poor swr

[philn]

I am not a ham and I don't know ham technology so I'll transfer info to him. But I have a blind friend, John, w2oez/4, ham who has an swr problem. Here's his problem as he dictated it to me. His swr is 7/3 at 39.99 Khz. He thinks it's an antenna problem. He has a 132 ft. long, #12 steel wire with insulation, antenna. He has a 100 ft. coax feeding. He has a dipole antenna. What could be causing his poor swr? I think if it is allright with anyone he can call anyone to correspond about the problem or call him at his ham station, w20ez/4. Just make arrangements with me to set this up.

[philn]

I am not a ham and I don't know ham technology so I'll transfer info to him. But I have a blind friend, John, w2oez/4, ham who has an swr problem. Here's his problem as he dictated it to me. His swr is 7/3 at 39.99 Khz. He thinks it's an antenna problem. He has a 132 ft. long, #12 steel wire with insulation, antenna. He has a 100 ft. coax feeding. He has a dipole antenna. What could be causing his poor swr? I think if it is allright with anyone he can call anyone to correspond about the problem or call him at his ham station, w20ez/4. Just make arrangements with me to set this up.

Correction in his call letters. It's not w20ez it's w2oez, w2 ocean easy zebra.

[5B4AJB]

Try replacing the steel wire with copper or aluminium, steel is pretty useless at RF.

Are you sure it's 39kHz and not 3.9MHz??? a 39kHz antenna is a LOT bigger than 132'

Also might be worth checking/replacing the coax for water leakage if it's more than 5 years old...

[philn]

Yes the frequency is 3.9 Mhz. He mentioned something aabout a formula that gives you the length, or 1/2 wavelength, something about dividing with 468 or something. Do you know of any such formula?

[5B4AJB]

Yes, that's it 468 divided by 3900 is 132 or thereabouts, the old formula, for length in feet.

For length in metres, use 300 divided by 3.9

(I think these formulae are correct, someone correct me if I'm wrong)

A point to remember, when cutting dipoles is to make them slightly (5%) longer. When you finally tune them up, you can always cut a little off either end if the SWR is still too high.

[WA9WVX]

Hello Phil,

It's OK to use insulated Steel wire for a Dipole antenna, in fact many Ham operators use antenna support structures, i.e. example Steel Towers and/or Copperweld Wire which is a combination of Steel Wire and a Copper coating applied during the manufacturing process. Copperweld has been available for 70 to 100 years used by railroads, utility companies (for the power grid / transmission lines), shortwave broadcast stations, the military and amateur radio operators (for non-stretchable antennas wires).

As far as a Poor VSWR at operating frequency 3.9 MHz or 3900 KHz depending on the way you want to view the frequency reference and using the 468 formula, the over all lenght of a Half Wave Dipole should be 120 feet +/- .25%. The Rule of Thumb for every frequency using this formula essentially dictates as frequency increases, the lenght of the Dipole decreases or as frequency decreases the lenght of the Dipole increases.

A 132 foot Dipole transmitting antenna will resonate at 3.5 MHz or 3500 KHz with a 1.1:1 VSWR but will have an extremely High VSWR at 3.9 MHz or 3900 KHz. Where as a 120 foot Dipole transmitting antenna will resonate at 3.9 MHz or 3900 KHz with a 1.1:1 VSWR but will have an extremely High VSWR at 3.5 MHz or 3500 KHz. The only method using a Dipole transmitting antenna covering the entire 80 meter bandwidth amd keeping the VSWR at a perfect 50-OHM match is to use an antenna tuner in the feedline to the antenna. The configuration is connected as follows, H.F. transceiver output to a Low Pass Harmonic Filter, then into a Wattmeter / VSWR Bridge, then into the Antenna Tuner / Matchbox and the Antenna Feedline Coaxial Cable or Open Ladder Line to the Antenna.

Mounting the 80 meter Dipole should be roughly 50 feet or higher above Ground Level for what is considered the transmitted RF “Take-Off-Angle” used for communicating with DX stations. Make absolutely sure that the feedline connections are well soldered and sealed with Coax-Seal and a good grade of 3M electrical tape so no corrosion can exist.

73,

Dan
WA9WVX