Low Dipole Antenna - How bad is it?

  • During the 2018 SARC FD, CW team tried to setup a "fan dipole antenna" on the very tall water tank.  The antenna resulted in high SWR on the 40 meters and the Ant was ditched.
  • ex: fan dipole cut to 40 & 20m. The insulated wires run nearly parallel, about 4" apart, along the top of a wooden arbor, with supports every 6'.  I don't think the "wood losses" are significant. The antenna is about 9' above pretty good ground (I'm in Portland OR, and the antenna is over a garden patch). I know I won't have a lot of low-angle radiation.  My tuner takes care of any SWR problems. Can someone estimate what my ground losses will be?
    • Ground losses will be very high and you'll be lucky to contact across town.  for 20 & 40, 9 feet above ground is too too low.  Change to a ground mounted vertical is you can't get that any higher, like around 30 ft.  
    • I have had some dipoles for 75, as low as 15 feet. It will work..but your signal will be at half strength.... But you can be heard. 30-40 feet is what you need to be effective.Big difference
    • The tuner may not be taking care of "all" your SWR issues.  It's making your transmitter happy, but unless the tuner is mounted right at the antenna feed point, you may still have losses due to SWR plus line length....unless you're using tuned feeders (open line, ladder line...).
    • I've made consistant weekly contact between VA and MI on 40M in the mornings running 5W with a loaded dipole between 10 and 15 feet off the ground. Sigs were usually about S5.
  • You will notice a BIG difference between the performance on 20m vs. on 40m.  On 40m the dipole will still be reasonably efficient - my guess is no more than 3dB of loss at high angles (but I'll have to check that number).  It won't have much low angle radiation, but the high angle should cover most of the Pacific NW (at least during the day until the critical
    frequency drops below 7 MHz.)  The SWR won't be too bad as the height above ground decreases the ground losses increase to keep the resistance at resonance between about 30 and 60 ohms.  This presumes, of course, that you have pruned the dipole to resonance - the major effect at low heights is that the resonant length is shorter.
    On 20m the efficiency is better but the antenna won't work as well.  This is because 20m is above the critical frequency, so NVIS isn't a useful propagation mode. That is not to say that you can't make any contacts, but that some extra height, or using vertical polarization, probably will make a significant improvement.
  • I can offer some real-world experience on both the low dipole and the ground mounted vertical.  I have a 40-meter dipole configured in a quadrant (see Cebik's web site) at the montsrous height of about 7 feet above ground (in Chicago).  Running 100 watts on CW, I have been able to work dependably out to about 1000-1200 miles on 40 meters during the daylight and early evening hours.  Once the band "goes long" in the evening, it is only good for local QSO's.  Not great, but good for lots of fun.
    I also have a ground mounted trap vertical that covers 40 through 10 meters.  It only has a modest radial system buried under the lawn but it seems to be at least a full S-unit stronger than the dipole on 40.  I am able to work most of the world, depending upon time of day and frequency of course, with the vertical.
    If I could keep only one antenna, I'd stay with the vertical.  Even though it is a noisier antenna, it covers more bands and has a lower angle of radiation.  In my case I was able to install the dipole during a mild winter but couldn't put in the vertical until the ground thawed, so my choices were somewhat externally driven.  This time of year you could probably do whichever you want.  They are both good for a lot of radio fun, depending upon what you want your station to do. 73 es C U on the bands.  Steve  WA9FZB
  • EZNEC has an average gain feature, and I calculated the average gain on 7MHz and 14MHz for this antenna at 9 and at 42 feet.
    You're going to get about 6dB more ground loss on 7MHz than an antenna at 42 feet (this is 1/4 wave higher, approx)
    You're going to get about 2dB more ground loss on 14MHz.
    6dB seems like a lot if you think of transmitter power heating the ground, but don't sweat it too much because it's not a lot in terms of the path loss on a communication link.  You'll make plenty of contacts with it. 
    DX is not going to pour in but on a quiet night it might not be so bad.  People put 160m dipoles at 40 feet all the time, and this is pretty much the same situation.
    Incidentally, -6dB is the average gain that EZNEC report s for a 1/4 wavelength vertical over average ground (with a good ground system.  this is just ground reflection loss)
    So if you put up a vertical, you're still putting 3/4 of your transmitter power into the dirt, but the radiated signal that doesn't warm the earth is at low angles.  
    73, Dan N3OX www.n3ox.net
  • Thanks to everyone. I agree with the comment about NVIS on 20m.  20m used to be my favorite band (with a horizontal dipole at 20' from land, and from a sailboat).   But with current band conditions, and mostly high-angle radiation, it's been disappointing.
    Whereas 40m was working well tonight -- Portland OR to the Mojave desert on 5 watts isn't DX, but it's not bad.  6 dB (ground loss) is _a lot_ of loss, when you're only running 5 watts!   Going from S9 to S8 is less important than going from S7 to S6.  I'll see if a vertical is possible.  Perhaps a Black Widow crappie rod is in my future . . . Thanks again.
  • Charles... If the low dipole is the best you can do on your particular piece of real estate, then so be it.  Very few hams have the luxury of the "perfect" antenna site and the financial capability to build it out.  Having worked with crummy random stealth wires in the past, I feel pretty good about my 70 foot tower and collection of tall pines in the Maine woods.  But compared to the guys who run 9 element vertical arrays or yagis on 80 meters, mine is a peasant station.  Your low dipoles will surprise you at times and disappoint you at others, but you're on the air and you can take comfort that your antenna is probably better than SOMEBODY'S!   I've said it before, but it bears repeating that the antenna you DO put up will be at least 80db better than the one you DON'T.  Just get on the air!
  • I use a vertical on 80M for DX - very simple antenna - costing a grand total of $7.  It works - worked lots of DX on it - including XU, ZL, HL and other neat stuff.  It also loads on 160M-10M.  It can be seen at:
    http://home.comcast.net/~kb0fhp/80M_Vertical/80M_Vertical.htm
    It all depends what you want to do - work the locals (then put up a low dipole) - work DX (then put up a vertical). Any antenna is better than no antenna - just depends on what you want to work.
  • Followup: I just bought 20' of 2" PVC pipe, and raised the dipole midpoint from 8' to 20' above ground. 20m has come alive!  And 40m is much improved.  $15 well-spent.  I'll give it a few more days before declaring victory over earth losses and high-angle radiation. Thanks for the ideas.  That 80m cap-hat vertical is very neat.