J-Pole Antenna Help Information
by Phil Karras, KE3FL
J-Pole Antenna Help Information
SWR: To measure the SWR correctly you should make sure that there is no RF
on the coax. One test is to see if the SWR changes when you use
different lengths of coax. (Don't use a full electrical wavelength
since this will hide any problems by simply moving you to another point
that is equal to the present measuring point.) We supply 20 feet of coax
in the KE3FL J-pole kit because the best readings are obtained when the
antenna is 2 wavelengths away from all objects. 2 WL of 146 MHz is about
13.5'. The antenna should be 13.5' up, and away from all other objects.
(1 m = 39.37" = 3.28'). Don't forget to take into account any coax used
INSIDE the PVC pipe if you are building the J-Pole inside PVC.
| The velocity Factor for RG-58 is 0.66, the wavelength is: WL = c/f, |
| c = 3E8 m/s, and f = 146 MHz = 1.46E8 cycles/s ==> 3E8/1.46E8 = 2.05 m |
| 1/2 WL = 1.03 m, (.66)1.03 m = 0.68 m ==> 26.70" == > 2.22' and 9*2.22 |
| = 20.02' ==> 20' 0.26" (NOTE: 9*2.22 = 19.98' because I was only |
| reporting 2 places but I was keeping more digits in the calculator.) |
If you are unable to find a match point for an SWR of 1:1 no matter where you place the contact points of the coax on the J-Pole, try adding a bit more length to the bottom end of the antenna. This will extend the distance between the shorted end of the antenna and the coax contact points. (For 2 meters, I add about 1 inch.) Then, place the coax contact points back to where the program said they should be. Move the shorting bar up and down until you find a place where the SWR is 1:1 (or as low as you can get it) at ANY frequency. Once this point has been found, move the coax contact points AND the shorting bar together until you can center the lowest SWR at your desired frequency. |
EXAMPLE: change:
+---c--------------------------------------------------------
|
+---s------------- ----------------------------------------
to: Diagram:
----+-*---c--------------------------------------------------------
|--- shorting bar
----+-*---s------------- ----------------------------------------
|
If you need to shift the frequency and moving the Tap point and
shorting bar together doesn't change it enough, you can cut the J-Pole
(hopefully you don't need to add wire). See the calculation screen
for the expected frequency shifts for the length listed. This is the
length change to the STUB. You will have to remove/add three times
that amount to the radiator.
==========================================================================
||* WARNING * WARNING * WARNING * WARNING * WARNING * WARNING * WARNING *||
|| ------------------------------------------------------------------- ||
|| When cutting the antenna shorter, I recommend making only HALF the ||
|| change you may need. In this way you may be able to prevent making ||
|| too large a change and having to undo it. ||
==========================================================================
|
| The first thing to check if you have a narrow bandwidth (BW) is the coax connection. If this connection is not long enough the BW will be narrow. The connection should be at least 1/2 inch long, and one inch is fine. Making it longer than one inch won't help matters. This will only help a little. If the coax connection is OK, within the above range, then the narrow BW is an indication that the antenna is too long. In my experience, a BW of 3.0 to 3.5 MHz indicates that the coax connection could be longer, but a BW of less (I've even seen a BW of 0.6 MHz!) indicates that the antenna is too long. For the above case (0.6 MHz BW) I shortened the antenna by 1.0 inch. I only removed this from the top of the antenna, not from the stub section. This brought the BW up to 4.0 MHz. Since I had to move the coax tap point to bring the SWR to 1.0 at 146.0 MHz the BW went down to 3.8MHz. Still, not bad and I left it at that. (I measure the BW between SWR of 1.7:1 points. I have built one wire J-pole with a BW of 4.0 MHz between SWR points of 1.2:1. This takes much time and experimentation to accomplish, this antenna took me three days to complete, about 9 hours of work. If you really want to do this, take very good notes and measurements so you can always return to where you were.) Also remember that a good dummy load has an SWR of 1:1 for a bandwidth of 100 MHz or more. The object here is not to make a dummy load but a radiating antenna. |
| This type of J-Pole was written about in QST magazine in 1994. The description has appeared in many of the local club letters. The J-Pole is not difficult to make, even for beginners. You will need to have some experience in soldering and in stripping wire without cutting through the wire. This antenna works well on 2-meters and also works on 440 MHz. I recommend that you read all of these instructions BEFORE beginning your construction project. Nothing is more frustrating than doing something only to find a hint afterwards that would have made the project go smoother. |
PARTS LIST:
1. ~5 feet of ladder line (for the antenna)
2. ~20 feet of coax
3. BNC connector
4. ~ 2" of Heat shrink tubing
5. Building and tuning instructions
6. Plastic carry bag (if available)
|
| Using ladder line is a bit different than using the solid TV twin-lead wire. Before cutting, stretch out the wire so that you can position the proposed cuts at a position that has center plastic support, and not at a position that has no center plastic. This may not be possible for both the 1/4 section and the total length position. I recommend selecting the support at the top if you must choose. This plastic melts well and can be melted back together. I have had to melt sections back together in both locations and the antennas work just fine and hold up to field rigors, but melting one section together is less time-consuming than melting both. Select the bottom of the antenna and strip off about 3 - 3 1/2" of insulation from both wires. Solder a shorting bar about 1" from the bottom of the antenna. (This bar may need to be moved.) The coax will be connected about 1 1/4" from the shorting bar to begin with. Again, this connection and the shorting bar may have to be moved in order to optain the best SWR and frequency match. |
Diagrams:
+-- coax connects here, Center, Shield
|
+---c---------------------------------------------------------
| gap
+---s------------- -----------------------------------------
| |
| +- tap +-
|+------stub----+|+--length of radiator-(above stub)--------+|
|
NOTES: Bend the coax about one inch from the end and score the
insulation with a sharp knife. This cuts into the insulation
without damaging the shield if done gently. Then rotate the
coax so you can continue scoring the coax until it is cut all
the way around. Cut the insulation from the new cut to the end
of the coax. You should now be able to pull off the insulation
with pliers.
