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Summary
HFTA is available bundled with the ARRL Antenna Book.
What is HFTA? HFTA or HF Terrain
Analysis is a software program developed by Dean Straw, N6BV when he was the handbook Editor at
ARRL is available bundled with the ARRL Antenna Book.
HFTA models horizontal antennas
(dipoles, simple yagis, stacked yagis) at specified heights over terrain and
plots the modeled antenna gain at different elevation angles.
A description is provided here of how HFTA by N6BV was used to evaluate
and implement antenna options at the N8RR site in West Virginia. Links are provided for other sources
describing the features of HFTA, how to acquire it, and how to interpret the
graphs so this information is not repeated.
Site evaluations for friends were performed using HFTA and some of the
results are described. The photo at the right shows a Google Earth image of N8RR's QTH. You can plainly see his three towers. To get a feel for his terrain, the red roofed house in the upper right is 152-feet below the base of his Rohn 45G tower and is just 447-feet away (at a heading of 43.4°). Click on any photo to see a larger image.
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| N8RR QTH - Red Roof House is 152' below N8RR's Tower |
Based on the experience gained using HFTA, the general
usefulness of the software is evaluated and the limitations are presented. General conclusions about antenna placement
on complex terrain are provided; these are the author’s conclusions only.
This work was not an attempt to scientifically validate HFTA
or the absolute accuracy of the software conclusions. Please see the later section on
Limitations.
A considerable amount of on-site A versus B testing was done
at the N8RR QTH over several years. The
testing was not conducted in a scientifically rigorous manner. The goal of the testing was to satisfy the author
that HFTA predictions of relative performance differences between local
antennas were valid. In nearly all cases, the A versus B tests were conducted
in receive mode. Thousands of tests were done.
HFTA predicts the takeoff angle and amplitude of the
transmitted lobe. Using received signals
may not always provide a true reciprocal comparison. When HFTA predicted an
antenna option would be best in terms of main lobe amplitude at useful angles,
the prediction was considered to be confirmed if the received DX signal was
consistently stronger on the receiver S meter. Signal to noise ratio improvement was not
considered as proof although there were many times when the local noise pickup
profiles of the antennas were drastically different.
Who Should Use HFTA?
Here is a beginner’s guide to using HFTA:
HFTA is useful for those who have irregular terrain
features, such as sloping ground, surrounding hills, etc., and who have options
for placing towers/antennas. If you
don’t have options for changing tower locations, changing antenna height on
existing towers, or stacking antennas on an existing tower, HFTA will provide
no actionable data.
HFTA does provide interesting results. After looking at dozens of sites, and
recognizing DX success is partially psychological, I no longer volunteer to use
HFTA if I think a site might have some issues. What is the point of telling a
friend his existing antenna is performing significantly below par because of a
terrain feature that he can do nothing about? DX can be worked from just about any
location. Operators should focus on that
and not conclude, based on a software prediction, their antenna is bad and thus
it is not worth trying to work DX.
The opposite is true if I think a location has terrain
enhancement. Sometimes I can convince a
friend his location is better than average based on HFTA results. He might become more motivated to chase DX as
a result.
For antennas situated on flat ground, with no terrain
features within 14,000’ of the tower, there is not much to be gained from HFTA
except for evaluating stacks. EZNEC
might be more accurate for modeling stacks over flat ground but HFTA will
calculate, using propagation statistics, a Figure of Merit (FOM) to a given
target area from each antenna; this rating might be interesting. For example, the FOM might predict your 150’
high yagi is too high for EU on average.
You likely already know that. You
can evaluate adding stacked antennas with HFTA to see the effect on the FOM or
on the main lobe amplitude and elevation angle.
You can also evaluate the individual antennas in a stack.
