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-   -   Dipole vs. Vertical? (http://www.risingsun4x4club.org/forum2/showthread.php?t=6294)

Groucho 02-27-2008 03:49 PM

Dipole vs. Vertical?
 
In the 3rd HAM class, we discussed these two antennas, and the formulas of both.

For a Dipole, the formula was:
Length (in feet)= 468 / Frequency in MhZ. That length was then divided in half, and that gave you how long your "legs" of the Dipole should be.

For the Vertical, the formulas was:
Length (in feet)= 234 / Frequency in MhZ. The 234 is just half of 468, indicating what we learned that a Vertical is just half of a Dipole stood on its end.

For a 1/4λ vertical on 2M, the length wil be around 20". Someone (I think it was Scott) asked after we figured this out why he didn't see too many 20" antennas on anyones rig, but my antenna (the homebrew 2M) and Bruce's seemed much longer. I didn't have an answer for it then, but I do now. We have the 2M, 5/8λ vertical on our rigs. For us to understand this, lets figure it out.

Take for example the RS adopted simplex frequency of 146.460MhZ.

To figure out the multiplier in the 5/8λ formula, we take the 1/2λ number of 468 and multiply by 2 (to figure out the 1/1λ number).
468 * 2 = 936.

Divide the 1/1λ by 8 (because the divisor in the 5/8 antenna is 8) , then multiply by 5 (because the multiplier is 5) and you get:
936 / 8 = 117
117 * 5 = 585.

Put 585 into out formula:
Length (in feet)= 585 / Frequency in MhZ
and for 146.460MhZ we get:
585 / 146.460 = 3.994 FEET, = 47.93 INCHES

So for the 5/8λ antenna at 146.460MhZ, the optimum length of the whip should be 47.93 inches. Mine is 48 inches.

Hopefully that helps.

subzali 02-27-2008 04:01 PM

But there are some people who run 1/4λ antennas, which would be around 19" tall. But also, as on CB, a 5/8λ antenna generally works better, thus the 4' 2m antenna. Can you wrap the conducting surface as they do with CB antennas? A 5/8λ CB antenna should be around 258 inches (21.5 feet) long, yet they shrink it into a 4' whip. I suppose you give up a lot of RF transfer by doing that...

DaveInDenver 02-27-2008 04:25 PM

Quote:

Originally Posted by subzali (Post 64106)
But there are some people who run 1/4λ antennas, which would be around 19" tall. But also, as on CB, a 5/8λ antenna generally works better, thus the 4' 2m antenna. Can you wrap the conducting surface as they do with CB antennas? A 5/8λ CB antenna should be around 246 inches (20.5 feet) long, yet they shrink it into a 4' whip. I suppose you give up a lot of RF transfer by doing that...

Don't forget that an antenna does not have to physically be 5/8th of a wavelength long to look like it's that long. In the case of a CB antenna that's electrically 5/8-wavelength but only say 50" tall there is a loading coil at the base (or sometimes in the middle or even near the top) that makes it seem longer than it truly is to the radio. You give up surface compared to a real antenna of the same length, but you get the benefits of the directionality, for example. The downside is efficiency, which is why it's compromise. Efficiency is the measure of how much of your signal fed to the antenna translates into RF energy sent out. An antenna loaded with a coil to be physically shorter will lose energy in the coil and has a lot less radiating length, so the quantum world is happy in that you are not violating the laws of physics. A perfect antenna is physically the right length, but like you can see with longer wavelengths you have inconveniently long antenna whips. So we compromise to make them manageable.

BTW, my shortie antenna is 1/4 on 2m and it's 19" tall. It's a NMO2/70SH, short thing with a spring and big coil in the middle. It's low profile (looks sorta like a cell antenna) and so it doesn't attract attention in the city and doesn't hang up on stuff on the trail since it's in the middle of my roof.

Tramontana 02-27-2008 05:34 PM

Ground plane, 1/2 dipole antenna idea...
 
...so I think it was Dave that gave me the idea after class, but I've got an idea for increasing antenna performance on my extra cab Tacoma.

Since I have a Can-bak topper, would adding a layer of copper mesh to the underside of the fabric be a significant improvement to the "mirror" surface, then just the roof of the cab?

I found a couple sources of mesh wire online, and would need to select the ideal mesh size/wire size combination.

This is what I'm considering;

http://www.twpinc.com/twpinc/control...B811FA624.jvm1

It is advertised as RF shielding, so I expect it would act like the needed reflector surface? 72" wide fabric would work, then I'll figure out how best to bond it to the underside of the Sunbrella fabric.

Any thoughts?

rock_wagon 02-28-2008 11:21 AM

This seems like a pretty good write up on Vehicle Bonding, and includes some homemade methods of making bonding straps.

http://www.k0bg.com/bonding.html

Groucho 02-28-2008 03:09 PM

Quote:

Originally Posted by subzali (Post 64106)
But there are some people who run 1/4λ antennas, which would be around 19" tall. But also, as on CB, a 5/8λ antenna generally works better, thus the 4' 2m antenna. Can you wrap the conducting surface as they do with CB antennas? A 5/8λ CB antenna should be around 246 inches (20.5 feet) long, yet they shrink it into a 4' whip. I suppose you give up a lot of RF transfer by doing that...

Matt,

Before the test I wanted to make sure you got the math correct for this idea. It is close, but off just enough.

From our example, the multiplier for 5/8λ is 585. The low end frequency in MhZ for CB (11 meter) is 26.965 MhZ, the high end is 27.405 MhZ.

Then, we find the length in feet = 585 / 27.405
which is 21.35 Feet.

For the other end, we find that the length in feet = 585 / 26.965
which is 21.69 Feet.

So for CB, the length of the whip of a 5/8λ vertical antenna needs to be between 256.2 inches and 260.3 inches long.

By my calculations, if you were using the 585 multiplier you were assuming that the CB frequency was in the middle of the 10 Meter HAM band (28.000-29.700 MhZ).

L = 585 / 28.53 MhZ.
L = 20.5 feet.

Did you use the 585 multiplier? Did you use 28.530 Mhz?

Groucho 02-28-2008 03:10 PM

Quote:

Originally Posted by rock_wagon (Post 64228)
This seems like a pretty good write up on Vehicle Bonding, and includes some homemade methods of making bonding straps.

http://www.k0bg.com/bonding.html


Alan is extremely knowledgable in this arena. I reccomend using his site for all types of mobile questions and direction. Everyone needs to read his points on mag-mount antennas.

subzali 02-28-2008 03:45 PM

Yeah I was confused, thought CB was in the 10m band :o

Quote:

Originally Posted by Groucho (Post 64259)
Matt, ...

Did you use 28.530 Mhz?



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