Home 
Search 
Today's Posts 
#1




5/8 wave ground plane
I want to construct a 5/8 wave ground plane for 24 MHz.
I am OK with the dimensions, but I can't get a grip on what the impedance at the feedpoint will be. Even the ARRL antenna book hasn't been much help. Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! Larry DiGioia N8KU N8KU at longwire.com 
#2




For a 5/8wave vertical above ground at 24 MHz, conductor diameter 10 mm. 
Base feedpoint resistance = 110 ohms Base feedpoint reactance = j386 ohms Base loading coil inductance = 2.6 uH Coil diameter = 25 mm Coil length = 33 mm Number of spaced turns = 13 Wire = 14 awg = 1.64 mm diameter. For other heights, frequencies and coil dimensions download in a few seconds program BOTLOAD for immediate answers.  .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... 
#3




Kraus (2nd edition, p. 375) gives a different answer.
Interpolating from the diagram on p. 375 (assuming length/diameter = 472) it is about 90j250. Why the difference?  Jim N8EE to email directly, send to my call sign at arrl dot net "Reg Edwards" wrote in message ... For a 5/8wave vertical above ground at 24 MHz, conductor diameter 10 mm.  Base feedpoint resistance = 110 ohms Base feedpoint reactance = j386 ohms Base loading coil inductance = 2.6 uH Coil diameter = 25 mm Coil length = 33 mm Number of spaced turns = 13 Wire = 14 awg = 1.64 mm diameter. For other heights, frequencies and coil dimensions download in a few seconds program BOTLOAD for immediate answers.  .................................................. ......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. ......... 
#4




Larry D wrote:
Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! The VERT1 vertical that comes with EZNEC is 1/4WL on 40m. It will be 5/8WL on 17m. (7.2 * 2.5 = 18) EZNEC sez the feedpoint impedance on 17m will be about 100j470 ohms. A loading coil is usually installed between the base of the antenna and the radial system. The coax braid is tied to the radial system and the coax center wire is tapped down on the coil at the 50 ohm point. Rad coil Gnd+////////5/8WL Rad ^  XMTR+ If you don't mind a relatively low SWR of 2:13:1, use good coax, a standard baseloading coil, and your tuner at the transmitter. There is plenty of 5/8WL information for 2m antennas in the ARRL publications and antennas are scalable.  73, Cecil http://www.qsl.net/w5dxp = Posted via Newsfeeds.Com, Uncensored Usenet News = http://www.newsfeeds.com  The #1 Newsgroup Service in the World! == Over 100,000 Newsgroups  19 Different Servers! = 
#5




Z is close to +58j138 ohms.
A small inductor of 0.88 uH in series with the antenna should provide a pretty good match to 50 ohm coax. Larry D wrote: I want to construct a 5/8 wave ground plane for 24 MHz. I am OK with the dimensions, but I can't get a grip on what the impedance at the feedpoint will be. Even the ARRL antenna book hasn't been much help. Can anyone tell me the impedance I can expect to see, also any tips on matching it? Thanks! Larry DiGioia N8KU N8KU at longwire.com 
#6




Kraus (2nd edition, p. 375) gives a different answer.
Interpolating from the diagram on p. 375 (assuming length/diameter = 472) it is about 90j250. Why the difference? ============================== The program takes into account antenna diameter. Does Kraus? And is he on the same band? Is he at the same height above ground? What sort of ground or radial system does he have? Have you interpolated correctly? Have you understood what Kraus is saying? Is the loading coil in the same location? Is the feedpoint at the same place. Are you comparing like with like? Spend more time thinking about it. Is Kraus infallible? Could be he's wrong again. Who's Kraus anyway? ;o)  Reg 
#7




There is plenty of 5/8WL information for 2m antennas
in the ARRL publications and antennas are scalable.  73, Cecil =========================== The trouble with scaling (as from 2m down to 160m) is that antenna conductor diameters are forgotten about which can cause appreciable errors.  Reg, G4FGQ 
#8




