W3XAF
08-29-2007, 04:41 PM
I purchased an MFJ-1775 rotatable dipole antenna and installed it on top of a Ham IV Rotor.
The dipole was very easy to put together and MFJ provided many spare parts with the MFJ-1775 kit.
The dipole consists of one 14-foot long aluminum element (tube) which has two coil assemblies attached to each end of the aluminum tube. #Each coil assembly consists of four coils, one coil each for 10, 15, 20 & 40 meters. #Connected to each coil is a metal ring which has four graphite spokes inserted into holes in the ring.
In order to create a resonant frequency on each band, one has to painstakingly cut each graphite spoke using an MFJ-259B SWR Analyzer until the resonant frequency is achieved. #That task took me about three hours.
After I hoisted the antenna into my Ham IV Rotor, and connected the RG-213 Coax to the antenna balun, I rechecked all the resonant frequencies and they had all changed. #This might have been due to the fact that I used five plastic covered steel guy wires to secure my antenna mast to the ground beneath.
I finally replaced all my guy wires with 3/8" Dacron rope and then had to re-cut the antenna spokes to the required resonant frequencies for each band. #No easy task standing on a sloping roof 50 feet above the ground.
Furthermore, I found it necessary to order more 30"-long spokes from MFJ in order to achieve the 40 Meter resonant frequency I needed.
I then ran into a major problem. #When I applied power to my antenna and tuned up using my FT-1000MP Mark IV Transceiver, AL-80B Amplifier and MFJ 989D Tuner, I found that the resonant frequencies for each band were not the same as those I had established with my MFJ-259B SWR Analyzer. #Therefore, I had to go back up on my roof and re-cut or lengthen the graphite spokes accordingly. #This task took many days!!!
The only other problem I have experienced now that the antenna spokes are all cut for the resonant frequencies that I want to operate on, is that when I use voice modulation my SWR varies between 1:1 and 1:4 depending on how long I speak into the microphone. #The longer I speak into my microphone, the higher the SWR goes. This problem may be the result of my balun overheating?http://www.qrz.com/iB_html/non-cgi/emoticons/rock.gif
The dipole was very easy to put together and MFJ provided many spare parts with the MFJ-1775 kit.
The dipole consists of one 14-foot long aluminum element (tube) which has two coil assemblies attached to each end of the aluminum tube. #Each coil assembly consists of four coils, one coil each for 10, 15, 20 & 40 meters. #Connected to each coil is a metal ring which has four graphite spokes inserted into holes in the ring.
In order to create a resonant frequency on each band, one has to painstakingly cut each graphite spoke using an MFJ-259B SWR Analyzer until the resonant frequency is achieved. #That task took me about three hours.
After I hoisted the antenna into my Ham IV Rotor, and connected the RG-213 Coax to the antenna balun, I rechecked all the resonant frequencies and they had all changed. #This might have been due to the fact that I used five plastic covered steel guy wires to secure my antenna mast to the ground beneath.
I finally replaced all my guy wires with 3/8" Dacron rope and then had to re-cut the antenna spokes to the required resonant frequencies for each band. #No easy task standing on a sloping roof 50 feet above the ground.
Furthermore, I found it necessary to order more 30"-long spokes from MFJ in order to achieve the 40 Meter resonant frequency I needed.
I then ran into a major problem. #When I applied power to my antenna and tuned up using my FT-1000MP Mark IV Transceiver, AL-80B Amplifier and MFJ 989D Tuner, I found that the resonant frequencies for each band were not the same as those I had established with my MFJ-259B SWR Analyzer. #Therefore, I had to go back up on my roof and re-cut or lengthen the graphite spokes accordingly. #This task took many days!!!
The only other problem I have experienced now that the antenna spokes are all cut for the resonant frequencies that I want to operate on, is that when I use voice modulation my SWR varies between 1:1 and 1:4 depending on how long I speak into the microphone. #The longer I speak into my microphone, the higher the SWR goes. This problem may be the result of my balun overheating?http://www.qrz.com/iB_html/non-cgi/emoticons/rock.gif