Question on 80m wire antenna

[Anthony Rizzolo]

I’m building (or trying to) an 80m dipole antenna.  I have a 20 ft mast that I put the center of the dipole on.

I am currently seeing a plot on the antenna that has the SWR at about 5 at the bottom of the 80m band and then it slopes down to about 1.5-2 in the end of the band.  It is probably about a 4.5 at the middle of the band.

Now I noticed something strange these are the SWR’s when the antenna is stretched out at length and hung about 15′ up in two trees at the far end of the about 67′ length.

I had taken the one end down and the SWR went to 2.0 in the middle of the band but the second I start to raise it to height it goes right back up to 4.5.

So my question is, is this telling me that the antenna wire is too long or too short for 80m.  80m is the band its tuning to because the other bands all have much higher SWR’s.

Also, what might explain the behaviour when the antenna is closer to the ground, I would have thought that might raise the SWR so was surprised when it went down.  Also, could the 20ft mast not have the antenna high enough?

I think for first though I would like to know if I cut the wire too short or if its too long.

Thanks,

Anthony Rizzolo/KC1DXL

 

[Fred Kemmerer]

Hi Anthony,

First, it’s very difficult to get a Dipole to cover the entire 80m band. It’s pretty typical that a dipole on this band might cover about 100 KHz or so of that band.

When you tune the antenna, first put it up and run an SWR curve and the height that you’ll be using it at. The lowest SWR frequency is where the antenna is resonant at. If that’s at the high end of the band, then your antenna may be too short if you want to use it for CW. Here are some suggested spots to tune for –

• For SSB – Tune to 3.795 center frequency
• For CW/Digital – tune for 3.550 Mhz or so

It’s hard to get an 80m dipole high enough. Ideally, it would be up about 65 ft but don’t worry too much about that. Just get it as high as you can.

I hope that this helps.

[Anthony Rizzolo]

Thanks Fred…I will try what you suggest…Just to clarify on the Antenna analyzer I set the center frequency for the test to 3.795 since I will be using it for SSB only right now and see what the SWR is through the center frequencey then.

The SWR is actually lower at the high ends of the band so I guess thats where I want it  to be, so might be the correct length for SSB.

Tony

 

[Nashua Area Radio Society]

I will assume that your exploring your options with an Antenna modeling program such as EZnec before constructing the antenna. I’ll list a few generalized comments that may be helpful in understanding some of what your seeing.

1) VSWR is never constant across any Amateur BAND (e.g. 80/75M, 40M, etc.) for almost all antenna designs.  The HF band in question is 500 Khz wide with a center frequency of 3.75 Mhz (or about 13% of the center freq).

2) Height above ground will change the antenna’s impedance and resonant frequency (as well as radiation pattern).  Height is usually evaluated in wavelengths ( 1/8, 1/4, 3/8, 1/2…..  of the average antenna height).  The variations look like a dampened sinewave that eventually becomes a constant as you approach free space conditions.  This pattern is sometimes drawn as a spiral in the ARRL handbook and antenna books.

3) Standing Waves always result when the antenna’s impedance does NOT match the transmission line impedance.  An antenna matching unit can correct the SWR that the radio will “see”, BUT the standing waves still exist on the transmission line between the antenna and the ATU, so higher transmission line losses (increased heating) will be sustained in that line segment.  Note: These losses can be minimized by locating a REMOTE ATU as close to the antenna as is practical.

4) The heights described are an extremely small percentage of 1 wavelength, making this design a NVIS (Near Vertical Incident Skywave) antenna – aka a Cloud Burner.  The antenna pattern is largely almost straight up and the radiation pattern is almost OMNI-DIRECTIONAL.  E.G. there is a lot of radiation even along the end to end axis of the wire.  QST ran an article on NVIS antennas several years back and discussed how very low height antennas could serve very well during a natural disaster event where good LOCAL (200 mile radius) “omnidirectional” communication served best for reaching local responders.

5) The part of the antenna with the largest current flow will have the greatest effect on antenna performance (radiation angle and pattern)… usually getting this “as high as practical” is a good practice.   For a dipole, this the CENTER feed point.  Try supporting the center feed point higher up from a tree, tower or other available anchor point.  The ends can be lowered some and secured to some other lower anchor points (think of the inverted Vee antenna used for 40M on field day.  Lowering the ends of the dipole into the Vee shape will also provide a lower antenna feedpoint impedance and thus better VSWR results.

6) Trim the antenna for resonance around the frequency where you expect to operate the most… CW segment or Phone segment?  Make the best compromise possible during the adjustment phase.

7) Consider other antenna styles for your space: loops: delta loop; end fed L or sloper antennas.  Every situation is different and you’ll need to find a good on that works at your location.

Last:  Don’t be afraid to experiment – that how we all learn – and ANY ANTENNA is better than NO ANTENNA at all!  One this is almost certain: It won’t be the last piece of wire you ever put up.

I hope this helps answer some of your questions, and perhaps gives you a useful idea or two.

Mike

 

[Anthony Rizzolo]

Thanks Mike…no didn’t use EZNEC…the copy that I have claims it has a virus and it won’t install.  Perhaps I should do that as well, just figured a wire antenna was simple enough that the length and height were the most important things and I could trim it by tuning it with the Antenna Analyzer, perhaps I am nieve then.

I will see if I can get a copy of EZNEC and see what it says.

Thanks,

Tony/KC1DXL

 

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