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Apr 21, 2012, 02:26 PM
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Joined Dec 2011
43 Posts
Building your own (bipole) antenna: The quarter wavelength roundup for beginners

Hi all,

I've been reading a lot about home made antenna's and have read some very helpfull tutorials. However there seems to be a lot of different and scattered information with different approaches and formula's which can be a bit overwhelming and may result in mistakes after all... Sometimes even the obvious for the seasoned guy can be quite confusing for the beginner. So I thought I'd make this post in an attempt to shine some light on the subject from a beginners perspective. Please note I'm not a electrical engineer of some sort, so I'm just trying to get things clear from a 'end user perspective' with moderate soldering skills, which in my case is making antenna's for a ground vehicle. I also might not get everything exactly right, so feel free to comment, correct or add!

Alright: Starting with IBcrazy's post here , a popular home made antenna is the quarter wavelength bipole. It's basically a T shape antenna where the vertical part of the T represents the coax input, and the horizontal top part of the T represents the radiating part and the ground part, split exactly in the middle. Ground part means connected to the outer shielding of the coax cable and radiating part means connected to the centre (core) of the coax cable. The coax input means the unmodified coax cable, so with shielding intact.

To compute the quarter wavelength of the frequency you use, IBcrazy presents the following formula:

7125 / frequency in Mhz = length in cm.

This is when you use 22AWG wire for the radiating element and the ground element. For 12 to 14 AWG wire the math is: 6985 / frequency in Mhz = length in cm. When this little math is applied when you want to make a 1200mhz antenna with 22AWG wire, the length of the radiating wire is 5.93 cm. The length of the ground part is also 5.93 cm. Total length would be 11.86 cm, which is half wavelength, so that's why it's called the 'half wave dipole' The two different formulas are presented because different thickness of wire have different propagation / velocity factors, so IBcrazy just gave us two common wire thicknessess with appropiate formula's. According to his own note he based those two formula's on a propagation factor (he calls it velocity factor) of 95%, which according to the second post in this website would mean solid copper wire.

Please note that the 22AWG wire in this example means the solid wire used for radiating 5.93 cm antenna and 5.93 cm ground part and not the coax cable that feeds the antenna! This one kind of got me, since I followed the link in that same post called Mr RC Cam's dipole This describes a dipole antenna, but made solely out of coax cable. It's a simple and clever design, but you need to calculate a different propagation factor because you use the coax cable itself for the antenna and not 'external' 22 AWG solid copper wire parts.

CORRECTION: I just found out on THIS site that the propagation factor of the inner core wire of coax is actually determined by the shield and insulation(!) As the last post there mentions:
Quote:
 In other words, the velocity factor of a bit of wire in space is 1. Put it near something and that VF falls a bit. Stuff it in a metal tube and it will fall a LOT, maybe even approaching half (0.66).
So the moment the inner core of coax leaves the shield, its propagation factor rockets up back to about 95%. So this way there's no need to compensate for lower propagation factor when you make your antenna element out of the core of RG174 coax as explained in the Mr RC Cam's dipole website.

I removed the parts below because they were apparently wrong, but I'll leave the websites I found cos they are relevant to the topic.

Here I found a few interesting sites/facts. Different propagation factors of different coax cables here. This site gives a formula where you can fill in your own propagation factor, in case you use other wire for the part of the antenna that radiates.

On this forum someone uses a formula which gives results in full wavelength in foot.

On a sidenote, using 95% propagation factor on the formula above gives:
300 / 1200 / 2 * .65 = 0.11875 meters. That's 11.875 cm half wavelength and 5.93 cm quarte wavelength which convienently matches with IBcrazy's formula for 95% propagation factor of 22 AWG solid copper wire. Here's another site to compute in feet. It also mentions a few coax cable propagation/velocity factors.

Alright, I hope this will be of some help for starters to understand or clear up some of the difficulties I at least encountered on this subject. And if I'm mistaken somewhere, please let me know to. Original credits go to IBcrazy's post!

p.s
I do have a few questions myself too: For maximum horizontal omnidirectional range, would this antenna design or this one be more suitable? Or perhaps just use a design where you create coils in between? Or maybe a 3/2 wave?
Last edited by Bram1982; Apr 21, 2012 at 05:25 PM.