Apr 21, 2012, 02:26 PM
Registered User
Joined Dec 2011
33 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.
 Apr 22, 2012, 07:30 AM Engineer for Christ Amherst, VA Joined Jun 2006 10,294 Posts Why do you need such a fancy antenna? Why do you think you need a high gain omni? Many of us are doing 2-3miles on circularly polarized antennas without any trouble. -Alex Latest blog entry: Project Covert Ops: Long range ground...
 Apr 22, 2012, 05:03 PM Registered User United States, IA, Cedar Rapids Joined Apr 2006 469 Posts I think something like the dipole on MrCam's site (referenced above) but possibly made to 1/8th wavelength per side (1/4 wave total length) might be the best solution for a 433MHz variometer. Slim and small to fit within a glider fuselage, and that would provide very good coverage when banking and circling a great deal almost overhead. I'm just wondering if a balun isn't really needed for best uniformity.
Apr 22, 2012, 05:16 PM
OSUFPV - KF7VFT
Corvallis, OR
Joined Apr 2010
1,702 Posts
Quote:
 Originally Posted by IBCrazy Why do you need such a fancy antenna? Why do you think you need a high gain omni? Many of us are doing 2-3miles on circularly polarized antennas without any trouble. -Alex
Quote:
 Originally Posted by Bram1982 which in my case is making antenna's for a ground vehicle.
I think thats why he's going for a high gain omni.

-Blues
 Apr 27, 2012, 12:05 PM Registered User Joined Dec 2011 33 Posts Yes, that's correct: this is about a ground vehicle. What I found out by the way that the Lawmate 1.2 ghz 1000mw performs really bad on the lower mhz's like the channel 5 setting (1010 mhz) The ground limit with the standard antenna's is a mere 100 meters in that case. Furthermore, these 1.2 ghz 7 dBi Omni antenna's perform even worse compared to the original antenna's on the Lawmate, either on the 1280 mhz setting or the 1010 mhz setting... They do improve on the recieving side a bit though, but it seems that 'longer is better' has a part in this too. Speaking of 1280 mhz the Lawmate performs way better, but ground range stays limited to about 300 meters, that is WITH line of sight. This is again with the standard rubber ducky's. 1280 mhz does limit my 2.4 ghz link a bit more. I'd say about 20% loss in range. The 1010 setting improves the 2.4 ghz link keeping the loss around 10% range. I did use a 1500 mhz low pass filter by the way, but it doesn't really seem to help though... So this is the reason I'm looking for antenna's with more horizontal gain. Of course I could get the 2500mw transmitter from Ready Made RC like this guy did with apparently great succes! However as I understand it's a bit frowned upon using an transmitter output this high, and it's preferred to use better (tuned) antenna's before jumping ship to more drastic measures (and additional cost's). Also the 2500mw transmitter might kill my 2.4 ghz reciever's range a lot more... So any idea's what antenna's best to built? Or buy? The 1.2 ghz 7dBi antenna's from ebay apparently don't do much good, and I was thinking I might need an antenna that's more resistant to ground reflection/multipath?
 Apr 27, 2012, 12:12 PM Registered User Lakewood, Colorado Joined Aug 2002 28,116 Posts I think you'll find that a high gain omni on a ground vehicle will give mixed results. Any time the vehicle pitches or rolls, you'll lose the signal. I think you'd have better results with a light weight low gain omni, placed as high as possible (on a mast). Getting your Rx antenna even higher (on a pole) will help with the inevitable loss of signal due to being so deep into the Fresnel zone (multi-pathing). I've lost signal at 100ft with an 600mW Tx due to that. ian Latest blog entry: Daemon Relay - Below Line of Sight FPV
 Apr 27, 2012, 04:12 PM Registered User United States, IA, Cedar Rapids Joined Apr 2006 469 Posts If you're using those omni/rubber duck antennas and just want more gain, cheap: http://www.freeantennas.com/projects...ate/index.html But I'd think even just a CP receiver antenna might help you some with all the multipath.