Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old August 24th 04, 06:46 PM
Airy R. Bean
 
Posts: n/a
Default Vertical Antennae matching

It seems to me that if I have a vertical antenna with the ATU at the
base of it with coax back to the shack, that all I need is a series
inductor to tune it for frequencies for which it is shorter than 1/4
wave and a series capacitor for frequencies for which it is longer
than 1/4 wave.

What do I do, however, for those frequencies for which it is close
to a half wave? PI network? Or L network?


  #2   Report Post  
Old August 24th 04, 08:57 PM
Reg Edwards
 
Posts: n/a
Default

What do I do, however, for those frequencies for which it is close
to a half wave? PI network? Or L network?

===================================

You very slightly de-tune (decrease) the antenna length, not enough to have
any effect on its radiating properties, such that a simple parallel tuned
matching circuit will cope.

You can connect a coax feedline line either in series with the coil, an
L-network. or tap the coax a few turns up the coil. A very modest ground
connection adjacent to the bottom end of the coil witll suffice.

The same circuit will cater efficiently for quite a wide range of antenna
heights.

I hope you are satisfied with the above description.
---
Reg, G4FGQ


  #3   Report Post  
Old August 25th 04, 02:06 AM
K9SQG
 
Posts: n/a
Default

It's not that simple. Getting the antenna resonent is one issue, getting it
matched for 50 ohm coax is the other issue. So a simple inductor or capacitor
won't always work by itself. Helps with resonence, but not impedance.
  #4   Report Post  
Old August 25th 04, 02:45 AM
Reg Edwards
 
Posts: n/a
Default

But a simple inductor AND capacitor always works.

All tuners need at least one capacitor and one inductor.


  #5   Report Post  
Old August 25th 04, 04:25 PM
Rick Karlquist N6RK
 
Posts: n/a
Default

"Airy R. Bean" wrote in message
...
What do I do, however, for those frequencies for which it is close
to a half wave? PI network? Or L network?


An L network with reactances of several hundred ohms for
both inductor and capacitor.

Rick N6RK




  #6   Report Post  
Old August 25th 04, 05:49 PM
Roy Lewallen
 
Posts: n/a
Default

A common solution is to use a parallel tuned circuit, approximately
resonant, with inductor and capacitor reactances of a couple hundred
ohms or so, and the capacitor being adjustable. Connect one end of the
network to ground and the other to the antenna. Connect the feedline
between ground and a tap near the bottom of the coil. Alternately adjust
the tap position and the variable capacitor for the best impedance match.

Roy Lewallen, W7EL

Rick Karlquist N6RK wrote:
"Airy R. Bean" wrote in message
...

What do I do, however, for those frequencies for which it is close
to a half wave? PI network? Or L network?



An L network with reactances of several hundred ohms for
both inductor and capacitor.

Rick N6RK


  #7   Report Post  
Old August 25th 04, 06:12 PM
Airy R. Bean
 
Posts: n/a
Default

I'm never satisfied, always driven by the spirit
of curiosity and discovery!

Changing the vertical length is not an option as its
an ally pole supporting a tv antenna; it just happens to be insulated
from the ground.

I was planning to put about 20 radials in green-and-yellow, splaying
out from a single copper stake.

"Reg Edwards" wrote in message
...
What do I do, however, for those frequencies for which it is close
to a half wave? PI network? Or L network?

===================================

You very slightly de-tune (decrease) the antenna length, not enough to

have
any effect on its radiating properties, such that a simple parallel tuned
matching circuit will cope.

You can connect a coax feedline line either in series with the coil, an
L-network. or tap the coax a few turns up the coil. A very modest ground
connection adjacent to the bottom end of the coil witll suffice.

The same circuit will cater efficiently for quite a wide range of antenna
heights.

I hope you are satisfied with the above description.
---
Reg, G4FGQ




  #8   Report Post  
Old August 25th 04, 06:12 PM
Airy R. Bean
 
Posts: n/a
Default

Hence the question about Pi and L networks.

"K9SQG" wrote in message
...
It's not that simple. Getting the antenna resonent is one issue, getting

it
matched for 50 ohm coax is the other issue. So a simple inductor or

capacitor
won't always work by itself. Helps with resonence, but not impedance.



  #9   Report Post  
Old August 25th 04, 06:13 PM
Airy R. Bean
 
Posts: n/a
Default

I've obtained roller coaster and 350 pF TX capacitor.

"Rick Karlquist N6RK" wrote in message
news:Nb2Xc.42797$9d6.33881@attbi_s54...
"Airy R. Bean" wrote in message
...
What do I do, however, for those frequencies for which it is close
to a half wave? PI network? Or L network?


An L network with reactances of several hundred ohms for
both inductor and capacitor.

Rick N6RK




  #10   Report Post  
Old August 25th 04, 06:15 PM
Airy R. Bean
 
Posts: n/a
Default

Presumably the roller coaster could be used for
a vari-tap, but then that means that the whole coil
will always be in circuit.

Time for a quikc .XLS for all bands.....

"Roy Lewallen" wrote in message
...
A common solution is to use a parallel tuned circuit, approximately
resonant, with inductor and capacitor reactances of a couple hundred
ohms or so, and the capacitor being adjustable. Connect one end of the
network to ground and the other to the antenna. Connect the feedline
between ground and a tap near the bottom of the coil. Alternately adjust
the tap position and the variable capacitor for the best impedance match.

Roy Lewallen, W7EL

Rick Karlquist N6RK wrote:
"Airy R. Bean" wrote in message
...

What do I do, however, for those frequencies for which it is close
to a half wave? PI network? Or L network?



An L network with reactances of several hundred ohms for
both inductor and capacitor.

Rick N6RK




Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Inverted ground plane antenna: compared with normal GP and low dipole. Serge Stroobandt, ON4BAA Antenna 8 February 24th 11 11:22 PM
15m Vertical Matching help J. Laub Antenna 18 November 1st 04 02:34 PM
QST Article: An Easy to Build, Dual-Band Collinear Antenna Serge Stroobandt, ON4BAA Antenna 12 October 16th 03 07:44 PM
Poor vertical performance on metal sheet roof - comments? Kristinn Andersen Antenna 23 August 8th 03 11:08 PM


All times are GMT +1. The time now is 02:59 AM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017