|
New to Ham Radio?
My Profile
Community
Articles
Forums
News
Reviews
Friends Remembered
Speak Out
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
|
|
1-8 of 8 messages
|
Page 1 of 1
|
|
Calculate Air-Core Balun?
|
Reply
|
|
by KI6LNG on November 5, 2009
|
Mail this to a friend!
|
Hey everyone,
I want to build an air-core (because I have all the parts I need for one). I've got a twin lead-folded dipole for 20 meters on my roof, so I need a 4:1. How can I calculate the number of turns I need for this balun if it is wound on a 1-1/2" piece of PVC pipe?
|
| |
|
RE: Calculate Air-Core Balun?
|
Reply
|
|
by N3OX on November 6, 2009
|
Mail this to a friend!
|
You might be disappointed with trying to get an air core balun to work.
It seems like most of the "air core" 4:1 designs I've seen on the web have never actually been tried to do a 200 ohm to 50 ohm impedance transformation.
Someone wires one up and sticks it in a system where they use a tuner.
Here's someone who actually built and tested one:
http://g8jnj.webs.com/Balun%20construction.pdf
You'll see far down in the document that the air core versions don't do a broadband transformation.
That said I guess you can get it to work in your application by experimenting. I don't have direct advice for that, just try some different turns ratios and total lengths.
If you get frustrated trying to find an air core transformer that works, find yourself a ferrite core. For a 20m balun for use with 100W you can probably wind one on a couple of snap on ferrites from Radio Shack or something. You don't need much ferrite for a low power 4:1 balun.
73
Dan
|
| |
|
RE: Calculate Air-Core Balun?
|
Reply
|
|
by WX7G on November 6, 2009
|
Mail this to a friend!
|
You might consider a 4:1 balun made with coax. It requires a 180 deg length of coax. 23' for solid polyethylene coax and 28.5' for foam coax.
Wind the coax on the PVC. Bring the coax feedline to the balun. Connect the braids together; the feedline and both ends of the balun. Connect the feedline center conductor to one balun center conductor. Connect the other balun center conductor to one side of the folded dipole. Connect the feedline center conductor to the other side of the folded dipole.
|
| |
|
RE: Calculate Air-Core Balun?
|
Reply
|
|
by WD4HXG on November 6, 2009
|
Mail this to a friend!
|
http://www.hamuniverse.com/balun.html
http://www.southgatearc.org/techtips/coax_balun.htm
Do not forget to account for the coax velocity factor.
For a single band with the coax cut for a quarter wavelength on the band of operation you will probably have pretty good service from the balun. However coax baluns are not noted for extra wideband performance so operation on adjacent bands may prove dissapointing.
Ideally you want to build the balun using a transmission line with a characteristic impedance that is the geometric mean of the input and output impedance. Thus for 50 ohm input and 200 ohm output the optimal coax impedance would be 100 ohms. If you build the balun for a 75 to 300 ohm transform then the optimal coax impedance would be 150 ohms. About as close as you are likely to find to 100 ohm coax is RG-180/U (95 ohm impedance). Next best choice would be 75 ohms. The coax impedance does not drive the impedance ratio as much as the length of the coax but will impact the bandwidth of the balun. You can use 50 or 75 ohm coax to make the balun but the bandwidth of the balun will suffer. This is likely to be a problem if you use the balun across multiple bands. The losses in the balun will be a little more with 50 ohm coax as compared to one built with 100 ohm coax but it should still be usable and provide the 4:1 impedance ratio you are looking for single band operation.
The nominal impedance of the folded dipole is around 280 ohms. With a 4:1 balun you will likely run a VSWR around 2:1 under the best of conditions so you will likely still need an antenna tuner if running a solid state rig.
Build it, characterize it and use it. All designs have tradeoffs. A balun with 1 to 2 dB loss may provide you with a system that is perfectly acceptable while I will grouse if I have more than 0.25 dB loss in my balun. The goal is to build what works for you and allows you to enjoy what you are doing.
|
| |
|
RE: Calculate Air-Core Balun?
|
Reply
|
|
by WX7G on November 6, 2009
|
Mail this to a friend!
|
Don't split hairs.
The coax velocity factor has been accounted for in the coaxial balun. With a 180 deg line the impedance of the line does not matter much. The impedance at the load end appears unchanged at the source end. The SWR of the line affects the loss but for a 50 ohm line used where a 75 ohm might be optimum we are splitting hairs.
|
| |
|
RE: Calculate Air-Core Balun?
|
Reply
|
|
by WX7G on November 6, 2009
|
Mail this to a friend!
|
|
WD4HXG: you are confusing a 180 deg balun with a 90 deg matching section. The correct impedance of the line in a 180 deg balun is 1/4 the load impedance.
|
| |
|
RE: Calculate Air-Core Balun?
|
Reply
|
|
by WX7G on November 7, 2009
|
Mail this to a friend!
|
KI6LNG: Air core balun, 4:1.
I know of two ways to build a 4:1 air core transformer. Galvanically isolated transformer and autotransformer.
1) Transformer: 17 uH inductor on a 2" form: 16 turns #14 THHN wire (house electrical wire) close wound. The coil length is 1.6 inches. Connect this to the folded dipole. Wind 8 turns over this and connect to the coax. This does not form a perfect balanced-to-unbalanced transformer. This is due to the capacitive coupling from primary-to-secondary. Build it and let us know how it works.
|
| |
|
RE: Calculate Air-Core Balun?
|
Reply
|
|
by WX7G on November 7, 2009
|
Mail this to a friend!
|
|
2) Autotransformer: Wind 16 turns close wound, #14 THHN wire on a 2" form. Connect the ends to the folded dipole. Connnect the coax shield to the center of the coil. Connect the coax inner conductor to one end of the coil.
|
| |
|
Email Subscription
You are not subscribed to this topic.
Subscribe!
My Subscriptions
Subscriptions Help
Check our help page for help using
Forum, or send questions, comments, or suggestions to the
Forum Manager.
|
|
|
