Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Wed 20, 2021 8:37 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 4 posts ] 
Author Message
 Post subject: Short antenna capacitance
PostPosted: Aug Mon 16, 2021 12:10 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4956
Location: Texas
I want to understand is how to calculate the self capacitance of a simple Part 15 antenna wire.

On my LM386 transmitter, the final is an MPSA18 NPN transistor. The collector is fed by the LM386 through a 2 mH choke. The collector also goes to the antenna via a 157 uH coil shunted to ground on the antenna end by a 4 pF capacitor, peaking at 1680 KHz.

Resonance calculators say the capacitance must be 56 pF. So where's the other 52 pF? Does the wire antenna have that much capacitance? Does the capacitance in the transistor figure in? If so, what part of the transistor contributes to the capacitance stack? THANKS.


Top
 Profile  
 
 Post subject: Re: Short antenna capacitance
PostPosted: Aug Mon 16, 2021 12:31 am 
Member
User avatar

Joined: Jan Fri 06, 2012 8:47 pm
Posts: 7479
Macrohenry wrote:
I want to understand is how to calculate the self capacitance of a simple Part 15 antenna wire.

On my LM386 transmitter, the final is an MPSA18 NPN transistor. The collector is fed by the LM386 through a 2 mH choke. The collector also goes to the antenna via a 157 uH coil shunted to ground on the antenna end by a 4 pF capacitor, peaking at 1680 KHz.

Resonance calculators say the capacitance must be 56 pF. So where's the other 52 pF? Does the wire antenna have that much capacitance? Does the capacitance in the transistor figure in? If so, what part of the transistor contributes to the capacitance stack? THANKS.
https://badw.de/fileadmin/members/R/368 ... tionMJ.pdf This article points out that electrically short antennas can easily amount to 25 pF/meter. So a 6 to 10 ft antenna could easily amount to a capacitance in the range you are estimating.

Curtis Eickerman

_________________
http://curtiseickerman.weebly.com


Top
 Profile  
 
 Post subject: Re: Short antenna capacitance
PostPosted: Aug Tue 17, 2021 6:15 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4672
Location: Perrysburg, OH, 43551 U.S.A.
Here you go: https://www.everythingrf.com/rf-calculators/whip-antenna-calculator
Here's a sample using a 118" (~3 meter), 1/8" dia. rod at 1680Khz:
Attachment:
Sample whip ant calc.jpg
Sample whip ant calc.jpg [ 68.15 KiB | Viewed 2963 times ]

John

_________________
“Never attribute to malice that which can be adequately explained by stupidity.”
― R. A. Heinlein


Top
 Profile  
 
 Post subject: Re: Short antenna capacitance
PostPosted: Aug Tue 17, 2021 6:59 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4956
Location: Texas
Well, there you go. I love definitive answers. I can see exactly where that missing capacitance is. THANKS Curtis and John.


Top
 Profile  
 
Post New Topic Post Reply  [ 4 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 7 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB