This page is an update to the presentation given by Peter Hutter, WW2Y at the 1997 Dayton Hamvention Antenna Forum. The material presented here is essentially the same given in the talk, with a bit more emphasis on practical implementation of the antennas.
These patterns do offer increased S/N ratio over simple transmitting antennas such as dipoles and verticals. However, more directive transmitting antennas at WW2Y often out performed single wire Beverages. A more directive antenna was desired; one that would offer more high angle rejection of local signals and improved F/B and F/S ratios to reject storm QRM from the southwest. Several methods can be used to improve performance of single wire Beverages. Victor Misek, W1WCR, describes a "Cone of Silence" mode1 which offers best low angle F/B ratios for Beverage lengths near one wavelength. Also, Brunemeir, KG6RT described a 40 meter Beverage which used reactive terminations to achieve the same goal.2 Elevation null control has been implemented on 1.8 MHz at WW2Y using a 500 foot Beverage terminated with variable resistance/inductance.
Here are the patterns of the broadside array, compared to a single wire Beverage:
The broadside array offers approximately 15 dB of F/S ratio improvement and a narrowing of the azimuth beamwidth. One disadvantage is the large amount of real estate required to implement this antenna. Many people have enough trouble finding the room for one Beverage, let alone a pair spaced 200 feet!
Here are the patterns of the offset array.
As you can see, this offset configuration offers a much smaller rear lobe compared to a single wire Beverage. One disadvantage of this configuration is that the quadrature feed requirement essentially makes this a single band antenna. At WW2Y, the Beverages are fed through a homebrew 90° hybrid similar to the diagram below.
In this system, each individual Beverage is fed through a 9:1 transformer and equal lengths of 50 ohm transmission line are brought to the ports of the hybrid splitter. For readers wishing to use different impedances, or implement this antenna on bands other than 1.8 MHz, additional design information for hybrid splitters can be found in ON4UN's Low Band DXing book.3
|
|
|
| Requires large amount of real estate | Real estate requirements similar to that of single Beverage |
| Easy to feed | Feed system more complex |
| Narrowing of azimuth beamwidth | Azimuth beamwidth preserved |
| Improved F/S ratio. 3 dB gain improvement over single Beverage | Improved F/B ratio, 3 dB gain improvement over single Beverage |
| Performance improvement realized over large
bandwidth - eg: 1.8-3.5 MHz |
Primarily a narrowband system - 1.8 MHz only |
If you feed these elements in quadrature with a binomial current distribution, (1:2:1) some amazing patterns result.
The binomial feed system for this antenna is quite complex and definitely limits this antenna to single band implementation. A description of the feed method is beyond the scope of this paper. Interested readers are referred to ON4UN's Low Band DXing book for information on one type of binomial feed.4 The binomial feed was implemented differently at WW2Y.
WW2Y has used this antenna toward Europe since late 1996. The performance has been extremely satisfactory. For example, during the 1997 CQ 160 CW contest, a storm in the southeast US plagued many northeast US stations with high QRN levels. The QRN from the storm was effectively nulled out with this antenna. Sometimes, we think the antenna works too well as many western stations are almost inaudible on this antenna. Many times while we have all antennas pointed toward Europe, a W6 will start CQing on our frequency, and we will not notice him until a different receive antenna is switched in. The bottom line is that although the implementation of this antenna required a large amount of work, a better European receiving antenna has never been had at WW2Y.
There are at least three different ways to implement endfire arrays of Beverages that will work on 2 bands. None can really be considered optimum for all situations, so you have to make the tradeoffs that work best for you.
The simplest system is to offset the Beverages 1/4 wavelength apart on 160 and feed them through a 0° splitter such as a Wilkinson Power Divider with an extra 90° (on 160) delay line on the forward Beverage. This gives extra front to back ratio on 160, and on 80 provides enhanced rejection off the sides and at high angles, but not off the back.
If you want enhanced front to back on both 160 and 80, then you have to compromise on the offset, using something in the neighborhood of 100 feet, or about 3/16 wavelength on 160. The pair of Beverages can then be fed via a single coax feedline and a broadband 90° hybrid. A broadband 90° hybrid, consisting of cascaded hybrids is described in a paper by R.E. Fisher.5 This single-feedline system can potentially save money on coax, and may be the best option for the single operator station, who may not need to use it on two bands at the same time.
With a 100 foot offset, you can also split the output of each Beverage first, and run one output from each antenna into single band 90° hybrids, and run 2 feedlines to the shack, one for 160, and one for 80. Having separate phasing networks for each band allows you to optimize phasing and amplitude on each band independently for best front to back ratio. This could involve adding additional delay lines or tweaking the hybrids for amplitude balance. An example would be to put an additional 22.5° delay line on the 160 hybrid to get 112.5° phasing. Having separate feedlines for 160 and 80 would be valuable in a multi-station environment.
The ultimate Beverage array would be a pair of end-fire offset Beverage arrays in a broadside array (four total Beverages). This gives you improvement in both the side and rear lobes. The real estate to implement this array is not available at WW2Y, but it sure would be interesting to try somewhere.
See you all on TOPBAND.