Megahertz-Radio

About Radiocommunication – VHF – UHF – HF

Posts Tagged vertical

The vertical groundplane (GP) antenna

Dec 30

Posted by admin in Antennas | No Comments

The GP antenna is a very popular antenna. You can use it for every freqency you want as long it’s cut to the correct length. It’s a excellent HF antenna or 27 mhz CB antenna (PR-27).  The typical groundplane antenna is 1/4, 1/2 or 5/8 wavelength long.  The different length have different characteristics. 

The 1/4 will be shorter than the 1/2 or 5/8 wavelength antenna and will have a higher take-off-angle (shorter skip distance). On the other hand the antenna may work  better in a valley. (The signals will shoot out of the valley.  A 5/8 will maybe shoot the signals straight in to the mountains). Read more about take-off-angle here: http://megahertz-radio.com/2008/11/28/skip-distance-and-radiation-angle-toa/ 

The design of the antenna:

gp1

Before you build this antenna, choose your desired length after your needs. If you have the space and area for a big antenna you should make the 1/2 or 5/8 antenna. It will be a better long distance antenna. If yo want to make local contacts in your area you go for the shorter 1/4. 

Here is a little calulator: http://www.csgnetwork.com/antennagpcalc.html

The calulator is for the 1/4 antenna. The 1/2 antenna is double the length and so on. 

Radials and where to install the antenna:

Many places I see a CB antenna without any radials. Well It will look good, but that is not a very efficient antenna. The radial system is HALF the antenna. So a decent radial system will make this to a good antenna. So a home made GP antenna will often be better than a one you buy. 

If you mount the antenna on the ground (GROUNDplane antenna…) you will need a lot of radials. The radials should be from 1/10 wavelength up to 1/2 – 1 wavelength long. The length is not important when them are on the ground. You should lay down ATLEAST 8 radials. If you have the land, the “optimal” is 120 radials (!).  

The best way to make an efficient GP antenna is to have elevated radials. (atleast 2m above the ground). A good way is a roof-mount. This will also place the antenna over noisy obstacles and give a better “line-of-sight” coverage. The most important note for elevated installation is the radials NEED to be correct lengt. (ex. 1/4 long). Opposite of the groundmounted antenna where you can use random length.  You dont need more than 4 radials with this setup and will give you an excellent radial system. 

Read more on radials: http://www.steppir.com/files/radial%20systems%20for%20vertical%20antennas.pdf

 

gp2gp3

The GP antenna is fun and easy to build. with some meters with wire and a fishing-pole you can make this in minutes and you dont need a balun or a matching network. Just hook  it up to your 50 ohm coax and straight to your radio.  Before you transmit you need to check your SWR or you can blow your finals. If you have a low/high SWR the antenna is to short or to long.  If you like to build antennas, I recommend that you buy a MFJ 259B  antenna analyzer. (link)

More about SWR (link)

Tags: , , , , , , , , , , , , , , ,

Homemade 1/2 wave vertical antenna for Ham radio and CB/Walkie-Talkie

Dec 22

Posted by admin in Antennas | No Comments

I made a blogpost about a cheap CB radio, but you will need an antenna for this. The best is to make one yourself. Homebrew antenna is easy to make, cheap and will give you a satisfaction.  Later I will post about other antenna-designs. 

 

Omnidirectional Antennas (From http://www.signalengineering.com/ultimate/verticals.html)

“Omnidirectional” is generic term for an antenna that radiates equally well in all directions. There are several antennas that are considered omnidirectional.

1/2 Wavelength Vertical

Most folks lump all vertical omnidirectional antennas into the same category and call them “Ground Planes”. A ground plane antenna is actually an antenna similar to the vertical dipole. Shown in figure 1, you can see the hollow tubing is now instead brought out at a 45 degree angle (and split into 3 sections) out from where it is on the vertical dipole. These rods are usually called “radials”. This type of antenna is really not a very high gain antenna.

Figure 1 – A ground plane antenna.

A much better type of antenna that has more gain is the 1/2 wavelength vertical (Shown in figure 2). We know that the impedance of the 1/2 dipole is 70 Ohms when we attach the coax in the middle, but what if we were to attach our coax directly to the end? The impedance at this point is high, very high, so we must make a matching device to match the antennas impedance to the 50 Ohm coax. What would happen if we did not use this matching device? Well if you have been reading along, you would know that this would result in a very very high SWR.

There are several commercial 1/2 vertical antennas available, the two that I can think of most easily is the Solarcon A99 and the Shakespeare Big Stick. They provide slightly higher gain than the vertical dipole antenna.

