Indoor dual-band design

[va3css]

Many thanks to Tony, va3atd, who gave me permission to post his design. This is his writeup that got published in the LARC (London Amateur Radio Club) newsletter a while back.

I'll have to post this in two parts, due to size.

Indoor 2m & 440 J-poles

By VA3-ATD, Anthony Duffy

Before I begin I’ll put a little background into it. I was looking for antennas for a dual band radio with out the need for a duplexer. Since the antennas were to be used indoors they needed to have a little gain and small foot print to be mounted near a window (and I sure didn’t complain about the wide bandwidth). Also being mounted indoors the antennas couldn’t be sensitive to the effects of near by objects. After looking at several types of antennas I decided that a j-pole antenna was best suited to my needs. I searched for plans for the antennas from several different sources - books, internet, news groups etc - but all I managed to find was plans designed around 300 or 400ohm twin-lead. This was fine and dandy if you wanted to hang the antenna in the window; but the effects of the steel in the window frame had a significant effect on the antenna (especially on 2m when I tried them) and the designs did not allow for adjustment of where the coax connected to the 1/4 wave stub for tuning. I also preferred to have the design free standing to some extent for easier mounting or at least more versatile mounting which brings us to the reason for this article.
A j-pole is simply a half wave radiator with a quarter wave tuning stub so the overall height of the antenna is roughly 3/4 of a wave length. The design of the tuning stub gives it the gain and large bandwidth. The materials I used to make the antenna are easily found and some of them may all ready be in your junk drawer. The design of the mount can be changed if you want to have a free standing model of the antennas. If so get more of the 1/2 x 3/4” wood strip and material for a suitable base - my design was for clamping to a narrow shelving unit near the window. Also the more advanced builder can experiment with the width of the tuning stub to increase the band width of the 2m section. Please note that you don’t have to build both antennas if only one is required - I just happened to need both for a duel band radio.


Here is a list of materials:

Item Qty.

8 ga. Terminal Block 2
(RS # 274-679 but check the local surplus store you could save around 75% - Also only 1 block is required for a single antenna)

8 ga. Aluminum. wire 6.5’ for 2m
(RS # 15-035) 2.2’ for 440

RG -8U coax (95% braid) 10’/ antenna
(use 1 length if building 1 antenna - also longer lengths and different types can be used although I do recommend thin coax)

PL-239 Connector 1/ antenna

#6 or #8 Spade Tonge 2/ antenna
Connectors

1/8” dowel 4”

1/2 x 3/4 wood strip 10”
(acquire more length if a free standing model is to be made)

Wood Glue

[va3css]

Construction

Cut a section of 3 connectors from one terminal block. Using a small screw driver remove the two screws from the center connector and push the copper tube out of the block. With the rest of the terminal block starting from the left side remove all but the 1st, 3rd, 7th and 9th connectors. If you are only going to make one antenna then cut two 3 connector strips and remove the center connector from each one. Trim the access plastic from the bottom of the terminal block that will be used for the coax connections leaving the centre one unchanged.
Measure off the appropriate length of wire as specified in the material list and straighten as best as possible.
Bend the wire to form a “J” with a width of 5/8” between the two sides. The short side of the “J” should be 6-1/2” for 440 and 19-1/4” for 2m to start. You may find it useful to use the
terminal block as a guide when straightening and forming the wire.

