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E-Z Coax Power Dividers for Two/ Four Antennas - PRINT OUT -




 
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Author Topic: E-Z Coax Power Dividers for Two/ Four Antennas - PRINT OUT -  (Read 4162 times)
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K1JJ
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"Let's go hiking in the woods, Tommy!" - Yaz


« on: March 10, 2005, 08:23:40 PM »

Hola Guys,

Stacking antennas and  phasing them together is not as hard as you may think. Whether they are phased dipoles or Yagis, here's a simple method of feeding for the proper power split and match to 50 ohm line. Four port = feeding four 50 ohm antennas.    Two port = feeding two 50 ohm antennas.

Excuse the caps type. I received this from an older antenna guru who says he does it to read easily...   Cheesy   Print it out and save for future use.

73,
Tom, K1JJ

----------------------------

50.1 MHZ examples:

A FOUR PORT COAX CABLE POWER DIVIDER IS MADE OF TWO PIECES 1/4 WAVE LONG AND WITH A 'T' IN THE MIDDLE AND AT EACH END. THEORY: ON EACH SIDE, YOU BRING TWO 50 OHM LINES TOGETHER IN PARALLEL BRINGING THE IMPEDANCE TO 25 OHMS. 25 OHMS PASSES THROUGH /4 WAVE OF 50 OHM CABLE AND BECOMES 100 OHMS AT THE OTHER END.  THE SAME THING HAPPENS ON THE OTHER SIDE SO AT THE CENTER 'T' YOU HAVE TWO 100 OHM IMPEDANCES COMING TOGETHER IN PARALLEL AND YOU ARE BACK TO 50 OHMS.  THIS SYSTEM USES NO 75 OHM CABLE.  

YOUR OTHER QUESTION WAS HOW LONG THE 50 OHM PHASING LINES SHOULD BE WITH THE ANTENNAS CONFIGURES IN A COLLINEAR FASHION.  THE SIMPLEST WAY TO DO IT IS TO KEEP THE LINES IN 1 WAVELENGTH MULTIPLES.  LIKE THE INNER TWO YAGIS MIGHT BE FEED WITH 2.5 WAVELENGTH CABLES AND THE OUTER YAGIS WOULD THEN BE FED WITH 3.5 , 4.5 OR 5.5 WAVELENGTH SECTIONS OF COAX.

HERE IS YOUR FORMULA.

11802.8/ FREQ IN MHZ = 1 WAVELENGTH IN INCHES IN FREE SPACE. SO 11802.8 / 50.2 = 235.116 INCHES.  FOR 1/4 WAVE YOU DIVIDE THAT NUMBER BY 4. 235.116 / 4 = 58.779".  

WHEN USING COAX YOU MUST MULTIPLY THIS 58.779 TIMES THE VELOCITY FACTOR OF THE COAX.  LETS SAY YOU ARE USING LMR 400 FOR ALL YOUR LINES LIKE I DO. LMR 400 VELOCITY FACTOR IS .85 SO 58.779 X .85 = 49.962".  WHEN WE BUILD THIS 4 PORT DIVIDER, WE PUT AN 'N' CONNECTOR ON  ONE END OF ABOUT 51" OF LMR 400 AND USING A SWEEPER AND SCOPE WE TRIM THE CABLE UNTIL IT IS A 1/2 WAVE RESONANT CABLE AT 50.2 X 2 = 100.4 MHZ.   NOW IN ACTUAL PRACTICE, WE TRIM ABOUT 1 INCH OFF THIS FINAL LENGTH TO COMPENSATE FOR THE LENGTH OF 1/2 OF THE CENTER 'T' CONNECTOR.  THIS SYSTEM IS SEVERAL MHZ WIDE TO THE 1.2:1 POINTS SO IF YOU MISS BY 0.5 MHZ IT WON'T MATTER AT ALL IN THE BIG PICTURE.  THE MOST IMPORTANT THIS IS THAT BOTH PIECES ARE THE SAME ELECTRICAL LENGTH IE RESONANT AT THE SAME FREQUENCY.

USE THE SAME BASIC FORMULA WHEN CUTTING THE PHASING LINES.  1 WAVELENGTH IS 235.116" X .85 = 199.85 INCHES.  PUT THAT NUMBER IN YOUR CALCULATOR MEMORY.  AFTER YOU HAVE  THE MINIMUM LENGTH OF THE CABLE REQUIRED TO REACH FROM THE CENTER POWER DIVIDER TO THE OUTER AND INNER YAGI FEED POINTS, DIVIDE THIS 199.85 INTO THE LENGTH YOU HAVE COME UP WITH. LET'S SAY THE OUTER LENGTH NEEDS TO BE 65 FEET OR 780 INCHES. SO 780 / 199.85 = 3.902 WAVELENGTHS.  THIS TELLS YOU TO MAKE THE CABLE JUST A BIT LONGER OR 4 WAVELENGTHS 4 X 119.85 = 799.39" OR ABOUT 67 FEET TO START BEFORE TRIMMING TO EXACTLY FOUR WAVELENGTHS AT 50.2 MHZ.