BACKGROUND
I retired from paid work at the end of July 2008. Two years before retiring, we moved to a WV
hilltop. These last two working years
required 100% travel except for weekends and holidays. Evening downtime on the road was used to work
DX from the mobile and plan the future antenna installation in WV. Before discovering HFTA a plan was devised to
install a 100' Rohn 45G tower and stack two C31XR's at 100' and 50' along with
a Force 12 Delta 40/30 on top of the mast.
The antennas were purchased in 2007.
My terrain fits the definition of complex. The elevated site looked like a good radio
location. This was confirmed with a 100W
mobile rig and screwdriver antenna in September 2006, my first experience with
HF mobile. Having always heard there
were HF hot spots, the new QTH seemed to be one. When traveling up the main driveway from the valley
highway below, the signals could be heard building in my receiver. The effect was dramatic. I couldn’t wait to try a real antenna.
HFTA was discovered in 2007 before any antenna work was done
and it provided a detailed understanding of the general opportunities and
problems with the site. Many dozens if
not hundreds of hours were put into planning with HFTA before the first tower
was erected.
Having lost interest in contesting I decided to focus on
serious DXing. After spending the time with
HFTA, my plan was modified after the software said a 100' tower was too high
for my terrain on any band 40 through 10 meters and 56' and 62' were optimum heights for 20 and
40 respectively, looking in my most favorable directions. HFTA also predicted 56’ was too high for 15
and 10 meters on my hill. I chose to
install a C31XR at this height anyway for the first tower install, just to get
on the bands.
The Rohn 45G was installed in June 2009, with a C31XR at
56', a Force 12 Delta 40/30 at 62' and a
7 element M2 6M7JHV for 6 meters at 68'.
The results from this system were great on 40/30/20. My antennas look down on STEEPLY sloping
terrain in all directions, and from about 300 degrees clockwise to about 50
degrees, there are no higher terrain features out a few thousand feet.
After about 50 degrees going clockwise around through west,
even though we have steep terrain drop under the antennas, there are very close
in hills which rise up steeply from the valley floor below; in some cases these
ridges go to an elevation 100' or more
higher than my hilltop. These are my
problem directions as predicted by HFTA.
The low angles tend to be cut off below 4 degrees or so, and the first
lobe is pushed up to a higher elevation angle.
The effect seems worse on the higher frequencies. On several azimuth headings
affected by the nearby higher hills, HFTA predicts my antenna performance is
significantly below that of an equivalent flat land installation.
In complex terrain like WV, there are three possible
outcomes. One can be the beneficiary of terrain enhancement, where the antenna
clearly outperforms. One can be a
situation where the rugged terrain is not a huge positive or negative factor,
and performance on a par with level ground can be achieved. Last, the terrain can be a huge negative, and
nothing short of a costly and heroic effort can achieve good DX
performance. See the K8KT example
discussed later.
Having no antennas with which to do side by side
comparisons, I judged the 40/30/20 meter performance of the initial setup to be
excellent. The signal felt loud in my
favorable directions as predicted by HFTA.
I could easily run EU/AS stations and break pileups with ease. The propagation was down on 15 and 10M the
first year, but I could work DX stations.
Perhaps it was psychological but the high band results did not seem
competitive with others in my area. Likewise, the results in my terrain
challenged directions were much inferior to my good directions, especially on
the high bands. I had to work HARD to
break pileups, especially on 10M. DX
could be worked, but not in the easy manner like in the favorable directions.
After using my first system for several months, I had to
test the HFTA prediction that a lower antenna would be better on the higher
bands in my favorable directions. A
reference 2 element quad was built on a 29' tower, which was placed in a location
80’ from the first tower, in a position closer to the cusp of the hill looking
N/NE. The quad, which was optimized for
gain on EZNEC, used an 8' boom and covered 17/15/12 and 10 meters.
Many hours were spent doing A versus B comparisons on 15 and
10 meters between this quad and the higher C31XR. The small quad was consistently better to EU
and AF on 10 and 15 meters than the C31XR, which has 4 elements on 15 and 7 on 10,
each band using 25' of the boom length.
