Chock or balun using QRP

[N0SOY]

I have 2 qrp radios that I wll be running on a rotatable dipole.  The antenna is a basically 2 HF MFJ "hamstick" forming a 14ft dipole.  I have 40 and 6 meters sticks. 

The issue is Do I have to run a choke or Balun in the feed line when I am running 12 watts or less?  If so Why?

Thanks

David

[WB6BYU]

On one hand, it doesn't matter what power level you are using:  the antenna is still
unbalanced by the way the coax is connected.  (You have half the antenna on one
side, and half + the coax shield on the other side.)  How much RF flows on the
outside of the coax will depend on the length of the coax and what is attached
at the other end, including how (or whether) it is grounded.

What impact this had on your operation, and whether you notice any problems,
depends on several factors.  Certainly, you are more likely to have interference
to household electronics due to the RF when you run higher power, but noise
pickup on receive from such appliances is independent of power level.  One of
my QRP rigs is particularly sensitive to stray RF, especially on 40m:  it causes
the dial lamps to blink when I transmit.  I've also had it cause my keyer to
stick on after stop sending, even using QRP levels.

Or it might not cause any noticeable symptoms at all in your particular
circumstances.

You don't have to use a balun:  I wouldn't let the lack of one keep you
from getting on the air.  But it does make behavior more predictable.  If you
find your rig behaving in odd ways, or have a lot of receive noise, or if the
SWR changes depending on how your cable is arranged, then a balun may
make life much easier.

[N0SOY]

Would a choke coil made from 30 ft length of coax like I have seen work or would a different length work?

I have not found a good explaination of how to make a choke/balun with the mathmatics to back it up.  (sorry the engineer in me does come out at times  )  A link would be nice. 

[N2RRA]

Would a choke coil made from 30 ft length of coax like I have seen work or would a different length work?

I have not found a good explaination of how to make a choke/balun with the mathmatics to back it up.  (sorry the engineer in me does come out at times  )  A link would be nice. 

Yes, A choke balun when done correctly and cleanly will work fine. Here are a few links. Enjoy!

http://www.k7mem.com/Electronic_Notebook/antennas/dipole.html

http://hamuniverse.com/balun.html

http://www.dxzone.com/tag-choke-balun/

73!

[WB6BYU]

A link would be nice. 

G3TXQ has a lot of information about choke effectiveness for different winding
diameters and cores here:

http://www.karinya.net/g3txq/chokes/