Part B:
The AL7X 20m Dipole
Updated 8/12/2013
Minor updates 8/29/2013
8/2/2013: Building the Dipole
So now I have the tower up, ready to receive a rotor and beam. But, I am out of money for now, so I decided to put a dipole at the top of the tower.
This is not going to be some ordinary wire dipole hanging off the top of the tower, but rather a rigid one-element dipole. It will be fixed mounted as I do not have a rotor yet. But it is built for rotation, if so desired.
This image shows the electrical connection. The RG-8X cable reaches to the station transceiver.
Materials and Construction
As this will be a 20 meter dipole, the length will be between 32 and 34 feet. The 20 meter band, when open, allows for worldwide communications.
Basically I am putting a pair of 20 foot long "crappie" fishing poles, back to back on a wooden plank. The wire will be inserted inside the poles. The very tip part of the pole is too small to pass wire, so about 35 feet is all that can be accomodated. Just right.
These poles are made by Shakespeare as "Wonderpole" and can be gotten from Amazon.com for about $20 each.
This picture shows one of the crappie poles mounted on the plank; a 2X4, shown here before painting. The blue label on the pole says "Shakespeare".
A pipe flange attached to the plank provides for mounting on the tower. A nipple screwed into the flange could in turn be inserted into a rotor to make this a rotatable dipole ... but, I do not have a rotor yet. (note 8/12: ordered and was shipped today out of HRO, Portland, OR)
This is a 1-1/2 inch flange and nipple, which fits perfectly into the tower's top. The tower has setscrews to prevent the nipple to turn into the tower. A small setscrew, not shown, is inserted into the pipe flange to secure the nipple. This way, a wind gust would not turn the antenna.
8/6/2013: Erecting the Dipole
Here is the dipole on top of my tower, before raising. A coat of white paint has been applied to prevent the wood from getting wet and possibly throwing the antenna tuning off.
Showing the MFJ 259B antenna analyzer set at 14.154 mHz resonance. What I did is sweep the analyzer from 13.0 to about 15.0 to find the resonant spot (no reactance), and trimmed equal amounts of wire from each side until resonance came in at 14.150. I used copper split bolts to facilitate the connections.
Although the calculated length is near 16.5 ft per side, I started out with 17.5 ft just in case. It is easier to cut than to add wire!
Note that SWR and resonance are not exactly the same thing. Resonance is indicated by zero reactance on X=0 at the LCD readout. The resistance is 53 ohms, very close to the ideal 50 ohms. SWR=1.0 on the LCD and the analog left hand meter.
If you do not have an MFJ-259B (or the 269B) meter, you can accomplish the same thing by setting your transmitter on 14.000 and shortening the wires equaly on both sides, until the SWR is at its lowest. It will be "close enough for government work."
Here is the antenna in place on the tower, and raised up to its
full height of 55 feet above ground. Here are the SWR results:
| Frequency | |||
| 13.560 | |||
| 13.800 | |||
| 14.000 | |||
| 14.050 | |||
| 14.100 | |||
| 14.150 | |||
| 14.200 | |||
| 14.250 | |||
| 14.300 | |||
| 14.350 | |||
| 14.500 | |||
| 14.775 |
Looks like a perfect antenna to me!
Update 8/20/2013
The rotor has arrived!
The rotor has arrived, and I have it calibrated per Yaesu's Instructions. This is the Yaesu G450 unit, the lightest/smallest they have. It looks like a beauty!
Along with the rotor, I got a 100 ft control cable and 100ft of Ultra Flex RG-8/RG-213 type of Coax for the antenna.
All I have to do now is set the antenna on the rotor, and be good to go. Rotating antennas, particularly those set on crankup towers, need to have the cables come down without tangling the mechanisms. This is done with "arms" that hold the cable away from the tower. (8/29 update). The arms are fabricated and will be installed along with the beam.