View Full Version : VHF antenna length??
I'm setting up an antenna to fit under the keel.
I know aircraft band VHF antennas need about 22", but where can I find out the exact length that would best suit the freqs I use the most -- 123.05 and 122.8?
I have also read that the thickness of the antenna has an effect on the width of the band covered, but I don't know where to dig that info up either. I'm using a stainless steel whip that is about 5/32" at the base tapering to 1/8" at the tip.
Two other questions: Does having a small metal ball at the tip of the antenna have any effect? Does bending back the last 6" of the antenna about 45 degrees have any effect?
This antenna will be driven by a Sporty's handheld navcom with a BNC connector at the radio.
Thanks for any info.
08-16-2005, 06:56 AM
Whip antennas are ¼ wavelength long, Bob.
Wavelength in meters is 300/F so the wavelength of 123 Mhz is 2.469 meters. One quarter of that is 24 inches.
Thickness plus a ball on the end lowers the resonant frequency slightly so 22” sounds about right.
The only way to obtain an exact fit is through the use of a VSWR indicator. Then the antenna can be trimmed for the lowest VSWR.
08-16-2005, 08:55 AM
I dont know if they still do but radio shack used to sell a reasonal priced swr meter.
08-16-2005, 10:32 AM
2808 / F (frequency in megahertz) is a good practical formula for pre-trim length in inches, and accounts for the physical properties of typical antenna materials. For a center frequency of 120 MHz, 23.4 inches. You'll want access to a good SWR meter to do the final trim, which should be done in very small increments! If your whip is stainless steel, is can be very quickly trimmed by notching with a file, then breaking it off at the notch in a vise.
The part often overlooked is that the whip needs a ground plane, a flat, perpendicular surface about 5 per cent larger in radius than the length of the whip, connected to the shield of the coax at the antenna base, to achieve the 50-ohm impedance your radio is tuned to accept. That's a 48-inch disc of conductive material, hard to achieve in any small aircraft. Cessnas use the aluminum skin. The best we can usually do is to use the keel.
The little ball on the end of the whip isn't put there to affect the tuning, but to reduce the "staticky" noise which results from the corona discharge of static electricity developed as the whip moves through the air. It would be easier to trim for SWR at the ball end, but save the ball if you can.
The SWR meters at Radio Shack are acceptable for CB, at 27 MHz, but may be wildly inaccurate at VHF. Find an amateur radio club in your area, and find someone who has an MFJ antenna analyzer. It's a ~$100 box that makes the job very easy, and checks SWR with only a tiny signal output, reducing potential interference.
Do the check away from an airport if you can, away from metal objects, and hang the gyro a couple feet in the air if there's any way. Proximity to the ground will affect tuning.
08-16-2005, 10:58 AM
Paul, does anyone make a dipole type antenae for this frequency range?
08-16-2005, 12:41 PM
Try this at Aircraft Spruce. I have one and it works pretty good. You have to make sure you get the proper connector, either a straight BNC, or one with a 90 degree bend. My application has the antenna mounted on the inside of the fiberglass cabin.
This type of antenna would not work mounted on metal.
08-16-2005, 01:39 PM
I second Gary's recommendation. It's what I use.
08-16-2005, 03:18 PM
I must look into that when I get further along. There must be somewhere inside a SparrowHawk to hide one of them!
08-17-2005, 09:47 AM
I stuck my AAE antenna to the slanted structure between the windscreen and the door opening, on the R/H side. Most people don't even notice it being there...not even the connector.
This antenna works best if positioned from a 45 deg. angle to vertical.
08-17-2005, 10:10 AM
In an environment like a gyro, experimenting with position as Harry did may allow you to optimize tuning to make the radio happy. Any antenna inside the cabin, however, will have its coverage shadowed in some directions, especially behind you.
08-17-2005, 11:11 AM
Your correct on that, Paul.
The one deficiency in my case, is to the rear. When I first installed the AAE antenna, I ran an evaluation test. I had a friend to operate his transceiver on the ground as I was airborne. He was to continually read a text, as I was orbiting about 2 miles away...with a clear line of sight. Then, I was to transmit continuously while orbiting.
The result?? of this test was...I received all of his xmts. except he *faded*somewhat when my tail was aimed at him. My xmts. were received by him except when my tail was aimed at him.
It appeared that I could receive much better than transmitting, in that *null* position off the tail.
I hope you understand what I'm trying to say?! :rolleyes:
08-17-2005, 11:12 AM
Just a thought, There is a consil that runs up the back of the cab in the middle and comes forward on the ceiling for a ways. I wonder if you could bend it like a boomarang and put it inside of that to keep it out of harms way? Or does it like to stay straight? Harry do you mean up high? How did you run the coax?
08-17-2005, 03:59 PM
I did like Harry did, only on the pilots side so it wasn't right on top of the radio (on the passenger side).
The top of the antenna starts about eye-level behind the carpet on the fiberglass door post (between windscreen and door opening). The connector pops out just in front of the instrument panel along the door post. The coax cable pops out the side of the instrument panel and connects to the middle of the dipole. The second half of the dipole goes alongside the instrument panel and along the curved cabin pod towards my feet.
There is some interference (close metal to the proximity of the antenna) in the way of the door mount, some of the instrument panel (wiring and such), and the proximity to the rudder pedals. But with all that, I think I'm doing a lot better than those people that use a whip without an adequate ground plane. I never did measure the SWR, but I have had great performance of my ICOM A200.
I also have noticed a shaddow area towards the front as found by getting into position on a runway and then not being able to hear the tower's clearance. I taxied off the runway and then could hear the tower fine. It was only in one spot that this happened to me. I have never noticed it anywhere else.
Is this what you call a hijacked thread?
08-19-2005, 12:49 PM
:confused: I don't think so, Bob.
You titled the thread...VHF Antenna Length...and that's what we were talking about...VHF Antenna, right?! ;) :(
If I missed your point...sorry. :( Then again, you may be better advised to use a dipole antenna?!
08-20-2005, 05:40 AM
Mobileone has a good dipole antenna, (airkit) is the part number.
MEDICAL MARIJUANA (http://medicalmarijuanacard.info)
08-20-2005, 02:20 PM
One more "tweak" to provide a little more efficiency: Coaxial cable, regardless of type, has a "Velocity Factor". A coaxial cable of any length will carry the RF to the antenna but if it is trimmed to an odd wavelength (1/4 wave, 3/4 wave, 5/8 wave) it will carry the RF more efficiently (less loss). But due to the nature of the cable it "slows down" the RF propogation down the cable. Every type of cable has a spec table that includes the "Velocity Factor" that is expressed in percentage. One of the more common types has a VF of 66%.
So, using Mr. Beaty's calculation of 24 inches: 1/4 wave = 15.8 inches, 3/4 wave = 47.5 inches and 5/8 wave = 39.6 inches.
I'm sure that any avionics shop could make up a cable for you.
......or am I being anal-retentive?
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