Next trim one side off of the connectors. This will allow the connector to be clamped to the wire using the lower terminal block screws.
Slide the lower terminal block onto the wire making sure the trimmed side is facing away from the bottom of the “J” section. Next place the upper mounting block on the wire and locate it so the top of the block is 1/2” below the top of the “J” section and tighten the screws to hold it in place.
Using a “flat file”; file across both sides of the “J” section starting 2” from the bottom on the 2m element and 1-1/2” from the bottom of the 440 element. File for a section of about 2-1/2” for 2m and 1-1/2” for 440. (What I did to make things easier was to place the lower terminal block just below the area to be filed and lightly tightened the closest screws. I then hooked it over the edge of a cutting board to help hold the wire steady while filling). Periodically use one of the connectors and see if it will fit in the copper tube of the terminal block sitting on top of the filed area of the wire (you’ll find that the diameter of the wire is roughly half to fit the connector).
Once you have finished filing remove the terminal blocks and turn the lower block so that the trimmed section is facing the bottom of the “J” section (make sure the screws are over top of the filed section.
Using a 1/8” diameter drill; drill through the centre segment of the upper terminal block using the empty screw holes as a guide.
For both the clamping style and free standing mounts the next section applies.
Cut a section 4” long from the 1/2 x 3/4” wood strip. Next using the 1/8” drill and terminal block drill two holes in one end of the wood using the terminal block as your guide. (you may find it better to drill the first hole and use a piece of the 1/8” doweling to help hold the block in place while the second one is being drilled). Next take the 1/8” doweling and push it through the terminal block until it bottoms out in the hole. Using side cutters cut the dowel
flush with the terminal block. Repeat for
the second hole. Remove the terminal block and wood doweling from the end of wood strip making note of which piece of doweling came from which hole. Place a liberal amount of wood glue in the holes in the wood strip and push the doweling back into the holes. Wipe the excess glue from around the doweling and allow to dry.
Next cut a 1/2 x 3/4” deep slot in the end of the remaining piece of wood strip taking care not to split the wood in the process. Coat the inside of the slot with wood glue and place the 4” piece of wood in the slot locating the end with doweling 2” from the side. Square up the wood joint and clamp to dry over night.
Using the 1/8” drill; drill through the side of the wood joint and glue in a piece of 1/8” wood dowel to give it more strength.
At this point you will have to have decided which style of mount is most suited to you particular situation. For my self I decided the mount would be clamped to the side of a 5’ tall shelving unit since the overall length of the 2m element was about 5’ long and didn’t fit in the window. If you are building a single antenna as a free standing model then drill a hole large enough for the coax at a comfortable level up from the base. If you are building both antennas or just one with the clamp style of mount then drill a hole(s) about 4 -5” down from the horizontal wood strip with the 1/8” dowel pins.
Next prepare the coax by stripping away about an inch of outer insulation form one end of the coax. Using a fine screw driver separate the braid slightly then fold the coax over while working the center conductor through the braid. Strip about 1/4” of insulation from the center conductor. Next take the modified connectors and attach to the braid and center conductor. Last but not least install the PL-239 connectors on the end of the coax.
Clamp or stand the mounting system in the area where the antenna(s) are to be used. Feed the coax through the holes drilled in the vertical piece of wood strip. Take the terminal block used for mounting the antenna(s) and feed the antenna(s) into the connectors so that the short side of the “J” is 1/2” above the top of the block. (If both antennas are being used I would suggest placing the 440 one towards the window since the 2m antenna may cause a slight null in the omni directional pattern of the 440 antenna)
Next place the mounting block on the two 1/8” dowels of the mount.
Slip the connectors under the screws of the connector block and lightly tighten.

Tuning

I have included a diagram and dimensions for the antennas that I built as a starting point/ guide. I did notice that the band width of the 2m antenna was less than half of the band width of the 440 (which was about 10 mHz). Experimenters may want to try increasing the width of the “J” section to have 2 empty slots in the mounting block between sides of the “J” section. When I get the time I’ll try this myself and report it in another addition of the news letter.



Connect an SWR between your radio and the antenna or if your lucky enough to own an SWR analyzer connect that to the antenna. (If both antenna(s) are being used than tune the 440 antenna first.)
Check the SWR at ether end of the band and note any difference. If the SWR is higher at the top of the band then move the connector block up slightly until the SWR is even at both the top & bottom of the band.
Next check the SWR in the centre of the band. With this antenna the SWR should be about 1:1 when tuned. If the SWR is high then clip about 1/8” from the top of the short side of the “J” section. Recheck the SWR at the top & bottom of the band moving the connector block as required to balance both ends of the band.
Repeat the pruning as required until the SWR at the center of the band is as low as possible for your particular location.
Once the first antenna is tuned then repeat the process for the second antenna if used.

Final Notes

In some cases after tuning the antenna as best as possible without getting the match flat you may be able to put a slight spread in the “J” section to compensate for the alignment of the wire in this section then readjust the position of the connector block.
I’m quite impressed at the performance of the antennas; not from a transmission point of view since being 60’ up in the high side of town I could use a wet noodle and still get out. But because at how well the antennas match and have such a wide bandwidth compared to some other designs I’ve tried. The 440 model covers 440 - 450 with a match of 1.7 :1 at the band edges and flat at 445 mHz.
Other interesting traits of the antennas is that they are mounted 1-1/4” apart with out interfering with each other. Also the SWR isn’t effected by objects in the vacinity which is great when used indoors. This also allows the antennas to be moved to other locations without any serious changes in the SWR.

73
VA3-ATD
(Anthony Duffy)