Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: Help: Need the Formula for spacing verticals one qtr wave apart  (Read 2865 times)
W3HKK
Member

Posts: 593




Ignore
« Reply #15 on: March 15, 2011, 02:32:02 PM »

JTE:  I have used $10 telescoping 10-25 ft fishing pole verticals with 4 elevated qtr wave radials on 40-30-20-17-15-12 and 10m
at two different qth's fpr tje [ast 8 years.  Results  were always very good, working  most anybody on a particular band.  These were my stealth antennas at the last qth, and my   present antennas here on 3 acres in th ecountry. ( No tower yet and no tall trees.)

I never found the 4 radials wanting, altho I never did any performance testing.  SWRs were usually around 1.5 to 1 or less.

My 40m  vertical  originally had 4 radials 2-3 ft off the ground and worked the world easily, even thru 40 m ssb pile ups at times. to ZS, VK6 KH6 and KL7.

But once I put up the phased array the long haul performance dropped while the domestic performace was outstanding.  The source of  my higher than expected angle of radiation is still uncertain altho several  guys have blamed my radial pattern and lack of more radials.  Apparently  what you can get away with wih a single vert you cant with a phased array of two. 


Once spring temps get to Ohio, I will resume the quest!
Logged
RFRY
Member

Posts: 236


WWW

Ignore
« Reply #16 on: March 15, 2011, 03:23:25 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.

Interesting, but would the accuracy of his calculations exceed those of NEC?  And if so, what optimal directivity did he determine using his approach to analyze this array?
Logged
W3HKK
Member

Posts: 593




Ignore
« Reply #17 on: March 15, 2011, 04:52:36 PM »

71 and 84 degrees are the lengths I used.  However, cutting them accurately  was a bit difficult since  measuring the  qtr wave frequency was via  a very slow dip and rise.  Using  new RG213 as ON4UN reco'd.

But the F/B results  look very good so I must be pretty close.  Just need to lower the radiation angles.  In effect, for DX,  the single vert works better than the phased array the way its currently configured.
Logged
N4JTE
Member

Posts: 1154




Ignore
« Reply #18 on: March 15, 2011, 04:57:21 PM »

RFRY my answer is this, build one with 90 degree phase lines and then build one to ON4UN phase angles, the differance will be obvious. NEC assumes unknown ground and a voltage match at both antennas, usually not my experience in the backyard.
I offer this as someone who has built them both ways along with many variations including 135 degrees, force fed 1/4/ 3/4 and many other variables, for the 90 degree cardoid pattern/ reversable, the Christman method is the best.
Regards,
Bob
Logged
RFRY
Member

Posts: 236


WWW

Ignore
« Reply #19 on: March 15, 2011, 05:19:33 PM »

RFRY my answer is this, build one with 90 degree phase lines and then build one to ON4UN phase angles, the differance will be obvious. NEC assumes unknown ground and a voltage match at both antennas, usually not my experience in the backyard.

My NEC model did not assume an unknown ground, and it was driven with equal currents -- as shown in my link to that NEC calculation.

Would you please post the specific advantage in the directivity of this 2-element array that you and/or ON4UN claim, compared to my NEC analysis?
Logged
N4JTE
Member

Posts: 1154




Ignore
« Reply #20 on: March 15, 2011, 06:00:34 PM »

RFRY, no problem, build one in your backyard and we can converse further.
Bob
Logged
W6RMK
Member

Posts: 649




Ignore
« Reply #21 on: March 16, 2011, 12:05:56 PM »

I think you're getting wrapped around the axle, as it were, because you're confusing the phasing of the EM waves and the lengths of the coax to do the phasing.

ON4UN's calculations are correct for the Christman phasing technique (which takes into account the mutual Z of the two antennas and the phase shift in the coax with non-50ohm termination)... you basically set the two element currents (90 degrees apart, equal magnitude), then calculate what length coax you need on both of them to get the voltages equal at the end of the coaxes. At that point, you use a simple coax T.  I'll note that the Z at that T is probably not 50 ohms.

The real burning question is what is the actual feedpoint Z matrix... NEC will happily calculate it, but my experience has been that for ground mounted verticals, it's never right (and it's not just a matter of adjusting soil parameters).  Is it "significantly different"?  Not really, at least in terms of the pattern "degradation" for a two elements 1/4 wavelength apart sort of thing. 

I suppose the other way to attack the feed problem is to use 1/4 wavelength "current forcing" lines and a lumped 90 degree phase shift network.  ON4UN describes that approach too.


The Christman feed is great for things like a pair of UHF or VHF dipoles (because their self Z and mutual Z is going to closer to the ideal free space vallues).  It might also be useful for a pair of 10m horizontal dipoles spaced 1/4 wave apart.

Here's a page with a link to an Excel spreadsheet and more info
http://home.earthlink.net/~w6rmk/antenna/phased/christman.htm
Logged
WX7G
Member

Posts: 5922




Ignore
« Reply #22 on: March 16, 2011, 12:47:22 PM »

The two phased verticals should provide a gain of about 3 dB or half an S-unit. The elevation pattern should be the same as a single vertical.
Logged
W6RMK
Member

Posts: 649




Ignore
« Reply #23 on: March 16, 2011, 01:14:11 PM »

I find that the fact that two phased antennas has a null (or two) is actually more useful than the small forward gain.  You can null an interfering source (or region).  Sort of the idea behind the cophased whips on each side of a vehicle.. put a null on signals coming from the side of the road.

(Yes, I know that a more practical reason is that when your antennas are essentially at the end of a 40 foot long metal box, it helps provide ANY propagation the rear)
Logged
N4JTE
Member

Posts: 1154




Ignore
« Reply #24 on: March 16, 2011, 09:47:04 PM »

WX7G; I always take your comments to heart, but in the case of 2 correctly phased verticals, I have to disagree with your comment that a single vertical has same pattern. 2 elements correctly phased in 90 degree quadrature, Christman, assuming equal currents/ voltage and phase, the tricky part, will give a very nice low angle take off, cardoid pattern, with a 3db bandwidth of about 120 degrees. Mine is showing about a 20 BD front to back when changing directions, aint gonna get that with a single vertical. perhaps I misread your post as I am going from memory, hi.
Regards,
Bob
Logged
RFRY
Member

Posts: 236


WWW

Ignore
« Reply #25 on: March 17, 2011, 07:52:59 AM »

The two phased verticals should provide a gain of about 3 dB or half an S-unit. The elevation pattern should be the same as a single vertical.

The elevation pattern from an array of two 1/4-wave monopoles spaced 1/4-wave apart in free space wavelengths, and driven with equal currents in phase quadrature produces an elevation pattern that is approximately the same as that of a single 1/4-wave monopole in the direction of maximum gain.  It isn't exactly the same, because the radiating sources are not coincident in space.

But at azimuth angles toward the minima of the azimuth pattern, the elevation pattern varies greatly from that of a single monopole used in that array.

The link below leads to a graphic showing this for the azimuth angle of maximum gain, and at +/- 45 degrees azimuth relative to maximum gain.

http://i62.photobucket.com/albums/h85/rfry-100/40-m_Array_Elev_Pats.gif
« Last Edit: March 17, 2011, 04:28:58 PM by RFRY » Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!