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Author Topic: Mobile Setup Idea (what coax?)  (Read 1536 times)
KD0IPJ
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« on: December 23, 2009, 11:57:04 AM »

I'm trying to setup my car so I can have a little better distance to repeaters than I currently get with my VX-8R at 5W. My thoughts are that I don't have the money currently to buy a new rig such as a 50 watt dual bander which is what I'd want. So instead, keeping it within my $250 budget, I could get a Mirage BD-35 amp with a Comet B-10 antenna. I like that antenna given its size and it looks like a standard cell antenna (I drive a BMW with no antennas on it so as discrete as possible is the goal). Then I was going to use a Diamond K600S trunk lip mount (which seems overkill but I can't seem to find another option, so I'm open to suggestions). My question is, for the coax that goes from the amp which will be in the trunk to the HT in the car, can I use a smaller coax like is on the K600S (RG316) so it's not so huge and easier to maneuver? Also, any other thoughts on my idea of a setup?
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WB2WIK
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« Reply #1 on: December 23, 2009, 02:53:08 PM »

>Mobile Setup Idea (what coax?)       Reply
by KD0IPJ on December 23, 2009    Mail this to a friend!
I'm trying to setup my car so I can have a little better distance to repeaters than I currently get with my VX-8R at 5W. My thoughts are that I don't have the money currently to buy a new rig such as a 50 watt dual bander which is what I'd want. So instead, keeping it within my $250 budget, I could get a Mirage BD-35 amp with a Comet B-10 antenna. I like that antenna given its size and it looks like a standard cell antenna (I drive a BMW with no antennas on it so as discrete as possible is the goal). Then I was going to use a Diamond K600S trunk lip mount (which seems overkill but I can't seem to find another option, so I'm open to suggestions).<

::The B-10's a pretty lossy antenna.  It will surely work better than a "rubber duckie" inside the car, but it will not perform as well as the Larsen 2/70 mobile NMO whip antennas which are somewhat longer and better designed.  The K600S isn't a bad mount, but as you say may be overkill for such a tiny antenna which will work much better if you put it up at roof level rather than trunk level.

I'd pop a 3/4" hole for a "real" (and extremely inexpensive -- under $20 for the Real McCoy, American-made, top quality mount from Larsen) low profile NMO mount on the roof, near the rear window, which is exactly where the "factory" would put a cellular telephone antenna if they did it.  It will work better, look less goofy, be less of a target for thieves or vandals, and cost far less.

>My question is, for the coax that goes from the amp which will be in the trunk to the HT in the car, can I use a smaller coax like is on the K600S (RG316) so it's not so huge and easier to maneuver?<

::This stuff is *really* lossy on 70cm (not that bad on 2m), unless you keep it very short.  I'd opt for high quality RG58C/U or better still high quality RG8X.  RG8X has less than half the loss of K600S and it's every bit as flexible (although larger in diameter: RG8X is .240" O.D.), and also costs less.

WB2WIK/6
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KD0IPJ
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« Reply #2 on: December 23, 2009, 03:02:07 PM »

Thank you for the reply.
Do you have a recommendation for a low-profile NMO mount antenna? I'm looking for something that would look somewhat stock (there is no existing antenna on the car so it could go right where the factory would put one now days for satellite). It is a 1994 BMW 525i so I want it to look half way decent. I'm hoping for maybe a stub like the police have or something 18" or less.

KD0IPJ
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KE4DRN
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« Reply #3 on: December 23, 2009, 07:06:47 PM »

hi Kyle,

try the external antenna before you invest in the amplifier.

you may find the amp is not needed.

the external antenna will 'talk' better as well as 'hear' better,
so you may pickup some unwanted signals as you drive around town,
because the HT will not filter out those signals like a true mobile radio will.

a 2m moblie radio could be a better investment and be sold off later
when you are ready for a dual band mobile radio.

73 james
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N8EKT
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« Reply #4 on: December 24, 2009, 06:06:24 AM »

The B-10 is a great little antenna, I use one every day.
Big performance from a very small antenna

Check out jefatech.com

they have 195 low loss for 37 cents a foot.
only 7.77db loss per 100ft at 450mhz
and same size as rg58
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K0BG
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« Reply #5 on: December 24, 2009, 06:44:22 AM »

There is no free lunch!

I agree with Steve, the B-10 is a very lossy antenna, and the use of a clip mount just adds insult to injury. However, there is more to the story.

