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Monday, March 27, 2017

Using HFTA for Site Evaluation - Part 3

This is the third and final installment of the article by Charlie, N8RR, on how to use HFTA software.


Other HFTA Studies/Uses

Over the years since discovering HFTA, I have reviewed dozens of sites. 

WT8V:  Bart has many friends in the Middle East.  His large triband yagi at 60’ on a nice ridge-top site was consistently down in signal strength on 20M about 10 to 20 dB from WD8CCC to 9K2GS, with both running equivalent power.    This was much more than the gain differential between the antennas. 

An HFTA review of Bart’s site showed a significant degradation of the main lobe amplitude looking at 40 degrees, while the WD8CCC main lobe was the beneficiary of major terrain enhancement.   Ben, WD8CCC, has long had one of the outstanding beacon signals out of eastern North America. 

HFTA was used to identify the potential terrain factor that was impacting performance at WT8V, and to evaluate optional tower sites/antenna heights to  achieve better results.    It was necessary to move Bart’s new tower location 600’ west of the existing tower site to remove the terrain feature that was degrading performance from the signal path, but the new site worked.   Bart had the opportunity to do A versus B tests on 20M and the antenna performance on the new tower, especially to the Middle East area,  was far superior to the tribander, much more than could be accounted for by the antenna gain differential. 

The tower height was optimized for 20M, and later the tower was modified to optimize the height of a JK 404 Grande 4 element 40M monobander on Bart’s  narrow ridge top.   Bart considered stacking two of these JK antennas, but HFTA predicted a single antenna at 70’ on the optimized tower site would equal the stacks on the non-optimized tower site, the only place where such a tower could be guyed.  Currently, Bart just about owns 40M as can be heard by several You Tube videos from the ME and EU on the net.  Here is a recording of WT8V on 40M by 9K2GS.

K8KT:   My friend Karl K8KT has always wanted a huge 40/20M antenna array and decided to install one.  Karl, who lives in a beautiful WV hillside location, has the most challenging terrain we have encountered on an HFTA evaluation.   Truthfully, this was a case where the terrain was so severe, HFTA did not predict the results Karl got with his new antenna; HFTA understated  the performance.  The software did get one thing right, it said for any of Karl’s potential tower sites that were considered,  the higher the antenna the better.  All of the potential base sites were substantially below the surrounding hill tops and were actually located on the side of a hill.

A 150’ self-supporting tower was installed, with a JK 6 element 20M monobander at 160’ on top of the mast and a JK 404 Grande 40M monobander at 150’ on the mast bottom.  These antennas are high enough to see over top of the blocking ridges.  It is clear these antennas are performing at a high level, again based on the recordings of Karl’s signal that are available on the net.  Here is a recording of K8KT and WT8V on 20M in the Middle East.

Karl’s case illustrates the difficulty of modeling the antenna performance in the presence of a severe up slope condition using HFTA.   In Karl’s case, heroic (and expensive) steps were taken to successfully mitigate a severe terrain issue.   HFTA did predict improving performance with height, but significantly understated how good the performance would be.   Not many folks are going to do what K8KT did to overcome a terrain situation.   

General Conclusions Regarding Terrain

HFTA is useful when you have irregular terrain features and options for deploying antennas.   It can be useful for minimizing negative terrain impacts or maximizing favorable terrain features, but only if you have options for placing the antenna. 

Here are some general statements which I believe are supported by experience and HFTA results. These factors can be considered when viewing a site and placing antennas, where there are any options.  Obviously, if there are no options, you make do with what you have.

Sloping Ground: Sloping ground toward the desired direction lowers the elevation angle of the main lobe and provides favorable DX performance in the down slope direction, compared to a flat land antenna at the same height.  This effect can be significant. 

If on a hilltop or plateau, the farther back on level ground from the cusp of the hill the antenna, the less the terrain advantage becomes, and the more height is required to maintain relative performance.  At some point it becomes like a flat land antenna.   The antenna really needs to "see" the down slope in the foreground. 

Any up slope in the terrain immediately in front of the antenna shifts the elevation pattern higher and degrades performance compared to a flat land antenna.  It does not take much up slope for HFTA to predict this performance shift. Generally, if facing a significant up slope in the desired direction, low angle performance can be significantly impacted on an HFTA model.  Going higher with the antenna can compensate somewhat, but cannot completely overcome the up slope degradation.   There is a possibility HFTA results may overstate the negative impact of the up slope in some cases.  See the K8KT discussion above.

If you are looking up hill in a direction, I think you can expect performance in that direction to be down compared to a terrain neutral direction and significantly down from a down slope direction in most cases.   With that said, you will still get out and make Q’s in an up slope direction. 

The optimum location for an antenna in a down slope situation in a given direction is  on the slope, perhaps just past the cusp of the hill.  W3CRA proved this by experimentation back in the 60's.  See "Station Design for DX", a four part article beginning in September 1966 QST.
The link is for Part 1.  From the above site, you can link to the other three parts.

A very good description of W3CRA’s terrain situation, including an HFTA analysis, is provided by Bill Tippett, W4ZV here:

If the antenna can't be on the slope, or to minimize the uphill effect in the reverse direction, put it as close to the cusp of the hill as possible.   To get terrain enhancement, the antenna needs to illuminate the ground slope. 

Although HFTA does not do vertical polarization, my experience is that having a vertical over the down slope in the desired direction enhances performance.  My 4 x 160M inverted L antennas are positioned in each hill quadrant to take advantage of this effect. I usually see several dB of increased signal in the hill slope direction compared to the other antennas.   Sometimes this effect can be dramatic when switching transmit antennas.  

Regarding height over ground, generally higher is better over flat ground or in an up slope situation.  In a really favorable down slope situation, it is easy to be too high.  HFTA predicted this at my QTH.

In a favorable terrain situation, optimum height can be considerably lower than for a level ground installation.

On some hilltop sites, such as mine, stacking is not beneficial according to HFTA.    Due to the poor performance of the higher antenna, the overall performance of a stack is degraded.  On some of the other hilltop sites evaluated, such as WD8CCC, stacking 20M monobanders at 100’ and 50’ worked out.  However, HFTA shows the optimum height on 20M for the WD8CCC hilltop is 70’.  Coincidentally, Ben has a 4 element Steppir antenna on a 70’ tower.  HFTA predicts little if any difference between these two antenna systems, and Ben reports there is seldom any detectable difference in A versus B switching.     The steepness of the slope and the distance to other terrain features seem to determine whether stacking is beneficial or not. 

Overall, I think HFTA despite the limitations was a helpful tool to use for optimizing the antennas on my hilltop site.  In general terms, the antenna performance in the favorable directions is in agreement with HFTA predictionsAfter almost 8 years of DX chasing here, I also conclude the performance in the problem directions is better than the software predicted.  The 12 and 10 meter bands seem most affected in the HFTA models and also in real life compared to the other bands/directions, the difference being I have to wait longer for the prop to favor me, or wait longer in the pileups. When the propagation is marginal, is when the difference between a problem direction and an enhanced direction is most apparent.  I can live with the situation.   

The optimizing work is unfinished.  Several other improvements are planned.  Stay tuned!  73, Charlie, N8RR

Post written by: Charlie, N8RR

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