|Build a 17 Meter Reduced Size Coaxial Moxon Rectangle
An exercise with an MFJ 259B
Plans by N0KHQ
"If you have tried the Moxon Generator program to design a Moxon Rectangle, then you are familiar with the Moxon antenna. For those of you who are not familiar with the Moxon, it resembles two letter "U"s with the open ends almost touching but is squared off at the bends. One director and one reflector, each with tails pointing at each other. This antenna can achieve a respectable gain of about 3 to 4dbd and good front to back ratio and is usually made from wire or small diameter tubing. It is usually designed and operated on the higher Hf bands due to physical sizes of Hf antennas on the lower bands.
A standard Moxon rectangle designed for 17 meters would be about 19 3/4 feet wide. Using these plans provided by N0KHQ, the same Moxon rectangle is reduced to about 11 feet wide! Man, that's a big reduction!
John, N0KHQ sent us these plans for a 17 Meter Coax Moxon along with the formulas for using the velocity factor of all Coax to our advantage in reducing antenna size up to 40%! The formulas and instructions provided by him, greatly reduces the size of a normal Moxon and can be used for any Hf antenna to reduce it's physical size!
Have fun" and give thanks to N0KHQ........N4UJW
|17 Meter Reduced Size Moxon Rectangle
|Unfortunately the MoxGen program was written only for normal wire sizes. In order for the program to generate values for different types of coax cable, the author would have to investigate different coax types. It could be done, but would require a lot of work.
Here is a trick I have learned about using coax for antenna elements utilizing an antenna analyzer.
If you have an MFJ 259B or better, plug a coax tee into the top, terminate one side of the tee with a 50 ohm resistor, cut a piece of RG-58/U VF .66 using the standard formula 234/freqmhz for one side of a dipole. You will find after following the instructions below that it is too long and will have to be trimmed.
(for example we will use 18.050). Connect the center conducter to the other side of the tee and the shield to the ground side of the tee, leave the other end of the coax open. You will notice that when the MFJ is adjusted to resonance(50 ohm,X=0 and1:1) it will be somewhere around 17.6 or 17.8. Divide that number by the desired frequency (18.050) you will probably end up with something like .90, take the length of the coax and multiply it by .90, then cut off the valued displayed, check for resonance again....repeat as required until your coax reaches resonance at 18.050. Note, I always leave mine a little long to make connections to dogbone insulators.
I have built a 75m coaxial inverted "V" using the same method. Works great!
The formula for using coax instead of wire for shorter antennas:
984 x .66 = 650/18.050 = 36' / 4 = 9' x 1.2 = 10.8'. Now start testing with the MFJ 259B or the old trial and error method with an SWR meter.
NOW JOHN CONTINUES:
Its a lot of fun, there is no HF antenna that cannot be built with a 30% to 40% reduction in size.
Its a very, very light weight antenna. With this kind of reduction in weight......now we can start stacking them............mmmmm......mm. What a DX antenna that would be.....I can see the gray line already.
Talk to you later.
Thanks for keeping the Sterba on your sight. Hams are building those things like crazy and are totally blown away at the performance. UPDATE.....John says the Coaxial Moxon is out performing the Sturba!
John / N0KHQ / St. Louis
Always on 17M
You can build 'em better than you can buy 'em.
Visit www. Hamuniverse.com - then click on Antennas
|Seal all connections and coax ends from water.|
|If you had rather build the standard size moxon, get out that looong tape measure and click here to leave the site and see the project!
Back to Antenna Lab for more antenna fun!
|This end attaches to center insulator and feed line|
|Use your scroll bars at bottom and right to see full drawing!|
|Graphic intensive.....allow time to load!|
So far the antenna seems to have a great front to back and receives as well as my Sterba.
(See John's Sterba on this site). Further on the air testing yielded results as expected. Works GREAT!
Coaxial Resonators are discussed in the ARRL Antenna Handbook and on Antennex.Com.
Questions....We get Questions!
Note: Questions in Red, Answers in Black
(Some questions edited for clarity and content length)
Is there a quick formula for cutting coax "close enough for Government work" for those who do not have the MFJ 259? Yes!
984 x .66 = 650. 650/freq. = 1 wavelength / 4 = 1/4 wavelength per side of driven element.
