OCF question

[jazzboypro]

Hello all,

I currently have an EFHW from myantenna, the antenna works well and I am pleased with it but unfortunately I can’t use it on 80 meter (I knew it from the start so it’s not a complaint about this antenna). An external tuner does not appear to be the solution got this antenna to work on 80.

Last week I had a discussion with a friend and he is using an OCF dipole that covers 1-54 and he claims that it work well. myantennas sells a transformer for an OCF that covers 1-54 and I have to provide the wire.

The manufacturer recommends to ratios for the wire:

33%/67%
20%/80

I would like to know is one ratio is better than the other.

Many thanks.
73 de VA2FCS

 

[AK9R]

Did you ask myantennas?

I tried an OCFD many years ago. The manufacturer recommended that it be mounted at least 32 feet above ground. I couldn't get it that high. The antenna performed poorly. When you ask myantennas about the lengths, ask them about the mounting height, too.

 

[prcguy]

Hello all,

I currently have an EFHW from myantenna, the antenna works well and I am pleased with it but unfortunately I can’t use it on 80 meter (I knew it from the start so it’s not a complaint about this antenna). An external tuner does not appear to be the solution got this antenna to work on 80.

Last week I had a discussion with a friend and he is using an OCF dipole that covers 1-54 and he claims that it work well. myantennas sells a transformer for an OCF that covers 1-54 and I have to provide the wire.

The manufacturer recommends to ratios for the wire:

33%/67%
20%/80

I would like to know is one ratio is better than the other.

Many thanks.
73 de VA2FCS

How long is

Hello all,

I currently have an EFHW from myantenna, the antenna works well and I am pleased with it but unfortunately I can’t use it on 80 meter (I knew it from the start so it’s not a complaint about this antenna). An external tuner does not appear to be the solution got this antenna to work on 80.

Last week I had a discussion with a friend and he is using an OCF dipole that covers 1-54 and he claims that it work well. myantennas sells a transformer for an OCF that covers 1-54 and I have to provide the wire.

The manufacturer recommends to ratios for the wire:

33%/67%
20%/80

I would like to know is one ratio is better than the other.

Many thanks.
73 de VA2FCS

A resonant 80m OCFD like MyAntennas (or I) would make is about 133ft long and the 40m version is about 64ft long with 4:1 current balun. Basically the same length as a resonant EFHW and I believe you have a 64ft 40m version. The MyAntennas transformer that covers 1-54MHz is a 4:1 and they say it can use either a 33/67% or 20/80% ratio but that means you need a full resonant 1/2 wavelength of wire at the lowest frequency and you place the transformer at the suggested ratio points to resonant on many different bands. Just because the transformer is rated 1-54Mhz doesn't mean your antenna will, that is still going to be a function of its length and whatever harmonically related bands you might get from that length of wire.

I would use whatever ratio MyAntennas uses (20/80%) because they get more bands from an OCFD than any other company I have seen. My everyday antenna is a MyAntennas 80m OCFD at about 30ft high and its a great antenna, no tuner needed on any band 80 through 10m but its 133ft long and very hard to fit on a big city lot and I think that might be why your looking at other antenna options.

 

[jazzboypro]

How long is

A resonant 80m OCFD like MyAntennas (or I) would make is about 133ft long and the 40m version is about 64ft long with 4:1 current balun. Basically the same length as a resonant EFHW and I believe you have a 64ft 40m version. The MyAntennas transformer that covers 1-54MHz is a 4:1 and they say it can use either a 33/67% or 20/80% ratio but that means you need a full resonant 1/2 wavelength of wire at the lowest frequency and you place the transformer at the suggested ratio points to resonant on many different bands. Just because the transformer is rated 1-54Mhz doesn't mean your antenna will, that is still going to be a function of its length and whatever harmonically related bands you might get from that length of wire.

I would use whatever ratio MyAntennas uses (20/80%) because they get more bands from an OCFD than any other company I have seen. My everyday antenna is a MyAntennas 80m OCFD at about 30ft high and its a great antenna, no tuner needed on any band 80 through 10m but its 133ft long and very hard to fit on a big city lot and I think that might be why your looking at other antenna options.

