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Author Topic: Which mobile antenna is better for EmComm? Dual, or 2m 5/8?  (Read 12264 times)
K7RNO
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Posts: 279




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« on: March 17, 2013, 04:31:27 PM »

So I have an NMO mount on my Corolla and both a 2m/70cm dual band (with 1/4 wave on 2m) and a 2m whip in 5/8. At the other end I have a dual band HT (Yaesu FT-60).

Which antenna should I have mounted when the fan got hit?

FTR, the dual is a Larsen 2/70 and the 5/8 is a MaxRad MMC150.

« Last Edit: March 17, 2013, 04:39:11 PM by KG7BJM » Logged

73,
aRNO
NAQCC #6870, SKCC #11131
KD8GTP
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Posts: 73




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« Reply #1 on: March 17, 2013, 06:59:26 PM »

Hello,
You don't tell us where on the car the antennas will be mounted.  If they are on the roof I suggest using something shorter than a 5/8.  I would be concerned about wind drag, between your light bar and other emergency lights on the roof it could slow you down when you respond to the scene.  When the sheeet hits the fan you need to get to the scene quickly. Don't let wind drag slow you down.
Thx
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K5LXP
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« Reply #2 on: March 18, 2013, 06:47:26 AM »

Just one antenna mount?  One radio?  Sounds like an emcomm wannabe.


Mark K5LXP
Albuquerque, NM


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K7RNO
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Posts: 279




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« Reply #3 on: March 18, 2013, 08:58:36 AM »

Just one antenna mount?  One radio?  Sounds like an emcomm wannabe.

I guess you're onto something. I should clarify a few things.

I am a new ham, also new to all the jargon you old farts use. When I said EmComm, you probably heard something different than what I meant. What I mean is wanting to be able to communicate in case of an emergency from inside the car, moving or not, communicating within my local ARES net. In such a situation, is it preferable to have the better 5/8 but be limited to 2m, or to be more flexible with 2/70 but have a less effective antenna, considering that I will only have 5W to work with.

If this scenario is too lowly for your highness, then please ignore it. Maybe there are other hams out there that can still remember how it was when they started.
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73,
aRNO
NAQCC #6870, SKCC #11131
KG4RUL
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« Reply #4 on: March 18, 2013, 12:00:30 PM »

With only 5W, the slight gain advantage of the 5/8 wave will be your better bet.
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N5TWB
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Posts: 35




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« Reply #5 on: March 18, 2013, 12:05:51 PM »

Now you've gone and done it...you made a reference to EmCom and the sharks are circling. (It's kinda like Beetlejuice where they only appear if you say it three times fast.) They'll not be satisfied until they take you down to Hiram Percy Maxim's locker where the pure RF flows through legal limit amps and hams are allowed to age until they are deemed ready for the 80 meter Hernia and Hemorrhoid Net. Life is more pleasant if you simply ignore their weak-sauce attempts to belittle an honest inquiry with their personal biases.

Now, as to your question: If all of the desired communications are going to occur strictly on 2m, then I'd go with just the 2m antenna. However, you do have a dual band radio so there may be some utility in being able to use both bands at some time or in some circumstances. If so, keep both in the vehicle and change to suit since making a change on a short vehicle on an NMO mount should be a breeze. KG4RUL also makes an excellent point about the HT power level being a factor.
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K7RNO
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Posts: 279




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« Reply #6 on: March 18, 2013, 07:51:32 PM »

Ha, I didn't know that there is an EmComm elite. Oh well, my crypto teacher would have said "poor souls".

Thank you for the comments. They confirm the inherent benefit of the 5/8 λ, possibly outweighing the loss of immediate UHF. Having one mounted and the other one always in the car is an excellent idea.
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73,
aRNO
NAQCC #6870, SKCC #11131
K1CJS
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Posts: 6042




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« Reply #7 on: March 19, 2013, 04:14:20 AM »

I've used both a 2 mtr. 1/4 wave whip (BTW, which is good for 70 cm also) and a 5/8 wave dual band antenna.  The 5/8 wave was a better antenna because of its gain factor.  The antenna is on a NMO mount on my trunk, so I leave the 5/8 wave on the vehicle all the time.  The 1/4 wave is a backup antenna only--in case something happens to the 5/8 wave antenna.

