Phased Stacked Halos
|
|
Phased Stacked Halos
|
|
|
After having great luck in building and using my single halo loop as built and described in my How To section (clicking on the 6M Halo Ant button on the left), I thought of just trying to see how easy it would be to have an phased stacked pair of halos. As I don't have the facilities to put up a 6M beam yet, a phased array which increased gain omni-directionally seemed a project of which I would like to experiment. If in three weeks the single halo has worked 22 states with a 100 watts, maybe more gain in my case may be better. This meant however that a phasing harness consisting of certain lengths of 50 ohm and 75 ohm coax for each antenna would have to be calculated and cut so that equal amounts of power are "forced" to each antenna. By doing so, some further gain is obtained but at this point I have not been able the measure this increase as I don't have the software to measure this. At this point I am going to assume that you have NOT made a 6M halo yet. In this case click on the 6M Halo Antenna button on the upper left. Make two of them. That page has directions, parts, photos, everything you need to build an inexpensive halo that works well from NAPA brake line. When you have completed the two antennas, return to this page for further directions, calculations, parts, and photos to make a phased harness for the two antennas and putting it all together. ___________________________________________________________________________________ At this point we need to obtain wavelengths based on 6M. I have done so
in the 50.100 band area. How Far Apart Should the Antennas Be? 5/8 wave stacking spacing is good for gain (It does NOT use the same formula as described under the next heading). 5/8 wave distance = about 12 feet at 6M (5/8)*(c/50MHZ) in Google), so I went with it. If you get them too far apart you'll get high angle lobes. I wanted low angle lobes as my antennas located on of the highest hills in Central MA. 6 feet will work if you can get the halos at EXACTLY the same impedance in the presence of each other and the mast, but the odd 1/4 wave sections will force equal currents in the two halos despite the antennas different impedances, (hence my choice of a 5/8 wavelength, a no-brainer). How Do I Calculate the Length of the Cables I Need? To find the needed wavelengths of 6M for our cables, we need
to use one or both of these formulas: IF .66 is the velocity
factor of your coax, it MUST be included
in your calculations for the correct length of cable needed in your 50 and
75 ohm cables. Plug in this formula in Google but add "in inches" after a
space after the .66 and you will get the 1/4 wavelength in inches. However
you will receive an answer in decimal form and you need to convert the
decimal measurement into actual inches. If you get and answer like 10.2
feet, not is not the same as 10 feet, two inches. You must multiply 12 by
the .2 to arrive the actual inches to make the correct cable lengths.
To figure out a 5/8 6M wavelength for your cables, just substitute the 1/4
in the formula with 5/8. The Velocity Factor: Why is it important in this project? Most coax cables have either a plastic or a foam
insulation around the center conductor. If it has a plastic insulation the
velocity factor is .66. If white foam, it is .85 That's it....a given with a
very small +/-. These figures MUST be considered in your calculations for
the correction length of cable sections for your phasing harness. This will
also help the rig see 50 ohms on your transmission cables to the antennas
and have the whole phased array work for you.
