I have 50ft of 9914f feeding a ground mounted 4-BTV. After a little tweaking
I have advertised swr across the 4 bands. (1.6:1 or less). The 4-BTV
instructions recommend a coaxial choke for "de-coupling the transmission
line".

I would like to be able to do some kind of analysis to see if I really need
to do this or not. Also with buried coax if I did so would the coax choke
get buried too? They (Hustler) recommend 10 turns of coax on a 6 inch form.
I am really pleased with the station so far ( about a week old!) and cannot
see any symptoms that I need this choke. I have a 6ft ground rod about 2 ft
from the rig and all is well best I can tell. What would some of the
symptoms be if I needed this rf choke?

TY
--

Charlie
Ham Radio - AD5TH
www.ad5th.com

If the coax is buried, you should not need any further decoupling. Any
RF current on the outside of the coax should end up amost entirely on
the ground.

I once had a 4BTV roof-mounted and didn't have many radials on it. I
didn't need decoupling for most bands, but 10 meters was downright
awful with high RF at the transmitter. I solved it by winding a few
turns of the coax to get a choke that was self-resonant at 10
meters...I seem to recall that I might have actually put a little extra
capacitance across the coil to tune it, but it was a long time ago.

If you put a neon light next to your coax near the transmitter and it
lights up when the transmitter is sending power to the antenna, it's an
indication you have lots of RF on the outside of your coax, and
decoupling is in order. You can do the same thing with greater
sensitivity using a small coil feeding a field strength meter...high
fields near the coax indicate current on the outside of the coax.
Cheers,
Tom

Thanks Tom that was a great reply. I was totally unaware that burying the
coax had that extra benefit. Adding the 75 meter resonator tomorrow and will
see how that affects things. Even without the resonator I hear DX stations
on 75M at night..

I do have quite few tuned radials with 20 on 10,15, and 20M respectively and
12 on 40m. I really do not room for any 75M radials. Hope I won't need them.
With over 50 inches of rain a year and the other 72 radials maybe I won't...

THX again - 73 / DX

--

Charlie
Ham Radio - AD5TH
www.ad5th.com
Live Blues Music
www.492acousticblues.com




"K7ITM" wrote in message
oups.com...
If the coax is buried, you should not need any further decoupling. Any
RF current on the outside of the coax should end up amost entirely on
the ground.

I once had a 4BTV roof-mounted and didn't have many radials on it. I
didn't need decoupling for most bands, but 10 meters was downright
awful with high RF at the transmitter. I solved it by winding a few
turns of the coax to get a choke that was self-resonant at 10
meters...I seem to recall that I might have actually put a little extra
capacitance across the coil to tune it, but it was a long time ago.

If you put a neon light next to your coax near the transmitter and it
lights up when the transmitter is sending power to the antenna, it's an
indication you have lots of RF on the outside of your coax, and
decoupling is in order. You can do the same thing with greater
sensitivity using a small coil feeding a field strength meter...high
fields near the coax indicate current on the outside of the coax.
Cheers,
Tom

Charlie, an antenna will need to have a choke only in the following
condition:
Check the Swr with feedline attached. Add another 1/4 wavelength of
transmission line with impdedance equal to the output of your
transmitter. If the Swr increases, then a choke is required. If Swr
goes down slightly, that only means that there is loss in the added
section of feedline.
The reason why this check works is that a choke is needed if current
exists on the outside of the coax. In that case, the current acts like
another wire attached to the antenna. This added wire changes the
effective length of the antenna causing a change in resonance with
added feedline.