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Author Topic: 30m Ground Mounted Verticle 1/4 or 5/8 wave??  (Read 473 times)
N2JDQ
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Posts: 60


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« on: January 12, 2004, 07:50:50 PM »

Hello,

Im just about to dive into my 1st QRP CW Home brew project.. a 1 watt trasmitter.. rcvr.. ect .

I'm just about certian that I want a Omni directional antenna.. hence the verticle idea. Now Im not very 'up' on all the fancy antenna theroy, but it seems to me that a ground monted verticle of 5/8 wave would have a lower take off angle, that the more common 1/4 wave. Is this a correct assumption?

I will have plenty of room for radials.. 25' 360 deg. around the antenna.. I just need some solid input of which way to go.. 1/4 or 5/8 wave. This WILL NOT be an elevated antenna... feed pt. will be right at ground level.. or just a few inches above.

If my memory is correct the 5/8 wave antenna on a car for lets say 2m , well out preforms a 1/4 wave .

Now lets say hypothetically that the 5/8 wave would be a better performer.. its design must be a bit more rugged due to size, not  a problem im handy mechanically, but what about impedeance matching?

This antenna will ONLY be for the CW portion of 30m.

Any input would be great!

- Steve Raas
N2JDQ
sraas@rochester.rr.com
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RobertKoernerExAE7G
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Posts: 1435




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« Reply #1 on: January 12, 2004, 09:35:27 PM »

There is quite a difference between VHF and HF antennas, and especially between VHF/UHF mobile antennas and ground mounted HF antennas.  

If I remember correctly, 5/8 wave verticals rely upon very good electrical earth, for many wave lengths, to out perform .25 wave length antennas.  I remember reading this in ON4UN’s book, “Low Band DXing”.  So, if your earth doesn’t have very good conductivity for a few wave lengths out, your 5/8 won’t out perform a ¼.  Plus, you will have to use some kind of matching network for the 5/8s, which might chew up some of your QRP signal.

You might find life a lot easier if you put up a Delta loop, and feed it .25 from the apex.  Many times, you can feed it with 75 ohm coax.  Plus, if you decide you want to be on 40 meters instead, if you can raise the center support, you can just ad more wire to the delta loop for 40 meters.

Have Fun
Bob
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KB0GU
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Posts: 130




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« Reply #2 on: January 12, 2004, 09:39:41 PM »

Steve, good luck with your endeavor!  What I have learned is the advantage of a 5/8ths wave antenna is lost if the ground + radial system is not adequate.  But you are correct, when an excellent ground system is under the 5/8ths wave antenna it should, rarely achieved in most hf installations, perform better than a 1/4 wave vetical.  You are going to need a lot of radials either way, and a way to match the base of the antenna to your feed line as a 5/8ths antenna does not represent a good match to 50 ohm coax.  Suggest you get a copy of ARRL Antenna Handbook and dig in!
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N2JDQ
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« Reply #3 on: January 12, 2004, 11:11:58 PM »

Hmmm... well after pondering a few things and reading a bit more.. Ill probabally go with the 1/4 wave.. now if i remember reading in the handbook ... if I mount this 1/4 wave right @ ground..with x ammnt of radials @ 90 deg to the antenna.. in theroy thats a 30 ohm load.. there is NO way I will be able to get the radials at a 45 deg angle to the radiator.. so what should I think abt now? 1) a base matching network of sorts or 2) play with the physical lenght of antenna? Or a little of both?

-Steve
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K7KBN
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« Reply #4 on: January 13, 2004, 12:00:25 AM »

Steve - you said that this antenna will only be for the CW portion of 30 meters.

That's good.  30 meters is ONLY CW and data - no phone subband at all.

That said, good luck on your project.  One watt is a challenge any time, and with the really lousy conditions we've seen recently (and for 3 or 4 years in the future), it might be REALLY slim pickin's.
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73
Pat K7KBN
CWO4 USNR Ret.
K0ZN
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Posts: 1553




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« Reply #5 on: January 13, 2004, 01:25:54 AM »


You are correct that a 5/8 wave vertical will provide better low angle radiation for DX (and about 3db gain) over a 1/4 wave if you have an extensive "Godzilla" radial system that is ALSO 5/8 wave or longer around the antenna. With a 5/8 wave vertical, the maximum ground current will occur about 27 feet out way from the base of the antenna, so this is where you would need the best ground conductivity (i.e. where the radials are most needed.)Remember, that with a ground mounted vertical, the ground provides a "mirror image" (in simple theory) of the vertical radiator. I would guess that a 5/8 wave vertical, even with a less than optimum ground radial system would still perform slightly better than a 1/4 only because there is a little more capture area (57 ft tall vs. 23.1 ft)and in all cases the radiation at low angles will be about equal to the 1/4 wave. Conversely, I would guess that a 1/4 wave vertical with a really good radial system (like 100+ radials) will work about as well as a 5/8 with just a few radials...and be a lot easier to build. The only negatives to the 5/8 vertical is that it may be poor for shorter distance contacts State side because there is considerably less higher angle radiation which is good for casual State side contacts. A 5/8's also will require a little more complicated matching network at the base. Personally, if I had the room and support structure for a 57ft. 5/8's antenna I would give it a try! Sounds cool!

73
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AC4FD
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« Reply #6 on: January 13, 2004, 11:47:41 AM »

Hi Steve,

I believe that even at 30 ohms... if the antenna is tuned properly, your swr will still be well below 2.0 to 1 and perhaps better than 1.5 to 1.  Depending on how critical you are on your SWR you may not need any turner at all.  I remember many times back in my CB days building quarter wave groundplane antennas.  And I was always able to attain an SWR lest than 1.5 to 1.

Good Luck on 30!

73

Randy
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WB6BYU
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« Reply #7 on: January 13, 2004, 04:59:07 PM »

First, some good reading on the subject is W4RNL's
article on 5/8 wave antennas:

http://www.cebik.com/58.html

Personally, I'd put up the 1/4 wave antenna and see how
it does, mostly because it is a lot simpler mechanically
to construct and mount.  (Or build one of each and
compare them.)  You can always use the same tubing for
the top section of the 5/8 wave antenna if you want to
try it later.

Matching shouldn't be a problem.  With just a few
radials on the quarter wave antenna, the added loss
resistance will increase the input impedance.  This is
why a quarter wave antenna often gives a very good
match to 50 ohms.  Of course, with plenty of radials
(and the resulting lower losses and higher efficiency)
the SWR may be over 1.5 : 1.

You can just ignore an SWR this low, or you can add a
simple coil or capacitor at the base of the antenna to
reduce it even further.  I'd start by connecting a 470
pf capacitor from the antenna to ground, then lengthing
the antenna a bit (perhaps close to 30'??) until the
SWR dips at the operating frequency.  Try different
values of capacitance:  at the 1 watt level, the old
mica "postage stamp" capacitors are quite adequate.
The other approach is to use add a coil from the antenna
to ground (perhaps around 0.5uH) and shorten the whip
to adjust the SWR.  Neither makes an appreciable change
in the radiated signal: there might be an advantage to
using the coil since it would provide a DC ground at the
feedpoint, and you can make it out of a scrap of wire.

The 5/8 wave whip is commonly matched by adding a coil
in series with the feedpoint of about 50 ohms reactance
at the operating frequency (in this case 0.7uH or so.)
But I think you will have better luck connecting a coil
of about the same size between the antenna and ground,
then tapping the coax center conductor up onto the coil
at the point of minimum SWR.  If you start with a coil
that is too large and tap it for both the antenna and
the coax, then adjusting the two taps alternately should
give you an SWR as low as you want.
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