Far too many folks select their antennas on one of two attributes, but never both; the size, or the gain. Without getting into a long winded discussion, probably your best bet, based on your (apparent) unwillingness to drill a hole, is to use a standard 1/4 wave ground plane antenna. The Larsen NMOQWFT is as good as any. It can be mounted either in a hood seam or trunk seam, using one of the various angle brackets Larsen supplies. You'll also need a cable kit which comes supplied with RG58B/U. For repeater use, it will perform much better than the B-10, and will easily handle 500 watts if necessary.

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WX7G
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« Reply #6 on: December 24, 2009, 07:33:54 AM »

The Comet B-10 antenna is "lossy"? No it is not.

It is rated for 50 watts. Given the size of the base loading coil it could safely dissipate no more than a couple of watts. Therefore it has a radiation efficiency of 95% or more. Additionally, the specified gain is 2.15/2.15 dBi. This tells us that  the efficiency is essentially 100%.
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WX7G
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« Reply #7 on: December 24, 2009, 07:41:46 AM »

Your question was can RG316 coax be used?

The loss at 146 MHz is about 10 dB/100'. A 10' length has a loss of 1 dB. At 50 watts the 10' length will dissipate 10 watts total and 1 watt per foot of length. Given that the surface area is 4 sq inches per foot the temperature rise will be roughly 10 deg C. It will work just fine.
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WX7G
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« Reply #8 on: December 24, 2009, 07:45:48 AM »

It would appear that you have been given inaccurate advice by another.

Beware of non-numerical, subjective answers.
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KC7YRA
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« Reply #9 on: December 24, 2009, 07:50:08 AM »

With a budget of $250, there is no reason you should not be able to get a rig AND antenna.

You may find a real nice dual band Yaesu (such as the ft-7800) for a great somewhere.  Or an IC-208H.  Should be within $20 of that budget.  It has an almost microscopic control face and it works great.

Couple that with a $10 coax kit from Larsen and a Larsen 1/4 wave antenna ($10) and you will have a real performing setup for dang close to your $250 price range.  

Brad
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K0BG
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« Reply #10 on: December 24, 2009, 09:42:36 AM »

David, you quoted the gain as 2.15 dBi. That's unity gain! In other words, just a plain old 1/4 wave vertical, and one that costs more than the Larsen.

What's more, the fact the antenna has a 50 watt power rating should also tell you something. Try this at home. Take a B-10, and put just 50 watts through it for 30 seconds. Then feel the base of the antenna.

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WX7G
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« Reply #11 on: December 24, 2009, 02:59:41 PM »

2.15 dBi is just short of the gain of a dipole. That is pretty efficient.

The base gets hot. That is what we expect from an antenna rated for 50 watts that has 50 watts applied. The matching/loading network will attain a temperature that the manufacturer has deemed to be hot enough.

To determine how much power is being 'lost' take the surface area of the base and the temperature when it reaches final temperature. Using a thermal resistance of 35 deg C per watt calculate the power loss.
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WX7G
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« Reply #12 on: December 24, 2009, 03:29:19 PM »

Or we can run a worst-case analysis of the antenna. The base looks to have a surface area of 6 sq inches. Given a temperature rise of 50 deg C tells us the dissipation is 9 watts. Why 50 deg C? The surface temperature is then 70 deg C which is the maximum allowed by UL.

9 watts dissipated out of 50 watts is just under 1 dB. Whether 1 dB is enough to steer the user to a less aesthetically desirable antenna is a decision for the user to make. Armed with numbers - 1 dB antenna loss and 1 dB coax loss (compared to zero loss coax) - he can make an informed decision rather than relying on subjective opinions.

My objection to subjective answers (I give them too) is that a person may be steered to an expensive and labor intensive 'solution' that performs little better than an inexpensive and less labor intensive solution. It is easy for someone to suggest 120 buried radials when he is not the one incurring the labor and expense.

In the VHF installation in question aesthetics, cost, performance, and ease of installation are the design parameters that are to be traded off.
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K0BG
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« Reply #13 on: December 24, 2009, 05:56:30 PM »

Apparently, you still don't get it.

Any amount of power which it turned to heat is wasted. You can have an antenna with 20 dB of gain, but if the majority of that power is used to heat the loading coil, the gain as it were is superfluous.

The real test is to measure the field strength between antenna one, and antenna two. The gain and/or power rating notwithstanding, the one with the higher field strength wins. If you compare the B-10 with a standard old 1/4 wave, using nothing more than S meter, you'll get the point rather quickly.
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WX7G
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« Reply #14 on: December 24, 2009, 10:40:06 PM »

K0BG, according to my calculations, and the specified gain of the B10, the difference between it and a full size 1/4 wavelength monopole is less than 1 dB.

Allow the user to balance electrical performance against aesthetics, ease of installation, cost and whatever other factors he has in mind. Provide numbers and let him decide.
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