This result was despite the quad having lower isotropic gain on each
band. There was never a time to EU or AF
where the higher C31XR beat the quad, and not many times when the result was
equal. HFTA successfully predicted this
performance favorability for the low antenna at this height and in these
directions.
HFTA also said the higher yagi would be better than the
lower quad looking straight north to Asia and also to the Pacific. This
prediction also proved to be consistently accurate. As far as I was concerned, the comparisons
on 15 and 10 meter with the reference quad and C31XR completely verified the
HFTA prediction.
For one practical example, ZD7XF was on CW and the Challenge
point was needed on 10 and 15M. HFTA
predicted the lower quad would be best, an unambiguous call. Sure enough, and without any doubt, the quad
beat the yagi. In fact, my Q's on 15 and
10 with ZD7XF had to be made on the quad, because the conditions were too
marginal for the higher antenna to hear ZD7XF very well.
Another practical example was particularly convincing. My C31XR at 56' worked well on 17M using a
tuner, and we made DXCC with it easily in a few months. The yagi models in
EZNEC to have about the same gain as a dipole on 17M, favoring the normal
reflector end of the antenna. However,
the 2 element quad on 17 walked all over the C31XR, in real life and the HFTA
model, in every direction except one.
HFTA predicted the C31XR, modeled as a 17M dipole, would consistently
beat the 2 element quad to the Pacific and VK/ZL. There was never a test where the quad beat
the yagi (dipole) on 17M to these directions.
Some testing was done on 6M. The M2 6M7JHV was (still is) at
68' on the Rohn 45G tower. A 6 element Hy-Gain yagi was temporarily installed
at 17' fixed NE on the same tower. HFTA
predicted the low antenna had much superior gain in the first lobe and a lower
elevation angle compared to the high antenna, looking NE. The install occurred after the EU Es
season. All of the North America AU and
Es signals from the NE that year were louder on the lower antenna. The second year, the first Es EU QSO's were
made on the low antenna, which was much better than the high one. After that, the low antenna developed a
problem and was removed from service.
The limited testing seemed to confirm the HFTA prediction that lower is
better on 6M on my hill. More future
testing is planned. In the meantime, the
6M7JHV produced a 6MDXCC on E skip from scratch in 6 years.
Although I was satisfied with the HFTA predictions, at least
for my hilltop, more optimization of the antennas was needed and this created
the opportunity for more HFTA validation.
A location was picked, using HFTA, for a TX-455 crankup tower. This spot seemed to have the best modeled
performance on the higher bands because the antenna could see the down sloping
terrain in all directions. The short
quad tower was situated on the NE lip of the hill, and it was looking across
the level yard to the west; the quad could not see the down slope on the west
side of the hill. The first antennas on
this crankup tower were a 5 element M2 17M5 yagi for 17M and a Hy-Gain 125CA 5
element 12M yagi. Now we could compare predictions
for the 2 element quad on 17M and 12M with the monoband yagis in A versus B
tests.
Generally, the HFTA predictions were confirmed as
valid. There were directions (NE) where
the quad was predicted to equal or be even slightly better than the big yagi on
17M, which has a 36' boom. However, the performance comparison between these
antennas did not justify keeping 17 and 12 meter elements on the quad. With a desire to increase gain on 15 and 10M,
the two element quad was rebuilt with a longer (18') boom and with three
elements covering only 15 and 10 meters.
EZNEC was used to manually optimize the gain at the expense of front to
back ratio. After the revised quad was
installed, comparisons with the C31XR in A versus B tests continued to confirm
the HFTA predictions; the lower antenna was better on 15 and 10 meters.
Before taking down the 2 element version of the quad, I
decided to rotate the boom 90 degrees to test the antenna with vertical
polarization compared to the big yagis.
