Antenna Engineering Question

Status
Not open for further replies.

Tim-B

Member
Joined
Nov 17, 2016
Messages
397
Location
Lafayette, LA
If I want to make my own antenna for a handheld to recieve VHF frequencies and I want to make it half wave but coil the antenna so it is shorter then is there a formula for what pitch and diameter the coils need to be to work best with a given frequency or does it not really matter?
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,090
Location
So Cal - Richardson, TX - Tewksbury, MA
There are formulas but at VHF there will still be a lot of experimenting to get the coil and whip lengths working together. The placement of the coil along the whip also makes a noticeable difference in the coil specs. Here is a link to a loading coil calculator for 1/4 antennas as an example: https://m0ukd.com/calculators/loaded-quarter-wave-antenna-inductance-calculator/

Even at HF frequencies where the calculator is intended to be used, there will still be a lot of tinkering with the coil windings to get the antenna to resonate where you want.

A 1/2 wavelength end fed antenna like your whip idea is a very high impedance antenna and you first need a circuit to match the roughly 3,000 ohm impedance of the 1/2 wave whip to your 50 ohm radio, which is not easy. Then you want to shorten the half wave antenna with a coil, which will reduce its efficiency and probably not make it worth using a half wave at that point.

Its much easier to just use a full size 1/4 wave whip, which is around 15 to 20 inches long depending on what part of the VHF band you want. Shortening a 1/2 wave antenna to a similar length will not work any better than a 1/4 wave whip of the same length due to all the efficiency problems you introduce with the coil and a shorter antenna. A full length half wave antenna with a proper 3,000 ohm to 50 ohm matching network will work noticeably better than a full length 1/4 wave if done right, but its going to be at least 3ft long.
prcguy
 

Tim-B

Member
Joined
Nov 17, 2016
Messages
397
Location
Lafayette, LA
Okay thanks, I may just stick with a straight whip antenna. Does the diameter of the antenna make any difference? Will a thick wire work better than a thin wire since resistance is inversely proportional to the cross sectional area of the conductor or does that not really matter much with antennas?
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,090
Location
So Cal - Richardson, TX - Tewksbury, MA
A thicker antenna will have more band width. For example, a 1/4 wave whip made of thin piano wire might have an acceptable match over a few MHz at VHF but a 1" diameter tube might work over 20MHz of BW or more.
prcguy

Okay thanks, I may just stick with a straight whip antenna. Does the diameter of the antenna make any difference? Will a thick wire work better than a thin wire since resistance is inversely proportional to the cross sectional area of the conductor or does that not really matter much with antennas?
 

lmrtek

Active Member
Joined
Feb 11, 2009
Messages
533
all you need is an 18 inch whip

you can add a wire counterpoise which will give you excellent performance

also you can easily make a 1/2 wave "sleeve dipole" and it will also work very well

18 inch radiator over a 18 inch sleeve
 
Status
Not open for further replies.
Top