Friday, June 24, 2011

Ground Mounted Vertical Antenna

Ground Mounted Vertical Antenna Notes for 7MHz and below

* Correctly configured ground mounted vertical antennas for 7MHz
and below will out perform horizontal antennas for DX work.

* A horizontally polarized antenna is better for local contacts up to approximately 1000km.

* A good ground plane is essential for a vertical to perform well.

* A vertical antenna has a lower radiation angle than a horizontal antenna.

* A good ground system has a critical angle of about 10 degrees,
where a bad ground will give a higher angle of about 30 degrees.

* Good ground systems have a low angle of radiation and greater field strength.

* Bad ground systems have a higher angle of radiation and up to 4dB poorer
field strength than an antenna with an excellent ground system.

* Tuned radials (i.e. ¼ wave for example) are the best choice and ½ wave
length radials will be an advantage over the ¼ wave.

* The more radials the better, although 120 radials seems to be the
maximum needed. At least try for 24 to 48 radials if possible.

* If you haven’t got the room to lay the radials straight, then bending them
around objects or trees is ok.

* A horizontally polarized antenna at frequencies above 7MHz maybe equivalent to,
but is normally a better performer than a vertical antenna.

* A tin roof with about 20-30 degrees sloop makes an excellent ground plane when
the vertical antenna is mounted on its peak.

Modifying a 27MHz CB Ringo Antenna for 80–6Mtrs

More Pictures added 22/8/09

Click here to download PDF document.

A 27MHz Ringo has been collecting dust in the shed for years, and after successfully
constructing a vertical for 6 meters, it was time to put the Ringo to good use.

The plan is to remove the ring and connect 4 x ¼wavelength radials for each band.
The center of the SO239 connector is connected to the 5.3 meter driven element
via a short piece of aluminum. The driven element is already a 1/4wavelength on 20
meters. A bracket was made from aluminum strip to bolt the radials to and was
screwed to the existing earth bracket left over after removal of the ring.

A 50mm x 500mm pipe is hammered into the ground leaving 200mm above the ground
for the base of the vertical to slip into. Radials were cut from 2.5mm diameter
aluminum wire scavenged from old power line cable.
Initially the 10 and 20 meter radials were bolted to the earth bracket.
A random length of RG58 coax was run from the antenna to the shack where a dip
meter coil was screwed on. One would have thought that 20 meters would have
tuned/dipped perfectly being a 1/4wavelength vertical. But not so. Adjusting
the length of the antenna had no real effect on the reading of 18MHz dip.

After several failed attempts on other frequencies, the coax was tested resulting in
– yes – a 18MHz dip. (It is interesting to note that the coax had little effect on
resonance when the antenna was setup for a 1/4wavelength on 28MHz, but played
havoc when used for other wavelengths and/or bands.) A new 27.85 meter length
of coax was cut and terminated with PL259 plugs. This cable now dipped on the
amateur bands and was duly plugged into the antenna. Hey presto, a perfect VSWR
was obtained on 14MHz as expected. The length of coax cut is the only length of
cable that is resonant on all HF ham bands. One could cut a single 1/4wavelength
piece to match one band but the aim here was to use all bands with one vertical.

Two radials for 7MHz were now attached and laid out across the ground. (The other
two are yet to be cut.) 10cm of each end was bent down and pushed into the soil to
hold them in place till a proper burial can take place. Further testing confirmed the
entire 14MHz band was 1:1 and 1.4:1 on 6 meters. With the aid of an ATU, all other
bands were tunable including 80 meters. Yet to add is 2 more radials for 7 MHz,
and radials for 21MHz. It will be interesting to see what improvements, if any, take
place once they are in place.

The following table was used as a guide for measurements.
Radials were cut using the measurements in the 1/4wavelength column.

300

Wavelength

FULL

5/8TH

3/4.

1/2.

1/4.

1/8.

1/16.

Freq

1.000

0.625

0.750

0.500

0.250

0.125

0.0625

52.000

5.769

3.606

4.327

2.885

1.442

0.721

0.361

28.000

10.714

6.696

8.036

5.357

2.679

1.339

0.670

21.000

14.286

8.929

10.714

7.143

3.571

1.786

0.893

14.000

21.429

13.393

16.071

10.714

5.357

2.679

1.339

7.000

42.857

26.786

32.143

21.429

10.714

5.357

2.679

3.570

84.034

52.521

63.025

42.017

21.008

10.504

5.252

1.825

164.384

102.740

123.288

82.192

41.096

20.548

10.274

Antenna length = 5.3mtrs

Performance

VK4 to ZL tests on 20mtrs were 1 to 2 S points lower from the vertical than the
TH6DXX beam. Feeding the antenna with RG213 instead of RG58 would give
better results due to lower line losses.

At around 0510z I worked VK7XX and the vertical was 1 to 2 S points better than
the beam. At 0530 the signal to VK7 was identical off both antennas. A VK3 joined
the now expanding group and comparisons were 1 S point in favour of the beam. Seems
polarisation must have played a role here. Tests returned good results on all other bands.

In conclusion the ‘all band’ CB antenna with 1/4wavelength radials for each band
surprised initial contacts on its performance and simplicity. It is small enough for
confined spaces and the longer radials can be bent around if insufficient space
exists to lay them out straight.

Good DX.


The Upgrade to a 40mtr 1/4Wave

After the success of the above antenna, I have now replaced it with a 40mtr 1/4 wave.

The bottom section is made of 50mm square aluminum and the top half is light duty round 50mm.
I had enough to make the full 10.3 meters in length. The bottom is hinged via a bolt through a
'u' shaped steel bracket. Another bracket on top of the post is used to hold it in place.
Hoping for lots of rain to soften the topsoil so I can bury the radials.

Above picture shows the radials completely buried.
Yep, the grass is dead due to drought!

I didn't have a full length of 10.3mtr aluminium
so I improvised using one 2" square heavy walled
piece, one thin walled 2" round that just didn't
make the height, so then I tek screwed a smaller
piece to it. All this was laying around collecting
dust and dirt, so it was good to put it to use.

It looks like crap but it works a trea
t.
1:1 Ugly balun added 28/11/09.

I removed it soon after, replaceing it

with a 7 turn RG58 coil on a 50mm

diameter plastic pipe. The vertical

tunes better on other bands now.

Anti-Static Discharge coil added
between driven element and earth.

The conduit is 8 inches long and I used
AW

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