[Antennas] SWR dipole

[George, W5YR]

Terry, I am not sure that I have the right picture here, but are you
feeding the *end* of a vertical dipole directly with 50-ohm coax through a
coiled-coax choke balun?

If so, how are you handling the enormous impedance mismatch between the
antenna (probably 1000 ohms and possibly much more at the end) and the coax
(50 ohms)?

If you are feeding directly, I cannot understand how the system could
operate at all, much less show a 1.7:1 SWR on the feedline . . .

Please give me a better description - you got me wonderin'!   <:}

As to your question, the usual way is to buy or build an r-f current probe.
Look in the Handbook or the Antenna Books or possibly in the ARRL Antenna
Compendium series. Roy Lewallen, W7EL, has described his design but I don't
recall where.

Placing your SWR meter at different point in the feedline can give a
first-order idea of current level, if any. The usual meter cannot work
properly if there is current on the outer braid of the coax. So, its
reading varies with position in the line. There will be some small change
in SWR with position due to line loss but it should be small for a fairly
short line. The measured SWR will decrease as you move away from the
antenna.

Measure the SWR near the antenna, wherever you can open the line and insert
the meter. Then again at the input. Then add a few feet of line at the
input and measure again. If you get essentially the same reading at all
locations, the chances are that you do not have enough outer braid current
to cause any problems.

But, how are you feeding that dipole?   <:}

73/72, George    
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!


Terry Fletcher wrote:
> 
> Yesterday I put up a 20M half wave vertical dipole with a coax choke
> balun at the bottom end of the shield side. . It's 32' long and  seems
> to work ok. The bottom end is off the ground about 15'. Good sig reports
> and it's 1.7:1 or less across the entire band.       I don't seem to
> have any rf on the feed line, no way to tell.
> 
> I wanted to use ferrite beads as a means of decoupling the shield (ala
> W2DU) from the bottom end of the dipole element. But I had some rg-8x on
> hand and there is no 73 series bead size to fit the OD of the coax, so I
> used the coil balun... . I may make another  using rg-58 and  fb-73-2401
> beads   The idea was  to  have a bottom fed dipole, thats easy to roll
> up and store, and to use as a sloper or vertical, depending on tree
> size.
> 
> Here's my question.
> Does anyone know of a good way to test for rf on the exterior of the
> feed line? A sensitive field strength meter perhaps?