dipole shape

[Ubbe]

I found the page for that that 1:4 guanella dual core balun. Balun Guanella Current 1:4
The schematic can be found on many other pages and some refer to pin 2 as "hot" and pin 8 as "cold".

When a dipole are tuned and in resonance if will have an impedance lower than 100 ohm. When it's out of tune and not resonant if will have several hundred or even thousands of ohm. A 1:4 balun used at the resonant frequency will just reduce the received signal 4 times but out of tune frequencies will work with a better match so it will create a more broadbanded antenna by sacrificing performance at the resonant frequencies.

A OFCD antenna will have one leg in resonance at low impedance and the other leg more or less as a passive element at a higher impedance and will begin to work more like a single element antenna with a counterpose depending of how much offset are used.

/Ubbe

 

[K8BBL]

I found the page for that that 1:4 guanella dual core balun. Balun Guanella Current 1:4
The schematic can be found on many other pages and some refer to pin 2 as "hot" and pin 8 as "cold".

When a dipole are tuned and in resonance if will have an impedance lower than 100 ohm. When it's out of tune and not resonant if will have several hundred or even thousands of ohm. A 1:4 balun used at the resonant frequency will just reduce the received signal 4 times but out of tune frequencies will work with a better match so it will create a more broadbanded antenna by sacrificing performance at the resonant frequencies.

A OFCD antenna will have one leg in resonance at low impedance and the other leg more or less as a passive element at a higher impedance and will begin to work more like a single element antenna with a counterpose depending of how much offset are used.

/Ubbe

That's how I plan to do it but have 1 question: does it matter which leg (short or long) is pin 2 & which is pin 8?

 

[prcguy]

That is a good project link except the ferrite cores are no longer available and there is no cross reference. With some searching I found a 31 mix ferrite is a good replacement and a pair of FT-240-31 toroid cores would handle a lot of power, probably 3kW or so within the resonant bands. The 43 mix should also work fine.

I found the page for that that 1:4 guanella dual core balun. Balun Guanella Current 1:4
The schematic can be found on many other pages and some refer to pin 2 as "hot" and pin 8 as "cold".

When a dipole are tuned and in resonance if will have an impedance lower than 100 ohm. When it's out of tune and not resonant if will have several hundred or even thousands of ohm. A 1:4 balun used at the resonant frequency will just reduce the received signal 4 times but out of tune frequencies will work with a better match so it will create a more broadbanded antenna by sacrificing performance at the resonant frequencies.

A OFCD antenna will have one leg in resonance at low impedance and the other leg more or less as a passive element at a higher impedance and will begin to work more like a single element antenna with a counterpose depending of how much offset are used.

/Ubbe

 

[prcguy]

The balanced end should be well isolated from the 50 ohm input so it shouldn't matter. The balun is just exciting the wire at a point where the impedance is close to 200 ohms on all useable bands and at any given point in time the long half of the antenna could have more of a positive going cycle or a negative, its a coin toss.

That's how I plan to do it but have 1 question: does it matter which leg (short or long) is pin 2 & which is pin 8?

 

[prcguy]

I don't agree with the statement below. What is really happening here is the 133ft long wire is resonant at around 3.55MHz as a fundamental half wave and also resonates on all harmonics like 7.1, 10.65, 14.2, 17.75, 21.3, 24.85, 28.4, etc. If you feed this 133ft wire in the center with 50 to 75 ohm coax you will have a good match on 3.55 and odd harmonics like 10.65, 17.75, etc. On even harmonics the impedance will be very high in the couple thousand ohm range and not useable if fed with coax.

You can feed this 133ft antenna in the center with high impedance ladder line on all bands with a tuner in the shack, or end feed it on one end with a transformer that will match the resulting 2500 to 3200 ohm impedance. In all cases you are exciting a resonant length of wire on all bands mentioned and the entire antenna wire is excited with RF and there are no passive parts of this antenna. The entire antenna is active and resonating on all bands. What you are doing in these various examples is feeding it in different places and accomplishing the same result with the same performance.

The OCFD is just another example of exciting a 133ft long wire resonant on all the bands mentioned above but you are choosing a place along the wire where its close to 200 ohms on as many bands as possible and using a 4:1 balun to provide a good match to 50 ohm coax. The end result is not broad banding the antenna or sacrificing performance, you are just feeding it in a different place. Some important things to consider is the balun should be low loss, it should provide a good match from 50 to 200 ohms across 3.5 to 29.7MHz and be tolerant of potential high voltage if used out of band where its at a high impedance point at some frequencies where you might accidently transmit.

When a dipole are tuned and in resonance if will have an impedance lower than 100 ohm. When it's out of tune and not resonant if will have several hundred or even thousands of ohm. A 1:4 balun used at the resonant frequency will just reduce the received signal 4 times but out of tune frequencies will work with a better match so it will create a more broadbanded antenna by sacrificing performance at the resonant frequencies.

A OFCD antenna will have one leg in resonance at low impedance and the other leg more or less as a passive element at a higher impedance and will begin to work more like a single element antenna with a counterpose depending of how much offset are used.

/Ubbe

 

[K8BBL]

OK, on the continuing saga of my OCFD antenna. Measuring the wires: look at this diagram and see if you can make sense of it.
The top drawing shows A-F as the total wire length with the required length being A-E. The distance E-F is about 6" and used to tie the wire to the insulator. The wire is stripped at E & F and they are joined. E-F is just a few wraps thru the insulator then joined back on itself.
The bottom drawing shows C-G as the required length and approx 6" of the required length is used to tie to the insulator.
Is either one correct, or should I be measuring between different points or should I simply run thru a tuner so it won't matter anyway?

 

[prcguy]

When a wire doubles back on itself it cancels out most of the section that is doubled back. So if your HF wire is supposed to be 64ft 3in cut it long enough so that the loop through the insulator is around 64ft 3in even though the total length will be several inches more. Actually its best to make it a foot longer then use a Ty-Rap to temporarily hold the wire at the insulator, then measure resonant freq and shorten the wire as needed to reach your target freq.

If your making an 80-10m OCFD I would say the target freqs are around 7.125MHz on 40m and 14.250 on 20m and that will give a good balance across all the other bands. If your using the MyAntennas formula of the feedpoint being at the 19%/81% junction then start with the overall length of 133ft, add maybe 2ft to each end for 137ft total, cut that at the 19%/81% point and install your balun and insulators. Then hang it up and measure the resonant spots on 40 and 20m. They should be low, maybe below 7MHz and at or below 14MHz. Then trim the exact same amount off each end until the entire antenna has the best match around 7.125 and 14.250Mhz. You can cut off three or four inches off each side to start with then go to smaller cuts when it gets close.

View attachment 106172


OK, on the continuing saga of my OCFD antenna. Measuring the wires: look at this diagram and see if you can make sense of it.
The top drawing shows A-F as the total wire length with the required length being A-E. The distance E-F is about 6" and used to tie the wire to the insulator. The wire is stripped at E & F and they are joined. E-F is just a few wraps thru the insulator then joined back on itself.
The bottom drawing shows C-G as the required length and approx 6" of the required length is used to tie to the insulator.
Is either one correct, or should I be measuring between different points or should I simply run thru a tuner so it won't matter anyway?

 

[Viking1]

Question for the wizards: I am planning to put up an HF dipole using wire between the trees. Due to various issues this is pretty much the only antenna option. The three points (center & two ends) will be almost even/flat as measured to the earth. The problem is that the three points are in an unequal triangle (as viewed from the sky), not a straight line. Will this cause me problems? View attachment 105716

It will work, it will just change the impedence and make it a bit directional. I've made worse work just fine.