and as good or better than the mag mount it replaced so it'll do for the time being.

Anyway, I just wanted to throw that into the mix as an option that falls somewhere between a mag mount and a roof mount.
Yet a NMO roof mounted 19" whip would only be -- 19 inches as opposed to 4 feet...  You tell them and tell them, but they refuse to listen, claiming all sorts of I can work whatever I hear...  Spending money on outlandish antennas and manufactured/ machined brackets, when there is but one solution.  Why is it again the reason police, fire, and EMS vehicles have properly mounted radios and antennas?

Mike
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K7RBW
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Posts: 387




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« Reply #24 on: August 28, 2011, 10:42:42 PM »

I guess you missed this part of my comment...

I'd like to do a roof-mount but I can't find the time to take the interior apart to find where to mount it right (there are places where it'll work with the sunroof and places where it'll completely mess up the roof and I can only see that with the headliner dropped).

Estimated time to install and tune glass-mount antenna: 30 minutes.
Actual time to install and tune glass-mount antenna: 30 minutes.

vs.

Estimated time to install and tune roof-mount antenna: 2 days (to include dropping the headliner, finding a spot that clears the sunroof, installing antenna, running cable around the car, tuning the antenna, replacing the headliner, etc.)

YMMV depending on your car and other things, but given that I had only 30 minutes to put an antenna on the car, the Kulglass was the logical choice. If I could have mounted a permanent antenna in 30 minutes, I would have, but that wasn't a choice.
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K0BG
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« Reply #25 on: August 29, 2011, 05:20:11 AM »

In most vehicles, it is not necessary (or advised) to remove or lower the headliner. If you use an NMO hole saw to drill the hole, all you need to do is fish the cable down the B or C pillar. If you're adapt at it like K6LXP, I suspect the total time is less than 30 minutes.

By the way, glass mounts don't work very well to start with (the coax cable does most of the radiating), and they're next to worthless mounted on passivated glass. That's about 99% of the vehicles made today.
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K7RBW
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Posts: 387




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« Reply #26 on: August 29, 2011, 06:59:06 AM »

I read all the warnings and I'm not saying a glass-mount antenna is more efficient, cheaper, or shorter than a roof mount. So, we're in violent agreement on all those points. All I'm saying is I had limited time, tools, and talent and put on one one to serve as a "good enough for the time being" antenna, which it is.

My car is a 2010 RAV4 with a sun roof and a luggage rack so there's about 8" of space between the sunroof and the rack bar where I could put the antenna. The dome light is a bit further back (due to the sunroof mechanism) than I'd like to put the antenna because I'd like to be able to use the roof rack area for luggage. I suppose I could peek up through the dome light hole to look around, but I don't want to poke a hole in the roof only to find I hit one of the stiffeners. I suspect there are few "right places" I could put a hole and many more "wrong places" given the location I'd like to mount it so I'd like to be careful. Also, because of the sunroof mechanism, I'd have to secure the coax to the roof (along one of the stiffeners, to keep it out of the way of the sunroof. I don't think I could just fish it down the B-pillar and let it hang loose. That's why I'm thinking I'll need to drop the headliner--to get in there and secure the coax to the sheet metal. All that plus my inexperience with punching a hole in the roof add up to the longer-than-30-min. estimate.

Another option would be to mount it towards the back, but that's where the radio antenna is and it's already in the way of loading and unloading the roof rack. A whip would be even more annoying in that location (even a 19" one).

One of these days, I'll have the time to figure all this out and do it better. For now, I'm on the air with a glass mount that's good enough for what I need. Again, it works (at least as good as the mag mount it replaced), it has <2.0 VSWR throughout the 2m band, and, if I put in another antenna some day, I can pop it off the windshield like it was never there.

BTW, I do like the roof rack as it provides the perfect surface for clamp-on antenna mounts. I put on a 3/8" CB mirror mount antenna (3/8" socket on the top and a SO239 on the bottom) on one of the rails. I use this for portable/mobile operations. I ran an extra ground wire and screwed it to the roof (under one of the trim pieces so the screw is hidden, but I do need to seal it before the rain returns). And it seems to be pretty solid, mechanically and electrically. It works well from 20m on up. 40 and down, I don't think I have enough car underneath it to serve as a good counterpoise or ground plane. I need to add some grounding points that I could use to connect the car to a ground rod, which might help.

For "picnic-table portable" operations, I can just screw in the whip, run the coax to the radio and I'm on the air in about 10-min after I park. I could probably do that faster if I was in a hurry. Again it's not the most efficient antenna in the world, but it goes up quickly so I spend less time setting up and tearing down and more time on the air.
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K5LXP
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« Reply #27 on: August 29, 2011, 07:23:46 AM »

I'd like to be able to use the roof rack area for luggage.

Practically speaking, just how often is this?  If it were once a week I might agree with you, but if it's a couple times a year I'd go ahead and just put it behind the sunroof where it belongs.  I've found even when I have used a luggage rack for actual luggage I can just pack around the whip or worst case, just take it off.  It's a whole lot easier to deal with working around it a few times a year than dealing with an unusual, complicated or compromise installation.


Mark K5LXP
Albuquerque, NM
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K7RBW
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« Reply #28 on: August 29, 2011, 09:28:54 AM »


Practically speaking, just how often is this?  If it were once a week I might agree with you, but if it's a couple times a year I'd go ahead and just put it behind the sunroof where it belongs.  I've found even when I have used a luggage rack for actual luggage I can just pack around the whip or worst case, just take it off.  It's a whole lot easier to deal with working around it a few times a year than dealing with an unusual, complicated or compromise installation.


Mark K5LXP
Albuquerque, NM


That's a good point.

If I left the glass mount stuck to the glass, I could just put an NMO cap on the roof mount for those 1 or 2 times a year I need the roof rack and use the 4' glass-mount. That way  I could leave the shorter (i.e. garage-door clearing) whip on for the other 350 days of they year.

Maybe I'll pop the dome light out and take a peek after all!

(then I could do an A-B-C comparison: Fixed mount, mag-mount, glass-mount)
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K7RBW
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« Reply #29 on: August 29, 2011, 08:20:10 PM »

I popped open the rear dome light and it's mounted to a roof stiffener just behind the sunroof mechanism. It looks like an NMO mount would fit quite easily just ahead of the stiffener and I could run the cable down the B or the C pillar to the floor.

For me, I'm guessing it looks like about an afternoon's project. More than 1/2 hour but much less than a couple of days.

But, I think I'll wait until next spring to find out. Thanks for the idea!
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