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Author Topic: Help: Need the Formula for spacing verticals one qtr wave apart  (Read 2822 times)
W3HKK
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« on: March 14, 2011, 12:56:53 PM »

Do you think I can find this anywhere? Nope. 

I'm looking for the formula to calculate the spacing for a pair of phased verticals the proverbial one qtr wave apart.

234/f is I think only for cutting bare wire to  a qtr wavelength. 

So for  calculating the spacing between two qtr wavelength verticals what  should be used?
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KA5N
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« Reply #1 on: March 14, 2011, 01:42:52 PM »

300,000,000 meters/second divided by frequency in Hertz (example 7,000,000 hz is 7 MHZ) multiply result by 0.25 convert meters (result) to measuring system you are using and write the answer.  If you return to first principles you don't have to memorize formulas.
Of course 234/ f is probably close enough.
Allen
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W0BTU
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« Reply #2 on: March 14, 2011, 02:01:07 PM »

246/f. You're spacing antennas, not making one.
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W3HKK
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« Reply #3 on: March 14, 2011, 05:12:31 PM »

Thanks!  Youd think I could find that in the ARRL Handbook, the ARRl Antenna Book, and both on ON4UN's Low Band DXing books.....
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N4JTE
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« Reply #4 on: March 14, 2011, 08:08:14 PM »

Check my eham artical if you plan on phasing, btw dont think you mentioned the band but 1/4wl is half of a dipole length and with verticals, spacing on 40 would be around 33 ft. not critical as far as a foot or so of seperation. More info you probaly don't want but a force fed 1/4 wl feed and a 3/4 wl fed with a tee connector will get you a 180 degree phased unidirectional gain about 3db over single vertical.
My artical shows proper phasing for 90 degree cardoid reversable pattern as per ON4UN calculations.
Bob
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N4JTE
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« Reply #5 on: March 14, 2011, 08:27:36 PM »

Just read a rather ponderous thread on qrz forum with your good questions, surprised your asking about spacing 4 months later, what exactly are you questioning here? Your spacing was perfect then.
Bob
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K4RVN
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« Reply #6 on: March 14, 2011, 09:50:30 PM »

Actually it is 246/F x velocity factor of the coax you are using to be exact for formula. Probably would not know the difference if you used 246/F.
If you use 234, the normal 1/4 wavelength, the spacing will be too close from what I have read.
It still might work OK.
Frank
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WX7G
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« Reply #7 on: March 15, 2011, 03:00:24 AM »

The spacing is not critical (at all). One wavelength in free space is found by this formula: L = 984/F, F in MHz, L in feet. One quarter of this is 246/F.
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RFRY
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« Reply #8 on: March 15, 2011, 03:48:57 AM »

Actually it is 246/F x velocity factor of the coax you are using to be exact for formula. Probably would not know the difference if you used 246/F.

To be exact in implementation, the verticals should be located 1/4-wave apart in free-space wavelength, so that their radiated fields will combine in space with the expected, net radiation pattern and directivity.

For an example with the quoted approach, using coax cable transmission line with a velocity of propagation of 0.66c, where the cable length to one vertical was twice the length to the other vertical would mean that the two verticals would be closer together than a 1/4 of a free-space wavelength.  But that would not provide the expected performance from the array. Free versions of NEC such as EZNEC will show the results for both approaches.

If both verticals need to be driven with equal r-f phase and equal power then it would be preferable to install them 1/4-wave apart in free-space wavelength and use equal lengths of identical coax cable between a 50/50 power divider and the feedpoint of both verticals.

RF http://rfry.org
« Last Edit: March 15, 2011, 05:34:30 AM by RFRY » Logged
W3HKK
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« Reply #9 on: March 15, 2011, 07:25:05 AM »

Thanks Bob.    I  somehow wound up with about 30 ft spacing ( which I am about to change to 34.4'.)  The resultant phased verticals worked great!......  out to about 2000 miles.  Huge signal reports in the NE and SW directions.  F/B ratios of at least 3 S units and  often 6-7 S units, from stations all over the US.  Usually 3 S units for DX.  Seems like my coax phasing lines are cut about right. But  sigs into Europe and VK/ZL  were down from when I ran a single vert.

IE  my vertical seems to work TOO WELL at higher angles and not as well at lower angles.

So after adding  ferrite beads to the coaxes, adding radials (now  up to 12 each, on the ground), I wanted to get the radiator spacing right to see if that would improve my low angle radiation. 

One other factor.  One radiator is about 8 ft from a 40 x 60 x 3.5 ft high wire garden fence.  Not sure how that factors in, or if its distorting my radiation pattern to a higher angle.  From what Ive read, this appears to be a greater factor on  vertical pairs than on a single stand alone vertical.

Bob
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W4VR
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« Reply #10 on: March 15, 2011, 07:38:37 AM »

Always use free space formula for spacing and include velocity factor when calculating phasing line length.
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K4RVN
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« Reply #11 on: March 15, 2011, 11:31:52 AM »

To W3Hkk

If you had the spacing at about 30 ft using RG8x coax, it was close as the velocity factor is .84 . Changing it to a greater number for spacing is going the wrong direction. If you use RG8, then you need to shorten the spacing to be correct by formula as mentioned by me and also W4VR. The velocity factor for RG8 is .66 which requires a closer spacing by formula.

K4RVN
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N4JTE
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« Reply #12 on: March 15, 2011, 11:59:13 AM »

HKK, Bob glad it's working well, couple of points to ponder as I have just put the same antenna back up for some experiments.
At my freq of choice 7.185, I ended up with about 34ft of spacing, as stated as long as you are within a foot or so your good to go. Never use a commercial 1 to 1 balun, throws off the quadrature matching even when in the circuit when cutting phase lines, that took days of contacts to figure out , the ferrite beads are the way to go for eliminating problems. I have experimented with 1,2 and 3 raised radials at 6ft above ground, limited by backyard dimentions. The third radial did nothing for saving ground losses but did end up giving me a more high angle take off in the direction it was pointing. The two radial system yeilded a Z of 50 ohms, nice match but loosing 25% of power in the ground. Gonna improve ground screen with a bunch of aluminum fence wire asap, looking for 30 ohms which would be equivilent to 100 buried radails, so work in progress. I also have at least an 18db of front to back, only good thing about FB is the help in measuring !
Keep on building !
Bob
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RFRY
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« Reply #13 on: March 15, 2011, 01:37:16 PM »

Below is a link to a NEC analysis of the h-plane azimuth pattern of an array of two 1/4-wave verticals spaced 1/4 of a free-space wavelength apart, and driven with equal power, 90 degrees out of phase.  The direction of the major lobe is reversible by changing the electrical phase between the verticals from -90 degrees to +90 degrees.

The peak directivity of the array is approximately 3 dB greater than that of either monopole, alone.

http://i62.photobucket.com/albums/h85/rfry-100/40m_Array.gif
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N4JTE
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« Reply #14 on: March 15, 2011, 02:21:19 PM »

RFRY, just for your info ON4UN did some precise calculations and found that 84 degree phase feed lines and an 71 degree lag line are much more accurate than the basic approach of 90 degrees .He calculated the exact point on the lines where the voltage are equal, makes it work as designed.
Bob
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