With the feedlines attached to the spreaders, the quad was in
significant mechanical stress. However,
some signal comparisons were made. I
wanted to see if vertical polarization made any improvement in the terrain
challenged directions. No improvement
was noted and, in fact, during the limited testing the vertical polarization on
the quad was never as good as any of the horizontal yagis on any band.
With the completion of the 3 element quad on 15 and 10, we
were in good shape antenna-wise, except the quad on the low tower could not see
the west hill slope. This negatively affected
the performance to the Pacific and VK/ZL.
The C31XR on the higher tower was best looking west on the high
bands. Raising the low tower so the antenna
could see the west slope eliminated the advantage the quad had for the E/NE. Another round of antenna changes was planned.
A decision was taken to improve 15/10 meters overall, while
also improving 20M. HFTA said the
optimum tower for 17/15/12/10 in all directions was the TX-455 crankup, because
it could see the hill slope in all directions.
The following changes were made:
1. The 17M yagi was
removed from the crankup tower.
2. The C31XR was
moved from the Rohn 45G tower to the crankup tower on the bottom mast
position. The 12M yagi remained in place
at the top of the mast.
3. A M2 20M5 5
element monobander for 20 meters was installed at 56' on the Rohn 45G,
replacing the C31XR.
4. The big 17M M2
antenna was moved over to the short tower on the east side of the hill,
replacing the 3 element quad.
The result of the changes was positive on all bands except
17M. The antenna was a world beater to
the N/NE on the low tower, but the performance was compromised to the west due
to not seeing the western hill slope. Raising the antenna would have fixed this
problem, but it would have negatively impacted performance to the N/NE. Occasionally the C31XR, as a 17M dipole,
will beat the big yagi looking west.
After the above changes, I could no longer do A versus B
comparisons on 15 or 12 meters. However, for the first time, tests with two 20M
antennas could be done.
The crankup tower was operated at a height of 30’ to 35’ to
maximize performance on 15/12/10 meters.
I thought this would detract from 20M performance, but HFTA said the 3
elements on the C31XR would work great on 20M.
There is 2.1 dB more isotropic gain on the big 5 element antenna. Depending on the direction, there is
sometimes no detectable difference by ear between these antennas. In other directions, the bigger, higher
antenna is best. The QSB cycle on a signal will sometimes favor one antenna
over the other momentarily, and then at the next moment switch over to favor
the other antenna. HFTA is able to
consistently predict which antenna will be best on average if there is a best
antenna on the path.
After a couple of years operating in this last mode, one
other change was made. I decided to
modify the small tower and placed a 105CA Hy-Gain 10M monobander at 25' and
raised the 17M yagi to the top of the mast at 35'. This later change negatively impacted the NE
performance on 17M slightly, but helped the performance to the west. It was a good compromise. On 10M, the low yagi on the east tower is
consistently better to the NE/E/SE than the C31XR over on the crankup
tower. The performance differences on
10M continue to be accurately predicted by HFTA between these two antennas,
with one exception. The C31XR cannot see
local earth SE of the tower due to the placement of my house. In all cases, signals from the SE/S looking
over my house from the C31XR are attenuated by 20 dB compared to the 10M
antenna on the east tower. HFTA does not
account for man-made obstacles! This
S/SE degradation from the C31XR only occurs on 10 meters; it does not seem to
be present on 15M or 12M.
Other potential improvements have been contemplated, such as
replacing the F12 Delta 40/30 with large monobanders. I run hot and cold on this. So far, I have not been able to justify the work
and cost considering how well the existing antennas perform. The terrain enhancement predicted by HFTA to
the N/NE seems to exist on 20/30/40 meters. N8RR was the first NA QSO with T6MO
(K9GY) when he first became active and finished leading the NA Clublog
leaderboard to Eric. When T6LC came on
the bands, he offered one of his combat medals to the first 5 NA QSO's. This terrain resulted in 2 of the first 5 NA
QSO’s (on 40 and 20M) with T6LC.
***** This description of HFTA will be continued in Part 2 *****
Post written by: Charlie, N8RR
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