HF + 6 OCFD - what works here, and maybe why.
Posted: Wed Nov 02, 2022 10:44 am
Backstory: I live out of town, with plenty of space between some high (Loblolly) pine trees.
I started with a commercial 258' OCFD, with a feedpoint @ 15%. The not very thick bare wire was at about 30' up for most of its length. The balun ratio was undisclosed. A fine get-started antenna, MFJ-2016. Even landed my first DXCC with it and just 100W.
As is the way with radio, one always wants whatever it is to work just a bit better. I looked at the MFJ and thought how should it be if it was reconstructed with best materials and better installation.
I used EZnec to model the ideal length for 160m and found that I should be using around 280' -significantly longer. The feedpoint was also moved to 20% as this seemed a better compromise for a 6:1 balun + common mode choke (Palomar) over all ham bands 160-10. Why 6:1? No scientific reason more than a tip of the hat to the popular Backmaster family of antennas. (By now a satisfactory homebrew Moxon beam was used for 6m, selected with a Diamond MX-610 duplexer.) A brief ponder on how the wire antenna works, resonating current and voltage to produce both magnetic and electrostatic components of the radio wave, mean that low resistance and best insulation are important. Also one should avoid capacitative losses from nearby objects like trees and metallic structures, by keeping the route of radiating elements high and well clear of such things.
The new antenna was constructed with vinyl-covered 12G stranded copper from Home Depot. Nylon insulators suspended on Dacron insulating rope now have the antenna up at about 80', with the final length being adjusted with a nanoVNA to give the best VSWR compromise on all ham bands. (No, I didn't climb pine trees. Solution: WalMart best slingshot, 1 oz fishing barrel weight on 20 lb fishing line. Shoot over the tree and pull up the rope. )
You'd expect this to work, right? Not so fast. When connected to the rig, I found that some bands tuned well, while 3 bands didn't tune at all! The culprit turned out to be the length of the long feedlne; just wrong it turned out for standing waves at the important frequencies to entirely screw up attempts to impedance match. I added lengths of coiled coax cable to the rig end of the feeder until every band matched easily. The magic length in this case turned out to be 12'added. (An auto-tuner at the antenna would probably have been a better solution, but I'd already spent too much at Home Depot!)
Now I have an antenna that is much better than before, with receive sensitivity better than most others on the nets I join; I hear most if not all net stations when others struggle. Transmit signal reports of 20=over and 30-over are not uncommon, leading many to suppose that I have an amp hidden away. No, just a wire antenna that really works.
How did I know that it's better? I used measurements from FT8 logs to give an approximate comparison in dB. -See separate post.
I started with a commercial 258' OCFD, with a feedpoint @ 15%. The not very thick bare wire was at about 30' up for most of its length. The balun ratio was undisclosed. A fine get-started antenna, MFJ-2016. Even landed my first DXCC with it and just 100W.
As is the way with radio, one always wants whatever it is to work just a bit better. I looked at the MFJ and thought how should it be if it was reconstructed with best materials and better installation.
I used EZnec to model the ideal length for 160m and found that I should be using around 280' -significantly longer. The feedpoint was also moved to 20% as this seemed a better compromise for a 6:1 balun + common mode choke (Palomar) over all ham bands 160-10. Why 6:1? No scientific reason more than a tip of the hat to the popular Backmaster family of antennas. (By now a satisfactory homebrew Moxon beam was used for 6m, selected with a Diamond MX-610 duplexer.) A brief ponder on how the wire antenna works, resonating current and voltage to produce both magnetic and electrostatic components of the radio wave, mean that low resistance and best insulation are important. Also one should avoid capacitative losses from nearby objects like trees and metallic structures, by keeping the route of radiating elements high and well clear of such things.
The new antenna was constructed with vinyl-covered 12G stranded copper from Home Depot. Nylon insulators suspended on Dacron insulating rope now have the antenna up at about 80', with the final length being adjusted with a nanoVNA to give the best VSWR compromise on all ham bands. (No, I didn't climb pine trees. Solution: WalMart best slingshot, 1 oz fishing barrel weight on 20 lb fishing line. Shoot over the tree and pull up the rope. )
You'd expect this to work, right? Not so fast. When connected to the rig, I found that some bands tuned well, while 3 bands didn't tune at all! The culprit turned out to be the length of the long feedlne; just wrong it turned out for standing waves at the important frequencies to entirely screw up attempts to impedance match. I added lengths of coiled coax cable to the rig end of the feeder until every band matched easily. The magic length in this case turned out to be 12'added. (An auto-tuner at the antenna would probably have been a better solution, but I'd already spent too much at Home Depot!)
Now I have an antenna that is much better than before, with receive sensitivity better than most others on the nets I join; I hear most if not all net stations when others struggle. Transmit signal reports of 20=over and 30-over are not uncommon, leading many to suppose that I have an amp hidden away. No, just a wire antenna that really works.
How did I know that it's better? I used measurements from FT8 logs to give an approximate comparison in dB. -See separate post.