80 Meter Buddipole Dipole
What happens if we take our Buddipole parts and make a dipole on 80 meters. This is now possible due to the new large coils and the TRSB (Triple Ratio Switch Balun), combined with the new long 9.5 whips (or the previous mil whips).
Some have said that this short loaded dipole is too small and not efficient. I did some experimenting with a similar setup several years ago, and here is a bit of modelling that will shed some light on what the issues are.
- support is tripod and 16 foot mast for 18 foot antenna elevation
- ground is high accuracy average (.005,13)
- software EZNec 4+
- loss is aluminum in the horizontal configurations (copper in the vertical due to dominance of counterpoise)
- coil Q approx 300 (arbitrary, don't have real Q data)
- component dimensions are close to Buddipole (models are not exact)
- antenna configuration is 2x22" arms, coil, 22" arm, 9.5' whip (each side) (antenna is approx 32 feet long)
- comparison antenna is 100 foot #14 wire dipole (nonresonant) at the same location
- Buddistick type vertical configuration - base at 10 feet, single sloping #20 radial 65 feet out 2 ft high at end; vertical going up: feed, large coil, 2x22" arm, 9.5 whip
- Buddipole type dipole -1.03 dBi gain straight up (NVIS)
- Wire dipole 2.58 dBi gain straight up (NVIS)
- Buddistick type vertical -1.4 dBi at 50 degrees, very broad lobe 3db down at 105 degrees (NVIS) and 12 degrees (DX), front/back ratio varies - up to 14 dB down
The horizontal antenna patterns appear to be nearly identical standard NVIS type patterns with the main radiation straight upwards and falling off to very little at low DX angles. Very nicely omnidirectional, good for local communications when the direction is not known.
So the Buddipole type dipole on 80 meters is down only about 3.6dB from a sizeable wire dipole in the same location. Not Bad!
The 3:1 SWR bandwidth of the small loaded dipole is about 30 khz. This is similar to a fairly high Q 80 meter mobile antenna, which should be no surprise due to the small size of the Buddipole type loaded dipole on 80 meters.
The similar configuration I field tested some time ago to check into an 80 meter net received excellent signal reports. Due to the high Q and narrow bandwidth it requires a bit of tuning (an antenna analyzer is highly recommended) and you need to know what frequency you are going to use, but the signal is very respectable. With a tuner you can get a bit more bandwidth, but the losses grow as you depart from resonance. This is a very appropriate antenna for local and regional communications on fixed frequency nets, such as emergency services where the portability is useful and there might be a lack of ready supports for a wire antenna.
Note that the TRSB is necessary to make this combination play, the low impedance needs to be stepped up to match 50 ohms. Note also that an antenna analyzer is also pretty much a necessity to maintain sanity while adjusting the resonant frequency. This antenna is narrow and it takes a bit of fiddling to get it resonant at the proper frequency.
Comparing to the Vertical Buddistick
The Vertical Buddistick type configuration for 80 meters (vertical with wire counterpoise) has greater bandwidth due to its larger footprint and greater conductor lengths (approx 50 khz 3:1), but the radiation pattern is quite different. It is much better for operation in one direction (220 degree horizontal beamwidth) and low angle DX operation and it has lower gain in the vertical NVIS local communications sense (-2.7 dBi at 90), but the largest difference is in the directionality. In the configuration modelled there is up to 14 db of front/back rejection which can be quite useful, or it can be a problem depending on the communications goals. For NVIS local communications such as an emergency or area net the drop in gain straight up and at high angles off the back side might be undesirable. (Note that the directionality of this antenna changes significantly with the ground conditions, much of the antenna is the radial and it is very to the ground.)(Note also that the directivity can be changed by bending the counterpoise 90 degrees, this also can reduce the footprint of the antenna to help fit it into a campsite, this was not modelled in this experiment.)
The traditional views of using verticals for DX and low dipoles for NVIS and reliable local net communications seem to apply here as well, even with relatively small loaded portable antennas. Small antennas can be fairly efficient if they use low loss components but the bandwidth is severely restricted - the more efficient the antenna, the narrower the bandwidth, as predicted from the physics.