Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: Problem with Cushcraft A3S 20M trap?  (Read 3909 times)
W9GB
Member

Posts: 2600




Ignore
« Reply #15 on: August 05, 2012, 08:46:07 AM »

Guy --

Let's start with theory and practical (A3S trap in hand).
This infomation is a review of Electronis 101 -- BASIC RF Knowledge required to be known by amateur radio amateurs.  IF you skipped it, now you know the shortcomings of memorizing answers.

The Cushcraft A3S is a Multi-band TRAP antenna.  
The traps are an L-C component (L = Inductance; C=Capacitance).
http://en.wikipedia.org/wiki/LC_circuit
RLC
http://en.wikipedia.org/wiki/RLC_circuit

Quote from: va3guy
I decided to take the trap off and inspect it again.  It still looked fine and the resistance reading was less than 1 ohm.  I decided to put it back up WITHOUT the aluminum casing, just to see what would happen.  No change...but when I brought it back down, put it back in the aluminum shell and reinstalled it on the antenna, the SWR came down to around 1.2.  So I am figuring that maybe I did not have a good enough contact with the tubing and the problem was not the trap after all.  Not really sure.

The Inductance is 74 turns of 12 AWG wire (HV enamel insulation) on a 3/4 inch non-conductive (plastic) form.  BUT, Where I the Capacitance?  We will get to that in a minute.

Let's first comment on your diagnostic tool, a DVM or VOM set to DC continuity.
That is fine to tell you that there is no breaks in the 74 turns of 12 AWG, BUT that is NOT enough !!

RADIO FREQUENCY (RF) follows AC THEORY,  NOT DC THEORY !!
A GRID DIP METER (GDO) is the diagnostic tool of choice for L-C TRAPS.

NOW, back to your trap's Capacitance.  The outer aluminum tube (shell) of the Trap is the outer shell of a COAXIAL CAPACITOR !!  That metal casing is an active component of the trap.
SO, a poor or loose mechanical or electrical connections between that outer tube and it's connection to the inner aluminum element tube -- means you have no capacitance OR a value different than desired (bad capacitor) -- the L-C trap does not work!

LAST, in the area of the trap (Inductance) where you had  ARCING (discoloration) that your neighbor saw!!
Carbon tracks from those HV arcs produced NEW conductive paths (NOT desirable).  
In this case you should clean, and apply a HIGH VOLTAGE Insulating Varnish (High Corona Dope) to prevent future arcs in this area.
http://shop.willyselectronics.com/browse.cfm/mg-super-corona-dope-high-voltage-insulating-varnish-2-oz-/4,7361.html

Quote from: va3guy
 Currently, there is no insulation inside that trap anymore but I am getting SWR around 1.4 at 14.010 up to 2.5 on 14.250.  Not too bad, really.  But is the insulation required?  What does it do?
Insulation can be reapplied, new adhesive heat shink products are very good for this application.
Readily available from McMaster-Carr, Grainger, and major electrical/Electronis suppliers.
« Last Edit: August 05, 2012, 09:11:12 AM by W9GB » Logged
VA3GUY
Member

Posts: 172




Ignore
« Reply #16 on: August 09, 2012, 08:40:17 PM »

Thanks Greg.  It has been 37 years since I got my license and I was off the air for around 30 of them so I have forgotten a lot.  Your info is welcomed and appreciated.  I will be acting on it also.  Thanks much...

- Guy

ps - thank you to all that replied.
Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!