The 1/4 wave would be better in one circumstance only--if the mount were on the roof of a fairly tall vehicle to begin with, with the 5/8 wave being in danger of hitting low hanging objects and getting damaged.  Of course, YMMV.  73!
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K7RNO
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Posts: 279




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« Reply #8 on: March 19, 2013, 08:02:45 AM »

I've used both a 2 mtr. 1/4 wave whip (BTW, which is good for 70 cm also) and a 5/8 wave dual band antenna.  The 5/8 wave was a better antenna because of its gain factor.  The antenna is on a NMO mount on my trunk, so I leave the 5/8 wave on the vehicle all the time.  The 1/4 wave is a backup antenna only--in case something happens to the 5/8 wave antenna.

The 1/4 wave would be better in one circumstance only--if the mount were on the roof of a fairly tall vehicle to begin with, with the 5/8 wave being in danger of hitting low hanging objects and getting damaged.  Of course, YMMV.  73!

Thank you. Your situation looks like the ideal case: the better performing antenna is also designed as a dual band. Short of height issues, this makes for a clear decision.
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73,
aRNO
NAQCC #6870, SKCC #11131
WB6BYU
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Posts: 13335




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« Reply #9 on: March 20, 2013, 11:02:08 AM »

If your local EmComm group uses 440, then it is worthwhile having that
band available.  Some groups do, some don't.

I've tested a number of antennas by switching between two of them
while driving through difficult terrain where the signal strengths bounces
up and down a lot.  The differences between a 5/8 wave and 1/4 wave
whips is less than the amount of variation as I drive down the road.
On average the difference is about 1dB or so:  basically you are looking
at changes the in relative amount of background noise.  The 5/8 wave
will vary depending on the speed, as maximum radiation is from the area
about 1/4 wave down from the top, and that can bend up to 45 degrees
backwards at speed.

If you are concerned with signal strength in an emergency, I recommend
carrying a portable mast of some sort (12' to 20' or so) and an antenna
that will mount on it:  getting the antenna up higher will allow you to
communicate further, even if it means stopping temporarily to pass a
message.  I use some old tent poles that I picked up at the Thrift Shop
for a mast and a homemade wire ground plane, but there are many
other options.
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K1CJS
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Posts: 6042




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« Reply #10 on: March 20, 2013, 01:11:51 PM »

If your local EmComm group uses 440, then it is worthwhile having that band available.  Some groups do, some don't....

It's nice to have the 440 option even if the emcomm group doesn't use it.  His radio has the band, so why not have the antenna for it!

Quote
...The differences between a 5/8 wave and 1/4 wave whips is less than the amount of variation as I drive down the road.  On average the difference is about 1dB or so:  basically you are looking at changes the in relative amount of background noise.  The 5/8 wave will vary depending on the speed, as maximum radiation is from the area about 1/4 wave down from the top, and that can bend up to 45 degrees backwards at speed....

There is also the question of gain to be considered.  A lot of the 5/8 wave dual band antennas have greater gain than a simple 1/4 wave 2 meter whip, and therefore a better signal going out--on both bands, AND greater reception strength.

Also, the better made 5/8 wave dual banders are much 'stiffer' than a 1/4 wave 2 mtr whip.  The one I have doesn't bend backward at speed much at all, less than a few degrees while the 1/4 wave whip does tend to bend back and flop around.  That doesn't come into play all that often, since most comms are made at slower speed--unless you're on a highway and using the rig to pass the time.  That is also why I said what I said about having the 5/8 wave on top of a tall vehicle such as a van or SUV.
« Last Edit: March 20, 2013, 01:17:08 PM by K1CJS » Logged
K7RNO
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Posts: 279




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« Reply #11 on: March 20, 2013, 01:29:51 PM »

Thank you, WB6BYU, very helpful information.

It confirms keeping the shorter (and stiffer) dual band mounted for mobile application, with the 2m 5/8w in the car for stationary use if needed.

This thread has been very informative for me, in more than one regard.
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73,
aRNO
NAQCC #6870, SKCC #11131
WB6BYU
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Posts: 13335




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« Reply #12 on: March 20, 2013, 02:43:55 PM »

Quote from: K1CJS

There is also the question of gain to be considered.  A lot of the 5/8 wave dual band antennas have greater gain than a simple 1/4 wave 2 meter whip, and therefore a better signal going out--on both bands, AND greater reception strength.



But not that much different, both by theory and actual measurement.  While a
5/8 wave whip does have 3dB gain over a 1/4 antenna over a perfect, infinite
ground plane, the roof of the typical vehicle is nowhere near large enough to
provide the ground reflection that is the cause of that difference.  The end
result is that the 5/8 wave has a small advantage due to the point of maximum
radiation being higher in the air for the same mounting height.