This will get you in the ballpark....
but you may have to take off an inch or two.
650/7.250mhz = 89.65 feet / 4 = 22.41 feet (1/4 wavelength per side for driven)
Reflector length = 5% longer than 1/4 wave X 2 = 23.53 feet X 2 (for each side added together) = 47.06 feet total reflector length
Is there a calculation or formula for the tail spacing?
There is no calculation for the distance between the tails. Doesn't make any difference.
The important things to remember :
1. The distance (center) between the Driven Elements
and the reflector (center) must be as shown on the MoxGen Software.
You should download the MoxGen Software from AC6LA and use it to calculate this distance for your particular frequency of operation..
In your case, from the MoxGen program, for 20m you will
want to use 9' spacing.
2. It is only important that the last 30% of the total element
lengths be used for the tail....no more....no less.
3. On driven elements only about the first 15%
contributes to the actual radiation. The length
on the elements sets the frequency.
My antenna design......even though its called Moxon, is a cross between the Hex Beam,
the X-Beam and the Moxon. (Editors note: The ex Hex X Moxon Beam?) HI!
The antenna your going to build will have more forward gain than the standard moxon and better front to back than the Hex Beam and the X-Beam.
Have fun and don't forget to use a good current balun. If you don't, you will have currents on the coax shield and the radiation pattern of the antenna will not be symmetrical (balanced).
Would it be okay to use an HQ-1 connector,
attach the coax and then bring the coax to underneath the hub, coil 8 turns
of coax on a 4" form (plastic soda bottle) to make a current balun?
Yes, just as long as it doesnt change the SWR.
Is a balun needed at the exact feedpoint of the antenna elements to the coax
I was intrigued by your moxon/hex/xbeam design conglomeration. (hexxxon beam)
I was going to scale it up in frequency for 6m use.....possibly adding a director or two.
Why not, this principle can be used for any HF/VHF design.
I saw no reference on the page to account for the dip of the reflector and driven element in toward each other in the center. Is there a guideline or design criteria used to determine how close they get ?
Yes there is. Use the normal spacing as outlined in the ARRL for Yagi style beam construction.
The impression I have is that the E dimension from the "calulator", (MoxGen Program), is used for the center and everything else is moved back to make it a square shape instead of the former rectangle thus reducing further the footprint.
Am I on the right track as long as I adhere to the proper tail dimensions (percentages as described 30% of the 1/4 electrical length).
You dont want the tails to be any more than 30%. The driven to reflector spacing is the important issue, the tail length will be what is left over.
I have built a moxon for 6m using 1/2" copper pipe and copper/cpvc fittings.
I works fairly well but I'd like something a bit lighter and better gain/directivity and this seems to fit the bill. I wonder if RG-58 is going to be too lossy at 50.200 MHz....I may try a second one with RG8x and RG8/u just for grins at some point in time.
I have not built one for 6m yet and probably wont, my Sterba Curtain works just fine on 6m. It really doesn't matter what type of coax you use as long as the VF is .66 and nothing higher.
Could I scale down the yagi-like directors too?
Use the element lengths for director and reflector as shown in the ARRL for Yagi beam styles.
If you are unable to locate a combination mast and element hub, they are available from me for $30 + $5 for packaging and shipping.
Any comments on the Moxon antenna for six meters?
The antenna is going to be so small and light there is no reason why you couldn't have a "REF-DR-DE-DE". Or, build two (2) element antennas and stack them.
I am thinking about building one, and just wanted to check with you for any guidelines.
By the way in one of your formulas you use 1.2, what is this for? ( 984X.66/1850=36/4X1.2)
The 1.2 was to allow 20% more coax so that you wouldn't cut the coax too short. The formula you have above is not correct. 984 x .66 = 650 / design freq. / 4 = 1/4 (90deg.) wave coax cut length. There really isn't any need to add the extra 20%. If I hadn't added the extra 20%, I probably would have gotten emails complaining that the coax was too short.
STAY TUNED FOR MORE QUESTIONS AS WE GET THEM
Email John with your questions here
Since this is the first antenna kit of this kind, I would like to donate it to a ham that would put the antenna up immediately after receipt and furnish Hamuniverse.com and me, N0KHQ, with his thoughts on assembly and operation to be published on Hamuniverse.com on this page.
