Off-center fed dipole offers significant performance boost to weather radios

[KB2GOM]

I've been writing a piece on NOAA Weather Radio for swling.com. Part the story involves comparing the sensitivity of various radios capable of receiving NWR, including scanners, ham hts, consumer radios, etc.

My SDS200 was the hands-down winner, with the ability to receive six local NWR stations, thanks to the recommendation of popnokick, who some time ago, suggested I build this: Homebrewed Off-Center Fed Dipole - The RadioReference Wiki I did so, hooked it up to my SDS200, and haven't looked back.

Then I wondered: what would happen if I connected that antenna to my Uniden BC125AT or my Icom V80? They both have BNC antenna connectors. Short answer: both were boosted from receiving three local NWR stations to receiving six local NWR stations.

So, if you want to hear more NWR stations or your reception is marginal, it might be worth giving Homebrewed Off-Center Fed Dipole - The RadioReference Wiki a try. I built the wire version with a modest investment in time and money . . . and a bit of help from a ham friend.

Perhaps some of the folks reading this can suggest ways to connect this antenna to NWR-capable radios that do NOT have BNC connectors.

 

[900mhz]

since the design calls for RG6 cable (75ohm) CATV coax, I would install a F-connector at the termination near the radio and use an appropriate adapter. I have a Midland weather radio that has a RCA jack for the external antenna connection on the radio. If your radio has a BNC jack, use a F to BNC adapter. By having several adapters handy, this antenna could be swapped to different radios with ease.

 

[prcguy]

What are you comparing the off center fed dipole to? What is it working better than? Or is it the only antenna you have tried?

I've been writing a piece on NOAA Weather Radio for swling.com. Part the story involves comparing the sensitivity of various radios capable of receiving NWR, including scanners, ham hts, consumer radios, etc.

My SDS200 was the hands-down winner, with the ability to receive six local NWR stations, thanks to the recommendation of popnokick, who some time ago, suggested I build this: Homebrewed Off-Center Fed Dipole - The RadioReference Wiki I did so, hooked it up to my SDS200, and haven't looked back.

Then I wondered: what would happen if I connected that antenna to my Uniden BC125AT or my Icom V80? They both have BNC antenna connectors. Short answer: both were boosted from receiving three local NWR stations to receiving six local NWR stations.

So, if you want to hear more NWR stations or your reception is marginal, it might be worth giving Homebrewed Off-Center Fed Dipole - The RadioReference Wiki a try. I built the wire version with a modest investment in time and money . . . and a bit of help from a ham friend.

Perhaps some of the folks reading this can suggest ways to connect this antenna to NWR-capable radios that do NOT have BNC connectors.

 

[900mhz]

What are you comparing the off center fed dipole to? What is it working better than? Or is it the only antenna you have tried?

I was going to make a response comment. But I do remember there are language interpretation issues.

 

[KB2GOM]

What are you comparing the off center fed dipole to? What is it working better than? Or is it the only antenna you have tried?

I was comparing to a Diamond 77 on the Uniden 125AT and a commercial high-gain 2 meter antenna on the Icom V80.

 

[vagrant]

Which antennas are being used outdoors?

 

[KB2GOM]

Which antennas are being used outdoors?

All indoors within 18 inches of each other.

 

[ladn]

I have two versions of the Wiki OCF dipole. One made from anodized aluminum tubing and another from stranded, insulated wire.

The tubing version is in the rafters of my garage and feeds a BC760XLT with mainly VHF aero and a few UHF LE frequencies. Performance is very good for what I'm monitoring.

The wire version is in my office and feeds a BC250D with VHF Lo/Hi, Aero VHF, Aero UHF, UHF and 800. It's pinned to the wooden paneling on the wall at about eye level. I'm pleased with the reception across all bands, except 800 (LA City FD) which is a bit spotty.

 

[burner50]

With you testing methodology, it would invalidate your results...

Changes in the antenna feed system alter the test parameters an a rather material way... Especially if you try to start changing connectors or using adapters.

 

[KB2GOM]

With you testing methodology, it would invalidate your results...

Changes in the antenna feed system alter the test parameters an a rather material way... Especially if you try to start changing connectors or using adapters.

Please explain.

Both radios have BNC connectors. All three antennas (including the off-center fed dipole) have BNC connectors. The off-center fed dipole has a feedline with a BNC connector; the other two antennas have direct connection to the radio through the BNC connectors without a feedline. No adaptors were used.

It is true that the off-center fed dipole has a feedline, but it seems to me that it would be impractical to use that antenna in a vertical orientation without a feedline.

The procedure was to turn on the radio, step through the NOAA weather radio presets, count the number of NWR stations that could be heard, then change antenna step through the NOAA weather radio presets, count the number of NWR stations that could be heard. The results were that more NWR stations could be heard with the off-center fed dipole.

Given that the NWR stations remain in the same location, transmit at constant power, and are not moving, and that the receivers are in the same location and not moving, and that I did not observe any tropospheric ducting, and I am not sure what variable I have not accounted for.

It seems to me that since the radios could receive twice as many NWR stations from the same location with the off-center fed dipole, that is a significant result.

 

[burner50]

You were asking for advice on connecting to NWR receivers that *do not* have BNC connections.

Sorry, That's what I was referring to.

 

[KB2GOM]

You were asking for advice on connecting to NWR receivers that *do not* have BNC connections.

Sorry, That's what I was referring to.

Right. Point taken.

 

[900mhz]

For those on a tight budget, this antenna definitely meets that threshold.
An adapter at the end of the coax will not have any impact on its operation.

 

[Ubbe]

One leg of that OFD are tuned to 170MHz and the other to 60MHz if they are 1/4 waves. The 300 ohm to 75 ohm balun will not work at it's optimum at those frequencies as the dipole will have a 70 ohm impedance. But when going off the tuned frequency the antenna will begin to raise it's impedance and the balun starts to work better and the antenna will work more like a broadband antenna with a less peak at its tuned frequency.

When the elements lengths match more to a 5/8 wave at 150MHz and 425MHz it will also have a higher impedance more closely to the baluns.

The balun works much like in the ST-2 antenna to adapt to the different impedances to create a broadband antenna. A standard 1/2 wave dipole would perform better at its tuned frequency but wouldn't be as good when going off frequency.

/Ubbe

 

[KB2GOM]

One leg of that OFD are tuned to 170MHz and the other to 60MHz if they are 1/4 waves. The 300 ohm to 75 ohm balun will not work at it's optimum at those frequencies as the dipole will have a 70 ohm impedance. But when going off the tuned frequency the antenna will begin to raise it's impedance and the balun starts to work better and the antenna will work more like a broadband antenna with a less peak at its tuned frequency.

When the elements lengths match more to a 5/8 wave at 150MHz and 425MHz it will also have a higher impedance more closely to the baluns.

The balun works much like in the ST-2 antenna to adapt to the different impedances to create a broadband antenna. A standard 1/2 wave dipole would perform better at its tuned frequency but wouldn't be as good when going off frequency.

/Ubbe

Thanks for the info. All I know is that it seems to work pretty darn well when attached to my Uniden 125AT or SDS200.

Cheers, Jock