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Effect of antenna height on 21MHz performance at GB2GP

12/30/2020

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The next contest band for HFTA analysis is 21MHz. The GB2GP antennas are either 3 elements on the 100ft tower or 4 elements on either the 80 or 60ft towers.
The graphs below show that the as wavelength increases, height becomes more important to achieve the lower take of angles. the absolute difference between the high 3 element and lower 4 element yagis is less than at 28MHz, but the 4 element antenna at 60ft has better average performance across a wide range of arrival angles, as indicated by the Figure of Merit.

The general terrain at GB2GP has a downward slope in several directions, with the only rise towards the North East - i.e. towards Japan. This is reflected in the HFTA analysis where the extra tower height seems to really help. You can see the general horizon with the screen shot from heywhatsthat.com below.

On 21Mhz, a lower antenna may be better overall, but my conclusion is that this band will not see any major improvement by moving antennas around.

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Effect of antenna height at GB2GP on performance at 28MHz

12/27/2020

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GB2GP has three towers at 60, 80 and 100ft. For 28MHz we currently have a 3 element yagi at 100ft, a 5 element yagi at 80ft, which could be moved to the 60ft tower if needed. There is no easy way to move the big yagi to 100ft.
The HFTA analysis presented in the gallery below compares these antennas against a flat earth model. I have assumed the following beam headings:
JA - 30 Deg
EU - 80 Deg
AF - 170 Deg
NA -300 Deg
SA - 235 Deg
AS - 80 Deg
Click through the gallery and let me know which height would would choose and why!

My conclusions at 28Mhz are:

1) Gain is more important than tower height at this site on this band.
2) The 5 element at 28MHz has a better figure of merit at 60 ft in most directions as it's gain doesn't fall away at medium arrival angles compared to higher antennas.
3) The take of angle to the EU is quite a lot more than 5 degrees, maybe EU isn't DX?
4) My on air experience is that the 3 el at 100 ft is able to open the band. I guess this is because the arrival angle is low and the combination of 100ft tower and the slope works to our advantage.

Please use the comments below or tag me in Twitter!

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A study on the effect of terrain on your antennas

12/27/2020

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A popular YouTuber was speaking recently about how we should use vertical antennas to “command DX” as their primary lobes are at low angles, he uses gain at 5 degrees to promote the antennas we sells. This reminded me of some work by N6BV published in the ARRL Antenna Handbook which demonstrated that your terrain also has a substantial impact on the lobes of your antenna. This led me to consider what the effects of the terrain at GB2GP has upon on the performance of our antennas.

There is nothing new here; N6BV created some modelling software called HF Terrain Analysis (HFTA) which plots the gain of typical horizontal antennas over your specific terrain and can compare it to flat earth. We are pretty lucky that Gilwell Park is on a local high spot so it will be interesting to see how the slopes that are in several directions have an impact.

The image below is a typical HFTA output and compares the gain of a 3 element, 28MHz yagi at 100ft over the terrain at Gilwell Park (blue line) beaming to north America, compared to the same antenna over flat ground (green line). The terrain give a clear advantage at 2 degree take off angle, +7dBi, but a 2.5bDi disadvantage at the 5 degrees.

Generating the terrain profiles for HFTA) has generally been a serious undertaking, but luckily K6TU has done us all a great service by automating this on his website https://k6tu.net/?q=TerrainProfiles. I have used these and thank him for making these publicly available.

Antenna height has a substantial impact on take of angles for horizontal antennas and we are fortunate at GB2GP to have three towers for HF yagis at 60, 80 and 100ft. Would swapping the antennas round offer any significant advantage on any particular contest band? The following posts simulates our current antennas at each height in key directions.

HFTA also includes take off (or arrival) angle data for signals over specific paths. Whilst 5 degrees cited in the opening paragraph might work as a rule of thumb, it’s a long way from the full picture as the Angle of Arrival changes with ionospheric conditions and HFTA plots the percentage probability of different arrival angles on a second y axis. This data suggests that we all need different antennas to suit every set of conditions, but acknowledging the real world, HFTA also includes a coarse judge of quality that amalgamates the gain over your terrain with the Angle of Arrival profile to help us decide which configuration is best.

The graph below shows the same set of antennas and direction with elevation statistics for the same path. 8% of signals arrive at 2 degrees where our 3 element yagi at 100ft has a terrain advantage, but similar percentage of signals arrive at 5 degrees where we have a disadvantage over flat terrain.

The following posts will show a band by band analysis for GB2GP.

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Effect of antenna height on 21MHz performance at GB2GP

On 21Mhz, a lower antenna may be better overall, but my conclusion is that this band will not see any major improvement by moving antennas around.

Effect of antenna height at GB2GP on performance at 28MHz

A study on the effect of terrain on your antennas

The following posts will show a band by band analysis for GB2GP.

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