Remember to always cut AWAY from yourself!
NEVER use wire strippers on the large portion of the coax; it only
damages the shield. If you have a tool designed for coax, use it.
Prepare one end of the coax as stated above in the NOTES. Then,
separate the coax shield and twist it together. Now strip off about
3/4" of insulation from the center conductor of the coax. (DO NOT
SOLDER at this time.)
Tie a gentle knot in the coax about a foot or two away from the other
end of the coax. This will keep the BNC parts from traveling all the
way down to the other end & off again. Slip on the nut, washer, and
rubber piece. Now, prepare this end as stated above in the NOTES
section.
Push the tapered metal piece from the BNC package onto the coax so that
the taper is towards the end of the coax and the piece stops at the
large insulation.
Fold the shield back over the tapered piece and cut off all shield that
folds back beyond the piece.
Strip off 3/4" or more of insulation from the center conductor and tin
(solder) this wire. (NOTICE the instructions that come with the BNC
connector and make the measurements as stated there if there is a
conflict with these instructions.)
Poke the center conductor into the gold BNC center connector and see
how far it goes. Then cut off all the extra center conductor (wire) so
you have just enough wire left to fit into the BNC center connector.
Tin this center wire. (To tin is to Melt some solder on it.)
Locate the hole in the side of the BNC center connector and rotate it
up so you can see it. Slip the center conductor into the wire and
place the soldering iron on the tip of the center connector. Feed
solder into the hole on the center connector. When some melts (and
this won't take long), remove the solder and slip the soldering iron
off the tip. DO NOT pull the soldering iron away from the tip as this
will pull the tip right off the center conductor in most cases. (There
is usually some solder on your soldering iron tip. This will attract
the BNC tip just enough to pull the BNC tip off the wire if you pull
the soldering iron tip away from the BNC tip.)
NOTE: If the tip does not have a hole in it, then use solder that fits
into the tip, cut off a small amount and slip it into the tip,
then push the center conductor wire into the tip after the
solder. Now follow the melting instructions above, when the
wire slips all the way in the solder is melted and you can slip
the soldering iron off the tip.
Push the tip into the BNC body, push all the other BNC parts up to the
body, and screw then together. Use tools here so that you can tighten
the parts together.
|
Wrap the shield 1.25" up from the bottom around the 17" side of the
twin lead. Wrap it in such a way that the distance from the coax to
the wire is the same for both the shield and the center conductor.
Solder the shield and twin lead wire together.
Wrap the center coax conductor around the longer twin lead wire up from
the bottom, the same distance that the shield is wrapped to the other
wire, and solder it.
Cut off the extra coax wire. Also, cut off all the excess twin lead at
the top except for a loop or two. These ladder steps are great for
hanging the antenna over a nail or hook, so leave at least one of them.
Your antenna is now ready to test. Get your SWR analyzer or meter.
Hang the antenna away from all objects (I hang mine from the top of a
window and this seems to work almost as well as from a tree.)
NOTE: For best SWR measurements the antenna should be at least 2
wavelengths away from any object. (For 2-meters this is 4
meters or about 13 feet - 4 yards and one foot.)
Set your radio for lowest power and 146.000 MHz simplex. Test out the
antenna for 144.000 and 148.000 as well. If all three are below 1.7
SWR and the SWR for 146 is about 1.2 or lower, you are done. If not,
see the (Help SWR, frequency, & BW) section of this program.
(shorten the antenna = shorter wavelength = higher MHz)
Once you are done, slip the shrink tubing onto the antenna over the
coax connections, squirt some RTV into the bottom of the shrink tubing,
and then heat up the tubing from the bottom up. This should push
(squeeze) some RTV all the way to the top of the shrink tubing. Wipe
off the excess and hang the antenna for 12 to 24 hours to let the RTV
dry.
NOTES
The SWR at 146.0 should be close to and below 1.3 to 1, for 144.0 and
1484.0 it should be 1.7 to 1 or lower. If you have difficulty
obtaining these results, see the (Help SWR, frequency, & BW) section of
this program.
At 445.0 MHz the antenna should read below 1.5 to 1. I have not
checked it out as completely as I have 2 meters.
Good-luck and let me know of any ideas you have for improvement to this
design.
73, Phil/KE3FL
|
Most of the time we find that the antenna does not tune up on the desired
frequency. This is generally because the wire is not the same as the wire
used for the initial calculations. It may also be that we simply do not
know what the velocity factor (vf) is for the wire we are using. The best
thing to do is guess at the vf, use: .83 or .84. Build the antenna as
specified, then find where it is tuned and use that information to
calculate the vf for your wire. This should work IF your SWR is 1.3:1
or better, but it will be suspect if it is larger. Other factors are
interacting if the SWR is not good. That said,
the equation to used is: vf = 4*stub*f/(factor)
where vf = velocity factor
stub = stub length (in appropriate units)
f = frequency (in MHz)
factor = appropriate numeric factor (11811 or 30000)
|
, my account is:
That's KE3FL but all lower case.