UPDATE October 18, 2003
K1XT, Bill, has written several articles for CQ Magazine and has the antenna.
His review will follow soon.....stay tuned
As always, it is with great gratitude that I thank my antenna Elmer,
Roger, W1VDE for his guidance, patients and help through out this project.
What a Ham!
Update Dec, 03
Pictured below is the WD7J installation along with his email:
I just completed construction of the coaxial 17M Moxon and it seems to work very well (see attached photo).
I can also use it on 10M with my tuner.
I got the plans off the Hamuniverse website and used half inch plywood, wood dowels and pvc pipe for materials.
I'll give you a shout if I hear you on 17.
John / N0KHQ / St. Louis
EMAIL ME AT N0KHQ@aol.com
Always on 17M
You can build 'em better than you can buy 'em.
Visit www. Hamuniverse.com - then click on Antennas
"Ham Radio - Just for the FUN of it"......N4UJW
Back to Antenna Lab for more fun!
17 Meter Coaxial Moxon design, original idea and plans Copyrighted by N0KHQ Oct 1, 2003
|7 feet from center of driven to center of reflector|
|The Coaxial Modified Moxon Rectangle
An explanation by N0KHQ/St. Louis
Using an MFJ 259B or better, connect a coaxial tee to the analyzer, on one side of the tee install a 50ohm termination resistor, the other side of the tee will be connected to the coax under test.
Example: We will cut a 1/4 wave length piece of coax to resonate at 18.050. When I say resonate, I'm saying that when we are done trimming the coax, the following will be displayed on the MFJ 259B = 49/52ohm, 1:1/0 SWR and X=0 (or real close to it)
Cut a piece of RG-58/U PE VF.66 10' long. At one end of the coax trim back a couple of inches of the PVC jacket, separate the shield from the center conductor, this end will feed into one side of the coaxial tee (center to center and shield to ground) on the MFJ. At the other end of the coax, remove a couple of inches of the PVC jacket slide the shield back, strip 1" of the PE from the center conductor, short the shield to the center conductor.
Turn the tune knob on the MFJ to 18.050, you will notice that the piece of coax is not resonate (too long). Turn the tune knob slowly down in frequency until the display reads 49/52ohm, 1:1 SWR and X=0, you should end up somewhere around 16.200.
Use this formula to determine how much of the 10' piece of coax you will have to trim off. This procedure is covered on page 20 of your MFJ-259B Operator’s Manual.
In this example:16.200/18.050 = .897. Take 10' x .897 = 8.970' this is the length that your 10' piece of coax should be trimmed to resonate at 18.050.
Check the frequency again with the MFJ, repeat trimming as necessary.
When you have resonated your piece of coax, only the center conductor of the driven elements (remove shield) will be connected at the feed point, the other end will remain shorted. How about that, an electrical ¼ wave length with over a 30% reduction in size verses using AWG. You will need 2 of these for the driven element. The reflector element will be .05% longer (lower in frequency), X will = 0 at 17.150 (both ends of coax are open under test). When installing the reflector element short the center to the shield on both ends.
The element spacing between the driven elements and the reflector element can remain at 7’.
When fabricated and tuned at 9’ (almost ¼ wavelength at 17m) the coaxial antenna produced the following readings: (using the MFJ - 259B), (some trimming or lengthening of the driven elements may be required)
18.068 18.130 18.168
SWR= 1.2 SWR= 1.0 SWR= 1.1
R = 59 R = 52 R = 51
X = 6 X = 0 X = 6
The power handling capabilities of this antenna is approximately 500 watts (continuous), this is due to the limitations of RG -58/U PE coax.
In my opinion, the mark of a good antenna system is how well it receives signals. Testing of this antenna at this QTH was done over a period of a few weeks. The receive of the coaxial antenna was compared to the receive of my All Band Sterba Curtain up at 50’. Because the Coaxial Antenna is only up at 9’ the take-off angle is pretty high, as a result the receive is a couple of “S” units down from the Sterba.
The Coaxial Antenna (weight 7 lbs.) should be raised to an operating height of 34’ to 40’ for optimal performance. A take-off angle of about 18 deg. To 15 deg. can be expected.
Copyright N0KHQ 10/03