Reg Edwards wrote:
The trouble with scaling (as from 2m down to 160m) is that antenna conductor diameters are forgotten about which can cause appreciable errors. I dunno, Reg. My 20m rotatable dipole diameter is about ten times bigger than my 2m 1/4WL ground plane. Seems about right to me.  73, Cecil http://www.qsl.net/w5dxp = Posted via Newsfeeds.Com, Uncensored Usenet News = http://www.newsfeeds.com  The #1 Newsgroup Service in the World! == Over 100,000 Newsgroups  19 Different Servers! = 
#9




Larry, N8KU wrote:
"I want to construct a 5/8 wave ground plane for 24 MHz.Even the ARRL antenna book hasn`t been much help." At 1/2wave, reactance nearly is zero and begins repeating as from zero.. With a 5/8wave, you have a 1/8wave connected to the feedpoint through a reactance repeating 1/2wave line. A 5/8wave antenna will have nearly the same reactance as a 1/8wave. Value of the reactance depends on the characteristic impedance of the antenna and this depends on length to diameter ratio. Resistance of a vertical antenna increases with its length until it reaches nearly 1/2wavelength where it peaks and starts to fall. Sharpness of this resonant peak depends on length to diameter ratio of the antenna. You might expect a radiation resistance of between 50 and 150 ohms. Reactance may be between 200 and 1200 ohms. It depends on how skinny the antenna is. Fat conductors have smaller impedance variations (a lower Q). My impedance numbers come from Capt. Paul H. Lee, USNR, K6TS`s "Vertical Antenna Handbook". 5/8wave vertical groundplanes were once popular on 27 MHz and often used an autotransformer at the base to step up the impedance from the 50 ohms of the coax. Best regards, Richard Harrison, KB5WZI 
#10




"Reg Edwards" wrote in message ... The program takes into account antenna diameter. Does Kraus? Yes. My post said length/diameter = 472 (One thing I don't like about the way Kraus wrote his book is that he gives the formula for only one part of the feed impedance (R or X) and a diagram for the complex impedance. He then refers you to someones paper somewhere for the 'complete' formula.. I have it around here but it would take 2 days to find it.) And is he on the same band? In the diagram I was interpolating from the dimensions are in wavelengths. Is he at the same height above ground? What sort of ground or radial system does he have? It is an infinite perfect ground planeground mounted. (We are talking about a 5/8 wave vertical remember). Have you interpolated correctly? Probably not. Have you understood what Kraus is saying? Yes. Is the loading coil in the same location? No 'loading coil' in the Kraus example. He is discussing the feed point impedance. A loading coil would be either across or in series with the feed point. Is the feedpoint at the same place. Since the original post concerned a 5/8 wave vertical, I assumed it is at the bottom, which is what Kraus is discussing. Are you comparing like with like? Hence my question. Are we? I recall that a 5/8 wave vertical was 'close' to 50 ohms at the feed point and could be 'adjusted' with an inductance across the feed point (with the feedline at a tap perhaps?). That is the whole point is using such a 'magic number' as 5/8 wavelength. If you look at the impedance curves, though, you find that they are quite steep at this point, so a small error in length causes a large error in impedance, and some tuning will be needed. The 5/8 wave is not a 'build it and go' antenna! Spend more time thinking about it. I have, which is why I looked it up. Is Kraus infallible? Could be he's wrong again. =8o Oh no!!! Kraus wrong!? Who's Kraus anyway? ;o) Oh, I don't know. Just some guy that signed my Master's Thesis 16 years ago.  Reg  Jim N8EE to email directly, send to my call sign at arrl dot net 
Reply 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Forum  
Inverted ground plane antenna: compared with normal GP and low dipole.  Antenna  
Plans for a 5/8 wave 2M ground plane  Antenna  
QST Article: An Easy to Build, DualBand Collinear Antenna  Antenna 