YOU CAN USE AN MFJ ANTENNA ANALYZER TO TRIM YOUR CABLES. PUT A 'T' ON THE MFJ. ON ONE SIDE YOU TERMINATE WITH A 50 OHM LOAD.  CONNECT YOUR CABLE WITH ONE CONNECTOR ON THE OTHER SIDE OF THE 'T' AND 'SWEEP' THE MFJ TUNING FOR THE BEST MATCH AT AROUND 50 MHZ.  NOW GO DOWN IN FREQUENCY UNTIL THE VSWR IS 1.5:1 AND NOTE THAT FREQUENCY.  NOW GO UP IN FREQUENCY UNTIL THE VSWR IF 1.5:1 AND NOTE THAT FREQUENCY.  THE MIDPOINT BETWEEN THE TWO 1.5:1 FREQUENCIES IS YOUR CABLES EXACT 1/2 WAVELENGTH MULTIPLE POINT.  

AT EACH 1/2 WAVE MULTIPLE AN OPEN CABLE REFLECTS AN OPEN BACK TO THE 'T' ON THE MFJ SO THE MFJ ONLY SEES THE 50 OHM LOAD. AT ANY OTHER FREQUENCY OTHER THAN A MULTIPLE OF THE FIRST, THE CABLE INTRODUCES EVERY OTHER IMPEDANCE ALL THE WAY UP TO A SHORT WHICH OCCURS ON AN ODD 1/4 WAVELENGTH LONG CABLE.   TO SUMMARIZE, AN OPEN QUARTERWAVE REFLECTS AN RF "SHORT" BACK TO THE 'T' ON THE MFJ AND AN OPEN HALF WAVE REFLECTS THAT SAME OPEN BACK AT THE 'T'.  THE MFJ DOES NOT LIKE TO 'SEE' A SHORT, SO WE GO TO TWICE THE FREQUENCY WHERE THAT CABLE IS NOW 1/2 WAVELENGTH LONG AND IT NOW REFLECTS AN "OPEN" AT THE 'T' AS IF IT WERE NOT EVEN THERE AND THE MFJ JUST 'SEES' THE 50 OHM LOAD AGAIN.


PHASING TWO 50 OHM ANTENNAS TOGETHER IS DONE THE SAME WAY AS BEFORE BUT YOU MUST USE (2) !/4 WAVES OF 75 OHM CABLE LIKE RG-11 ON EITHER SIDE OF THE CENTER 'T'. BARRELS GO AT THE OUTER END OF THE 1/4 WAVE SECTIONS TO COUPLE TO THE TWO EQUAL LENGTH 50 OHM PHASE LINES.  

I SHOULD MENTION THAT YOU DO NOT NEED TO BE IN 1 WAVELENGTH MULTIPLES FOR THE PHASING LINES IN THIS SETUP; JUST TWO EQUAL LENGTHS.  

BACK ON THE 4 PORT SYSTEM WE DISCUSSED YESTERDAY, IF YOU HAD A CLASSIC CONFIGURATION, WHERE 4 EQUAL LENGTH PHASING LINES COULD BE USED, YOU WOULD NOT HAVE TO MAKE THEM ANY PARTICULAR WAVELENGTH MULTIPLE. YOU COULD JUST CUT THEM ALL EQUAL AND BE DONE WITH IT.  WE LIKE TO DO THINGS AT 1/2 AND ONE WAVELENGTH MULTIPLES, SO WE CAN BE SURE THEY ARE ALL "ELECTRICLLY " THE SAME LENGTH.  ANOTHER SMALL ADVANTAGE OF USING 1/2 AND 1 WAVELENGTH MULTIPLES IS THAT THE TRUE IMPEDANCE OF THE ANTENNA FEEDPOINT IS MIRRORED AT EACH 1/2 WAVE DOWN THE LINE SO WHEN YOU LOOK UP EACH INDIVIDUAL PHASING LINE YOU SEE THE REAL ANTENNA EXCEPT FOR SOME SLIGHT IMPROVEMENT DUE TO FEEDLINE LOSS.  IN A GOOD QUALITY PHASING LINE THIS EFFECT IS IS MINIMAL.
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Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
Steve - WB3HUZ
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« Reply #1 on: March 10, 2005, 08:49:43 PM »

Good stuff Tom. Thanks for posting. You might want to have that guy check out the Universal Access (I think that's what it's called) preference panel. There are options in there for "magnifying" otherwise normal text so those who are sight impaired can more easily read and won't have to resort to ALL CAPS.

For those who don't like reading ALL CAPS, here is a reformatted version.