Quote

Also, the better made 5/8 wave dual banders are much 'stiffer' than a 1/4 wave 2 mtr whip.  The one I have doesn't bend backward at speed much at all, less than a few degrees while the 1/4 wave whip does tend to bend back and flop around.



Maximum radiation from the 1/4 wave whip is from the base:  it doesn't matter
much how much the top part flexes, the polarization doesn't change much.  By
contrast maximum radiation from the 5/8 wave whip is 1/4 wave down from the
top, and it isn't uncommon for that portion of the antenna to flex up to 45 degrees,
with a resulting shift in polarization that is much greater than when a 1/4 wave
whip flexes by the same amount.

There was a study many years back with regards to antennas on police cars,
which tested various commercial offerings (which were stiffer than most ham
antennas) at speeds of 100mph, and came to the conclusion that there was little
or no advantage to the 5/8 wave whip, and it could actually be detrimental at
high speeds.

Some of the dual-band antennas that are about 5/8 wave on 2m are certainly
much stiffer than a plain whip, but my experience has been that the more joints
in the antenna the higher the chance that it develops an intermittent connection
somewhere.  The Larsen where the radiator is bent into the 440 phasing coil is
one of the best that I've found
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M6GOM
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Posts: 944




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« Reply #13 on: March 20, 2013, 04:53:04 PM »


There is also the question of gain to be considered.  A lot of the 5/8 wave dual band antennas have greater gain than a simple 1/4 wave 2 meter whip, and therefore a better signal going out--on both bands, AND greater reception strength.


It is so little as to be not noticeable. A S point on the meter is 6dB. 5/8 wave verticals are not 6dB better than 1/4 waves and on higher take off angles they can be worse than 1/4 wave.

I have done A/B testing extensively on my car with a 1/4 wave 2m antenna and a 5/8 wave 2m antenna to a repeater 55 miles away that has an ERP of 10W which is about 200 ft higher than where I am. My antenna is installed in the middle of the roof of the car via hole drilled through roof method and doors/trunk/hood bonded to the car body. I have found absolutely no discernable difference between the two antennas. Audio sounds the same, S meter sounds the same and I can talk through that repeater just fine with 10W on both antennas.

I may do a Youtube video at some point to show this.


Maximum radiation from the 1/4 wave whip is from the base:  it doesn't matter
much how much the top part flexes, the polarization doesn't change much.  By
contrast maximum radiation from the 5/8 wave whip is 1/4 wave down from the
top, and it isn't uncommon for that portion of the antenna to flex up to 45 degrees,
with a resulting shift in polarization that is much greater than when a 1/4 wave
whip flexes by the same amount.



This is definitely the case travelling at 60MPH or more or even lower speeds if you're driving into a strong headwind.
« Last Edit: March 20, 2013, 05:00:02 PM by M6GOM » Logged
K1CJS
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Posts: 6042




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« Reply #14 on: March 20, 2013, 07:33:10 PM »

...the roof of the typical vehicle is nowhere near large enough to
provide the ground reflection that is the cause of that difference.  The end
result is that the 5/8 wave has a small advantage due to the point of maximum
radiation being higher in the air for the same mounting height.

You're kidding on this, right?  The 'average' ground plane requires only a 38 inch diameter circle.  Car roofs are always much larger than that--unless you drive a smart car!

Quote
There was a study many years back with regards to antennas on police cars,
which tested various commercial offerings (which were stiffer than most ham
antennas) at speeds of 100mph, and came to the conclusion that there was little
or no advantage to the 5/8 wave whip, and it could actually be detrimental at
high speeds.

Some of the dual-band antennas that are about 5/8 wave on 2m are certainly
much stiffer than a plain whip, but my experience has been that the more joints
in the antenna the higher the chance that it develops an intermittent connection
somewhere... 

You seem to be shovelling it a bit thick here.  How about a link to that study--I think it's either a study by the makers of a thin whip antenna or is just simply a lot of bally-hoo by a group that wanted to cast doubt on a better antenna.  BTW, there are a lot of differently designed 5/8 wave 2 mtr/440 antennas out there. 

The only detriment to a stiffer antenna is that any sort of contact from an overhanging object would tend top snap it off.  The only advantage to a thin 1/4 wave 2 mtr. whip is its flexibility.
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