The Vertical Budd-I-Pole Antenna
Vertical Dipoles are very effective, but mechanically somewhat unwieldy due to their height. Various techniques are used to reduce the height, but many of these techniques reduce efficiency and bandwidth, or are mechanically complex. The Vertical I-Pole provides a solution to the height problem without significant loss of bandwidth or efficiency and retains mechanical simplicity. This result is realized by using capacity hats on both ends. Using single conductor hats maintains the mechanical simplicity required for straightforward field deployment without significant impact on omnidirectionality. This page describes the Buddipole version of a Vertical I-Pole and some variants. The larger low frequency wire versions are described on the nearby VerticalIPoleAntenna page.
For fullsize resonant operation the width plus height should be a little less than a half wave. A modelling program can be used to fine tune the dimensions in advance. One coil is used to resonate this less than fullsize antenna on 20m, but due to the size of this antenna not much coil is required. For 17m no coil is required. There is only a coil in the lower section, and the feed is low (off center) for convenience. The lower whips should be high enough to minimize coupling to earth and avoid proximity to personnel. Eight to ten feet is the recommended minimum height. Other heights will work. The feedline should be routed off at 45 degrees from horizontal and equidistant from the lower hat whips. An effective balun is necessary to maintain the pattern and avoid common mode currents. The impedance is low so the Buddipole TRSB with 12 or 25 ohm settings works well here.
This Buddipole variant of the Vertical I-Pole was inspired by a posting on the Yahoo Buddipole group by W9ASH:
http://groups.yahoo.com/group/Buddipole/message/13059∞
I did some design and modelling for a slightly different set of components.
Note - I have not tested this yet, please provide feedback if you try it
Example 20 meter Dimensions (approximate, modelled)
- Described from the top down
- Top hat - two Buddipole 9.5 foot whips fully extended
- iTee Coupler in the middle of whips (or shorted VersaTee upside down and 3/8-24 stud)
- Four 22" arms going down
- VersaTee feedpoint, TRSB (balun) set at 12 or 25 ohms (for best swr)
- Small (standard) coil, 0.96uH predicted from model
- 11 inch arm
- iTee Coupler for lower hat whips (or shorted VersaTee)
- Two 9.5 whips fully extended
- Insulating section (with iTee, not required with lower VersaTee), 1/2 IP female threaded on bottom, long 3/8-24 stud on top *
- 8 foot mast
- Tripod
- Guys should be used, antenna is 19 ft tall
- gain 0.6 dBi omni at 20 degrees elevation (EZNec, high accuracy average .005,13 ground, aluminum loss, coil Q 100)
- (note that the top and bottom hats could be regular 6 foot whips instead, of course more coil would be used)
17 Meter Version (approximate, modelled)
- as above but
- shorten hat whips to approx 7.5 feet, adjust for resonance
- short coil (zero turns needed)
- gain 0.9 dBi
* Bottom Support Interconnect
- There are two approaches to this. One is to use another VersaTee here. This provides the necessary insulation fromt the mast. The other is to use an iTee and an insulating section, described next.
- An insulating section can be fabricated from PVC. Drill and tap a hole in a 1" endcap and thread a 3/8-24 bolt from the inside out through the hole (you can drill the hole small and tap with the bolt if you don't have a real 3/8-24 tap). Epoxy in place. Attach this to a 2" piece of 3/4" PVC. Put a 1/2" female IP threaded adapter on the bottom of this 1" piece (I used a 1" coupler and a 1/2" theaded to 1" bushing). This adapter connects from the top of the mast to the vertical arms through the lower hat iTee.
Variations on a Theme
Lou ke4uyp sent me some email about potentially higher gain with the Buddistick configuration, so I did some more modelling. Sticking with the basic pieces employed in the I-Pole I modelled an Inverted Tee configuration. Essentially just rotating one 9.5 top whip straight up, and removing the other. This increased the gain slightly (+0.4 dB to 0.98 dBi at 20 degrees) and the pattern appears the same. This is a bit easier to do mechanically, but in some cases might be less desirable. For example, in military or antenna restricted neighborhood use it raises the height of the top section where it might more likely be seen, or where wind might catch it. But if this is not a concern it takes one less 9.5 whip and is simpler to set up, and lighter. It is almost 30 feet tall!
The Buddistick with a single radial is more like an L, so I changed the model to an L configuration. 9.5 whip out one side only. I left the rest the same as the inverted Tee, so the vertical part was still 11" plus coil & Tee plus 4x22" plus 9.5'. In this configuration the pattern shifts and gain goes up a smidge more, now 0.8 dB higher than the I-Pole (1.4 dBi). There is somewhat more vertical high angle radiation (which picks up more noise also), and the back side is about 3db weaker than forward. Mechanically the L is a bit harder to keep up, with all the weight of the whip on one side only. It still avoids the counterpoise wire which in some cases is convenient. It gives a slight bit of directivity and gain, though in the past I have found this directivity effect is very dependent on the type of ground and does not always pan out.
Now for a more standard Buddistick configuration to compare with, I moved the bottom down to 4 feet with a 16 foot radial sloping from 4 to 2 feet of #20 wire, going up - 11" arm, feed, coil, 2x22 arms, 9.5 foot whip. So this is still a bit larger than a standard Buddistick configuration due to the taller whip. Gain 0.6 dBi at 30 degrees. Not quite as good at DX, somewhat better for local stuff. Approx 5dB front to back. About 6dB down straight up, so it is not really a good NVIS antenna.
Pushing this Buddistick up to 10 feet at the base (radial sloping to 2') brings up the gain slightly to 1.35 dBi, lowers the lobe to 25 degrees, front to back 4dB and otherwise looks pretty identical to the lower version in the paragraph above. Compared to the inverted T there is little difference in the gain. The primary differences are in the pattern and in the mechanical convenience of setup, and the RF and physical safety of people near the antenna. The gain trade is between the back side and higher angles to the front side. Depending on where the noise is coming from, and which directions the useful signals are coming from, either antenna may have advantages.
All of these configurations appear to work nicely. The biggest difference is mechanical, is there a good place to run the counterpoise and keep it both elevated and free of the local fauna? The patterns differ also though the maximum gain does not change all that much, the gain in some directions (back, up) does change substantially. The patterns of the Buddistick and the L configurations were very similar, not surprising since they were both L configurations. Again the mechanical/physical differences may lend themselves better to some installations than others.
If your goal is to work DX in all directions and avoid wires near the ground, the I-Pole or Inverted Tee would be good choices. If you want some higher angles or a little rear rejection and no radials then the L looks interesting. If you don't mind wires near the ground the standard Buddistick has very close to the same performance as the L. For use on the edge of saltwater wires are likely problematic. There the I, inverted T or L would avoid the wires. The front to back ratio of the L might be good to reduce noise from behind, if the mechanical torque from the unbalanced counterpoise can be managed. An I or inverted Tee with a second antenna as a reflector could provide both additional gain and front to back ratio.
References
- VerticalIPoleAntenna larger wire version
- Pneumatic Antenna Launchers∞
- W4RNL Antenna Analysis Papers∞
- AntennaModelling
- www.Buddipole.com∞
Files
| Attachment | Size | Date Added | |
| VertIpoleBP20.EZ | 1.97 KB | 1/12/2007 4:21 pm | |
AlanB, WB6ZQZ
CategoryHamRadio
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