Velocity factor for cable lengths at 6M
OK, if you have read this far into this page, you still must be interested, so the simple stuff now begins. I will detail my in-phase array of the two 6M halos as I have built them. Here are the calculations I used to complete the project. For reasons I stated above, I picked the 5/8 wavelength as the distance apart I have to have my halos on the mast. This distance calculates to 12 feet, (5 / 8) * (c / (50 MHz)) = 12.2946382 feet) so just 12 feet even is good enough for me. Cables: You will need one 1/2 wave RG8 50 ohm and one 1/4 wave RG11 75 ohm. I had 50 ohm foam insulated cable and 75 ohm plastic insulated cable available. Therefore I needed one 50 ohm foam cable at 89" which is 7.4' or 7' 5'' and a 1/4 wave 75 ohm plastic insulated 75 ohm cable was 38.9" or 3' 3" close enough. (If I had plastic 50 ohm cable then I would only have to have had 77.9" or 6' 5"). You need to make two sets of each of these; the one 50 ohm 1/2 wave and the one 75 ohm 1/4 wave which compromise one side of the harnesses for one halo for a total of 10' 8". Parts needed: TX cable; sections of RG8U (50 ohm) and RG11 (75 ohm) HRO does NOT carry any 75 ohm cable but I managed to find a source from the Wireman website, http://thewireman.com/index.shtml who shipped it up promptly. 3 Barrel connectors for PL-259's (female to female) 1 PL-259 T connector 8 Amphenol Connex PL-259 connectors from HRO, part #182120. These are the high quality PL-259 connectors & worth the money. Good quality U-V and WX-proof electrical tape that actually sticks when the end of the tape is applied and doesn't curl up. (Again Radio Shack but I don't have the part number) WX-proof clear silicone from Radio Shack (P/N 64-2314b) The Hard Part: Connecting the PL-259's: This part of the project is tedious and but very important you get it done right but first you have to cut the cable to the proper length. First lay out your cable in the straightest line possible and measure your particular cable. I have found a little cutting device at a flea years ago that works very well but don't know where to get one now. Below is a photo from the ARRL Handbook that shows you how much of the center conductor should show and how much outer braid should be exposed.
After cutting clean cuts on your cables, be sure to flux the center conductor and outer braid and apply solder to tin them. Tinning the outer braid is especially important as the PL-259 can "bite" in the tinned braid and have a nice solid connection. as seen below. Don't forget to place the base screw onto the coax first before you soldier the center connection.
Here you can see that I have soldered the small holes to the braid and the center pin to the center conductor. Be careful in applying heat so that the plastic piece that surrounds the center pin does not melt. After you have soldered one end, be sure to take an ohm meter and make certain that the pin and the outer casing do not short. The basic cause of a short like this can be caused by just one loose strand of outer braid to the pin. A Warning About RG11 Center Conductor: The center conductor of this cable is VERY fragile as it's individual wire strand size and number of strands, being much smaller than in RG8, be very careful when stripping off the center insulation. One hard cut too many and three to four strands will be cut leaving you with just a couple of tiny strands making this piece of the cable useless. Take your time and pretend you are a surgeon. This can not be a rush job!
Here is the final product...the harness with each half of the harness attached to the center T. The shorter RG11 cable goes onto each side of the T first then with a barrel connector connect the longer RG8 50 ohm cable whose other end will attach to the antenna.
The next step is simply finding the center 6 FT mark of your mast. Secure the center T connector to it by using the black electrical tape. Attach from the connector to the. You will have more than 6 FT of cable and that is best taken care of by coiling under the antenna and tape it off. Do the same to the other antenna. The Job Is Done!
Here is the final photo (close-up) of the project. Below the top halo is my 2/440 yagi then the mast guy lines. 12 feet down is halo #2. Notice the coil of RG8 under each halo. If you look closely, you'll see the T connection near the center of the mast weathered proofed in electrical tape and clear rubber silicone to protect ALL connections from the weather. The top antenna is the same antenna I built first and shown on the single 6M halo link The cable you see in the foreground is my twin lead to the G5RV. Because it is so new and didn't have a chance to build it before the VHF contest because of the massive amounts of heavy rain we had up here, I have only managed to make one contact into ME where he worked me off the BACK of his beam giving me an S1 but it worked! That's state #23 on this type of antenna and I suspect (really hope) that when conditions are good again, I'll have even better contacts to brag about which will be updated here. If you have any questions on the project, please feel free to e-mail at NG1I@arrl.net. I would appreciate hearing from anyone reading this page. Preliminary Results: Being very busy during the summer, I haven't got around to ham radio for awhile until 7/12/06 when I happened to see France and Italy spotted on the DX cluster. With a 100W and this stacked array I was able to work IK5 and F8 on SSB on a double hop and FG on CW. Maybe I'll stay with this antenna arrangement awhile. Next on the Distant Horizon: Stacking a yagi and a halo in a non-delayed phased array having a lobe looking like an old key lock which allows local and long distance DX at the same time. Stayed tuned and film at 11! Hope to work you on 6M. GL, Gud DX, and 73
Frank
|
This site was last updated
03/08/08
