Antenna: Helical Ant and L-match

begin @ 2014/7/17

The Antenna is the most mystery thing to me in the whole RF system. many type, many size, many shape. i stuck at choosing a antenna for final stage of the TX chain, after get PA work.

Rubber ducky Antenna / Helical

Most of the commercial handy device carry with a little rubber duck antenna(wiki), they are most of VHF/UHF device , beyond 100-500Mhz, it's performance is poor.

this also call Helical Antenna. but this is not Axial-mode helical, i am confused while search the helical on WWW.

some notes from wiki/ham:

* electrically short quarter-wave antenna, typically 4% to 15% of a wavelength long.

* poor efficiency, < less than quarter-wave whip, >better than same length base loading whip

* High Q, narrow bandwidth

* Most of Helical antenna mount on a ground plane, normally the handy device don't have.

An experimental improve handy antenna

as a roughly comparable number, helical normally -5db, “negative gain” compared to a full sized quarter wave whip held at face level5W transmiter will only 1W to air. handy as ground will generate 10-20dB attenuation, oh, almost 95% transmitter power is loss.

one way to improve is add a like C. Edward Harris, KE4SKY, add a wire counterpoise, as the ground plane. the counterpoise prevents transmitted RF from coupling with your body. Your antenna now performs like a center-fed dipole.

(picture from C. Edward Harris, KE4SKY)

(24" long for AM air band, 19.5" long for 2-meters, 18" for VHF "high" and marine band; 11” for 222 MHz or 6.5" for the 70 cm band and 6" for GMRS).

on6mu, DIY your own rubber Helical (which will very likely better than commercial one):

http://users.belgacom.net/hamradio/schemas/on6mu_vhfrubberduck.htm

Radiation resistance and Antenna Match

@2015/9/06

Antenna radiation resistance confuse me long time, because i always think in a wrong way that i assume the radiation resistance is the target resistance the transmitter want to match.

i.e, i wound a 1/4 wave length short whip for 10M, the helical wounded antenna, base on some calculation, or the knowledge, the radiation resistance is very small for a short whip or helical whip, which is even shorter.

while searching for a match unit, the L match is the most simpler one, i should use this, i told myself.

the left below picture, the basic, typical version of L match, I had suspect this match for long time, why antenna impedance will big than transmitter's 50ohm output impedance(capacitor on the antenna side)? and i successfully tune the SWR to almost 1 use this configuration. why?

Fig.2

and some match just like fig.2 make me even confused for long time, what you done when you connect a antenna to a LC tank?

until today, i realized i am wrong about what relationship between Rrad and it's impermanence when transmitter not working exactly the antenna's resonant frequency. following is a short whip equivalent circuit:

while we are not working on it's resonant frequency, we got antenna like this:

L match exploit the fact when the circuits resonant, from the input end, transmitter will see a difference impedance value than the antenna actually be.

this also apply to the antenna equivalent circuits, while working on it's resonant frequency, Rp=Rs=Rrad+Rloss

if not, Rp=(Rrad+Rlos)*(Q^2+1), Xp also have this transformation with Xs.

when deploy the L-match, in this situation, can imagine, the antenna not resonant at the transmit frequency, with the external coil and capacitor, the antenna will be tuned to the resonant at the transmitter frequency, the important things is, we see the Antenna's Rp not the Rs(Rrad+Rloss), so it's big than 50 ohm when working on this frequency, in my configuration, it's 28.6 Mhz.

now it's pretty clear, even the Rrad is very small, actually, small Rrad present a high Q resonant Antenna tank, then you transform it's Rs to Rp with a even big factor: Q*Q+1, the L match, absorb the reactance impedance of the antenna , and transform the Rp to 50 ohm.

OK, then at the end, here is the single band version of 'miracle whip''s match circuits. i could better understanding this circuit, whenever it work well or not.

Finally, this article, which give me confidence to think my understanding is reasonable, it's great , you should read:

AN APPROACH TO ANTENNA TUNING

by Lloyd Butler VK5BR

http://users.tpg.com.au/users/ldbutler/Approach_Ant_Tuning.htm

L-match Z-plane

@2015-9/10

let's continue the talk about the "Radiation resistance and Antenna Match". G3ynh is expert about this. let me pull 2 picture from his great site:

http://www.g3ynh.info/zdocs/z_matcing/part_2.html

So, this accurately explain everything about L-match behavior. this is the more 'text book' explain, previous explain, just another language to understanding what happens.

-----------------//tmp

shorted whip (bottom loaded)

1. 20cm whip serial with 2.2uH code inductor, (49Mhz toy walktalker)

2. 90cm whip, serial with 1uH code inductor (wireless telephone)

http://www.smeter.net/antennas/short-helical.php

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