The bandwidth of these antennas are good, they can easily span all the CB channels and more with a low SWR.

5/8 Wavelength Vertical

A higher gain antenna than the 1/2 vertical antenna is the 5/8 vertical antenna. As we can figure from the 5/8 wavelength rating the antenna is about 22 feet long (5/8 of 36 feet). This antenna is similar to the 1/2, it needs a matching device at the base to match it to the coax, it cannot be attached directly. This antenna has about 1.2 db gain over the dipole antenna and 1/2 vertical. Figure 2 shows both a 1/2 Wave vertical and a 5/8 Wave vertical antenna. It achieves this extra gain by concentrating its pattern out more at right angles from the antenna instead of wasting signal at high angles, see figure 3.

Figure 2 – A 1/2 Wave vertical is on the left the 5/8 Wave vertical is on the right. A matching device is required to match the high impedance feedpoint of these two antennas to the 50 Ohm coax. The 5/8 Wave vertical has 1.2db more gain over the 1/2 Wave vertical.

I have just heard Solarcon is coming out with a new 5/8 antenna (its about time, this antenna design has been around since the beginning of time!). There are several 5/8 antennas, Hustler 27JR, Antenna Specialist Sigma 5/8, Maco V 5/8. They offer slightly higher gain (about 1.2db) than the popular A99 and Big Stick. Shakespeare was committing a crime claiming their ABS 1600 5/8 wave antennas had 12.5 dbi gain (the “i” means over an isotropic antenna). This was plain ridiculous. A 5/8 wave antennas is a 5/8 wave antenna. Do you think a vertical could have more gain than a beam? Never! This is the perfect example of how out of hand antenna manufactures have gotten with their advertising. Now, everyone thinks their A99 is a low gain piece of junk because Shakespeare claimed 12.5 dbi with their “new” antenna. Shakespeare no longer claims the ABS 1600 has 12.5 dbi gain, but everyone who sells it is still advertising that gain figure.

Even if you do not know the manufacture or anything about the antenna you are looking at, you can tell if the antenna is a 1/2 or 5/8 wave by its length, again 1/2 is about 18 feet and 5/8 wave is about 22 feet (at CB frequencies).

Figure 3 – How one omnidirectional antenna achieves gain over another. Note this is a view of the radiation pattern from the side (as if we were standing on the ground looking at the antenna).

Tags: , , , , , , , ,

Skip distance and Radiation angle (TOA)

Nov 28

Posted by admin in Antennas | 1 Comment

This will be a very easy explantion of how Radiation angle work and how this will effect your HF radiocommunication below 30 mhz. 

Every antenna have a radiation-pattern. This pattern describe wich angle the signal will leave/enter your antenna.  So before you make/buy an antenna, you need to know the purpose of your radiocommunication. If you want to make long distance contacts (DX) over etc 3000 miles, you will need an antenna with low radiation angle (Take off angle = TOA). If your purpose is domestic contacts in your local area you want an antenna with high angle of radiation. That way the signal will go straight up to the atmosphere and right down.

The picture below will illustrate this. A and B are good for long distance contacts. That could be a vertical antenna with low radiation angle or an dipole that’s elevated atleast 1 wavelength above ground. The C example may be a low elevated dipole (< 1/2 WL over ground) for local contacts. For you can see the signal will go straight up, and straight down.

Horisontal dipoles, long wire’s , yagi’s and other horisontal antennas will work so good you want it to work by decide the hight of the antenna.  A thumb rule are if you want a long skip distance, you have to put the antenna atleast 1 wavelength above ground. (20 meters on 14 mhz).

If you want to rag-chew on the 80m band with your local HAM’s you hang the dipole on a low height. (10-60 feet f.e)

 

Under you can see radiation pattern of different antennas on different hights.

 

 

Tags: , , , , , , , ,

N2JEU Web controlled IC IC-R75

Nov 27

Posted by admin in Receivers | No Comments

One day I stumbeled over this great website. Here you can web controll a HF receiver. It covers from 0,03 to 60 mhz and the antenna is a 43ft Zero-five vertical antenna (160-6m). Just tune in the frequency and listen away on your computer. Love to to this when i’m not in front of my own radio.  Remember that multiple people can listen at the same time, so do not change frequency to often.

Remember to set the correct Mode for the band. The radio support AM, FM, LSB, USB, CW and RTTY. You find the site here: http://remote.n2jeu.net/ (direct link: http://remote.n2jeu.net/cgi-bin/icomr75.exe )

Tags: , , , , , , , ,