Since their 8010 EFHW is 133 feet long i assume an OCF for the same bands would be the same length. In my case the advantage of an OCF is that one leg would go to a tree in front on my house and the other leg would go in the back yard. I might the able to run 133 feet of wire that way. But the longer leg would not be in a straight line and the antenna would not be 30 feet high. Worse case it does not work well and i put back the 4010.

 

[prcguy]

Since their 8010 EFHW is 133 feet long i assume an OCF for the same bands would be the same length. In my case the advantage of an OCF is that one leg would go to a tree in front on my house and the other leg would go in the back yard. I might the able to run 133 feet of wire that way. But the longer leg would not be in a straight line and the antenna would not be 30 feet high. Worse case it does not work well and i put back the 4010.

My 133ft OCFD is held up with a tower at the transformer and the long end is horizontal at about 30ft and the short end tapers down to about 20ft with a pushup mast. I'm very happy with its performance and an EFHW of the same length would have been a challenge getting the coax back to the house.

 

[jazzboypro]

My 133ft OCFD is held up with a tower at the transformer and the long end is horizontal at about 30ft and the short end tapers down to about 20ft with a pushup mast. I'm very happy with its performance and an EFHW of the same length would have been a challenge getting the coax back to the house.

In my case i would remove the 4010. The OCF transformer would be at the exact same place so i won't have to replace/move the coax. I will take some measurements to see which of the two ratios is the easiest to implement. I would have bought their OCF-8010E-3K but it's out of stock. I sent them a e-mail to see when it will be back in stock.

 

[popnokick]

Some OCF examples, SWR plots, and application notes from Palomar Engineers that may be of some help -
Off Center Fed Dipole Notes - Palomar Engineers®

 

[K6GBW]

If you decide to go with the OCFD just know that you almost certainly will have to manage common mode current on the coax. Making sure the coax is grouded and choked before it enters the house will be important. Also, there's a strong possibility that you will need an external tuner to make it work. I ran an OCFD and it worked fairly well, but it constantly tripped my GFCI circuit breakers. Eventually, I went back to a dipole and found it to be a lot less problematic.

 

[vagrant]

The mention of an RF choke is a very good point. I use one at the feed point which significantly reduced around 2 S units of RX noise on some bands. Others have installed an additional one before it enters the shack to reduce common mode current with their OCFD. I use one of the MyAntenna RF chokes.

 

[popnokick]

In any case with proper choke baluns such as those from MyAntennas or Palomar (which you plan to install), you will not find an OCFD any more difficult to manage than your current EFHW. And as you noted, you can place the 4:1 balun of the OCFD on the support previously used by the EFHW and take advantage of the coax already being there. You’re getting an OCFD to get an additional band (80M). Why be forced to give up anything that your EFHW was getting you? The OCFD will handle all that… and more.

 

[jazzboypro]

Some OCF examples, SWR plots, and application notes from Palomar Engineers that may be of some help -
Off Center Fed Dipole Notes - Palomar Engineers®

Thanks for the info, i will look into it

If you decide to go with the OCFD just know that you almost certainly will have to manage common mode current on the coax. Making sure the coax is grouded and choked before it enters the house will be important. Also, there's a strong possibility that you will need an external tuner to make it work. I ran an OCFD and it worked fairly well, but it constantly tripped my GFCI circuit breakers. Eventually, I went back to a dipole and found it to be a lot less problematic.

I would just be changing the antenna and keep the existing coax. The choke and polyphaser are already in place juste before the coax enters the house. According to MyAntennas no tuner is required. If need be i think the internal tuner of the 7610 will do the job.

The mention of an RF choke is a very good point. I use one at the feed point which significantly reduced around 2 S units of RX noise on some bands. Others have installed an additional one before it enters the shack to reduce common mode current with their OCFD. I use one of the MyAntenna RF chokes.

I did use the rf choke from MyAntennas

In any case with proper choke baluns such as those from MyAntennas or Palomar (which you plan to install), you will not find an OCFD any more difficult to manage than your current EFHW. And as you noted, you can place the 4:1 balun of the OCFD on the support previously used by the EFHW and take advantage of the coax already being there. You’re getting an OCFD to get an additional band (80M). Why be forced to give up anything that your EFHW was getting you? The OCFD will handle all that… and more.

The only drawback is that the OCF will not be installed in an optimum configuration. I guess i will have to try and see what happens.