 A four port coax cable power divider is made of two pieces 1/4 wave long and with a 'T' in the middle and at each end. Theory: on each side, you bring two 50 ohm lines together in parallel bringing the impedance to 25 ohms. 25 ohms passes through /4 wave of 50 ohm cable and becomes 100 ohms at the other end. The same thing happens on the other side so at the center 't' you have two 100 ohm impedances coming together in parallel and you are back to 50 ohms. This system uses no 75 ohm cable.

 Your other question was how long the 50 ohm phasing lines should be with the antennas configures in a collinear fashion. The simplest way to do it is to keep the lines in 1 wavelength multiples. Like the inner two Yagis might be feed with 2.5 wavelength cables and the outer Yagis would then be fed with 3.5 , 4.5 or 5.5 wavelength sections of coax.

Here is your formula.

 11802.8/ freq in MHz = 1 wavelength in inches in free space. So 11802.8 / 50.2 = 235.116 inches. For 1/4 wave you divide that number by 4. 235.116 / 4 = 58.779".

 When using coax you must multiply this 58.779 times the velocity factor of the coax. Letís say you are using LMR 400 for all your lines like I do. LMR 400 velocity factor is 0.85 so 58.779 x 0.85 = 49.962". When we build this 4 port divider, we put an 'N' connector on one end of about 51" of LMR 400 and using a sweeper and scope we trim the cable until it is a 1/2 wave resonant cable at 50.2 x 2 = 100.4 MHz. Now in actual practice, we trim about 1 inch off this final length to compensate for the length of 1/2 of the center 'T' connector. This system is several MHz wide to the 1.2:1 points so if you miss by 0.5 MHz it won't matter at all in the big picture. The most important thing is that both pieces are the same electrical length ie resonant at the same frequency.

 Use the same basic formula when cutting the phasing lines. 1 wavelength is 235.116" x 0.85 = 199.85 inches. Put that number in your calculator memory. After you have the minimum length of the cable required to reach from the center power divider to the outer and inner Yagi feed points, divide this 199.85 into the length you have come up with. Let's say the outer length needs to be 65 feet or 780 inches. So 780 / 199.85 = 3.902 wavelengths. This tells you to make the cable just a bit longer or 4 wavelengths 4 x 119.85 = 799.39" or about 67 feet to start before trimming to exactly four wavelengths at 50.2 MHz.

 You can use an MFJ antenna analyzer to trim your cables. Put a 'T' on the MFJ. On one side you terminate with a 50 ohm load. Connect your cable with one connector on the other side of the 'T' and 'sweep' the MFJ tuning for the best match at around 50 MHz. Now go down in frequency until the VSWR is 1.5:1 and note that frequency. Now go up in frequency until the VSWR if 1.5:1 and note that frequency. The midpoint between the two 1.5:1 frequencies is your cableís exact 1/2 wavelength multiple point.

 At each 1/2 wave multiple an open cable reflects an open back to the 'T' on the MFJ so the MFJ only sees the 50 ohm load. At any other frequency other than a multiple of the first, the cable introduces every other impedance all the way up to a short which occurs on an odd 1/4 wavelength long cable. To summarize, an open quarter wave reflects an RF" short" back to the 'T' on the MFJ and an open half wave reflects that same open back at the 'T'. The MFJ does not like to 'see' a short, so we go to twice the frequency where that cable is now 1/2 wavelength long and it now reflects an "open" at the 'T' as if it were not even there and the MFJ just 'sees' the 50 ohm load again.


 Phasing two 50 ohm antennas together is done the same way as before but you must use (2) 1/4 waves of 75 ohm cable like RG-11 on either side of the center 'Tí. Barrels go at the outer end of the 1/4 wave sections to couple to the two equal length 50 ohm phase lines.

 I should mention that you do not need to be in 1 wavelength multiples for the phasing lines in this setup, just two equal lengths.

 Back on the 4 port system we discussed yesterday, if you had a classic configuration, where 4 equal length phasing lines could be used, you would not have to make them any particular wavelength multiple. You could just cut them all equal and be done with it. We like to do things at 1/2 and one wavelength multiples, so we can be sure they are all "electrically " the same length. Another small advantage of using 1/2 and 1 wavelength multiples is that the true impedance of the antenna feedpoint is mirrored at each 1/2 wave down the line so when you look up each individual phasing line you see the real antenna except for some slight improvement due to feedline loss. In a good quality phasing line this effect is is minimal.
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K1JJ
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"Let's go hiking in the woods, Tommy!" - Yaz


« Reply #2 on: March 10, 2005, 09:59:26 PM »

TNX, Steve.

Yes, much easier to read. I will probably use it as an article to post in the East Coast Sound area.  There is good info about testing the coax lengths here with the MFJ-259B  that I've not seen elsewhere.

G, you can use the 2 port divider for your new pair of homebrew 6M Yagis.  Huz, when you get up your four Moonrakers on the roof, the four porter will do the trick, caw mawn.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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