Tuesday, August 27, 2013

Parabolic UHF Troposcatter Antenna for TV and Amateur Radio

A parabolic UHF antenna
For scale, top of the 4-element feed is chest-height.
to investigate the feasibility and challanges of TV and amateur radio over troposcatter using parabolic reflectors was designed and built.

Troposcatter enables UHF and microwave signals to be received beyond the horizon limit (~70mi), which is traditionally considered the practical limit for these frequencies. The downside of tropo is signal fading with severe degradation in signal quality and strength which require extreme measures to receive.

Antenna is dismantled because the 1+1/4" galv. steel rotor mast pipe has bent from stress, causing it to increasingly point toward the ground. With the reflector's narrow beamwidth, this factor became non-negligible fairly quickly and it eventually ceased to perform. Because this started happening sleetfall, ice weight is suspected.

An extremely well-performing antenna which shows that parabolic reflectors are still good for very high gains in the UHF range. It is, however, very high-maintenance and a hazard challenge to install. Its tropo reception is unreliable and a lot of time has to be put into its assembly and use. Fade behavior is comparable to that of shortwave, but with ranges of hundreds of miles instead of thousands. Tropo performance likely will vary by location; third-party verification or improvements on this design would be interesting. The experience from this project may be used for fine-tuning future projects.
WZMQRF1962mi80% night, 30% dayTransmitter is beam-steered away from antenna
WJMNRF48104mi60%Night Only
WBAYRF23188mi15%Choppy day/night
WCWFRF21~180mi15%Choppy day/night
WFRVRF39~180mi15%Choppy day/night
With deep fading of tropo, figures are given in percent reliability (how often a tuner lock can be acquired) instead of signal strength. Antenna stands 25ft AGL on the side of a shallow <50ft hill.

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This is a 10'x10' offset-feed parabolic reflector made of 2"x4"-grid galvanized steel screen supported by a painted, heat-formed 1.5" ABS DWV pipe frame. The feed consists of four AntennasDirect C1s (SWR <1.5, UHF only, ~30dB attenuation in VHF) spaced approximately 1.5 feet apart and held in position by nylon rope. Each C1's output passes through a Laird Technologies 28A0640-0A2 ferrite choke and converges to a 4-way combiner followed by a Research Communications 9261 LNA (0.4dB NF, UHF only, with upper and lower boundary filtering) prior to downlead, connected via Aquatight PPC. Testing tuner is HDHomeRun. Total calculated weight is approx. 37 pounds. Reflector inclination, feed distance and position are adjustable. Including tripod mount, the structure rises approx. 13 feet off the roof.

Overdrive is a serious concern, which is why the feed and LNA are UHF-only and were selected to reject out-of-band signals. Despite this, there was still some overdrive and resulting harmonic LNA overdrive from WGLI (98.7MHz 200kW@4mi.), possibly due to coax shield spillover making it through the choke at the C1s. Another strong emitter were the various cell towers broadcasting in the upper UHF "white space." These bright stations created blind angles which could not be used for reception of tropo.

This antenna is inspired by the Channel Master 4251 and commercial antennas developed by Wade in an attempt to develop a very light-duty antenna of a similar performance caliber.

Extremely good performance: Stations too weak to be received by the stack can be received with this reflector. The calculated (before LNA) gain is approximately 20-22dB at 550MHz with a full beamwidth of about 13 degrees at 3dB contour down. Actual performance is roughly within 3 dB of this calculation. In practice it is aggressively directional and exceeds the granularity of the rotor. It also has a 10dB backlobe and two 12dB rear sidelobes from feed spillover at about 40 degrees.  There is a signal strength gap between where line-of-sight reception is lost and the maximum strength of typical tropo. This antenna successfully traverses that gap but with issues more complex than signal strength alone.

Wind Profile
Wind profile is surprisingly good for this antenna because of the minimal materials used, but the feed has a more significant wind profile with its broader surfaces and the added torsion of 7 feet which has no counter-torsion behind the reflector. Leverage, spring action of the mast, and elasticity of the nylon ropes also caused the entire feed to bounce (details in next section) but these did not greatly affect reception.

Bolstering supportive structure increases wind profile and could potentially decrease durability, particularly near the edges where wind torsion potential is greatest. This antenna's lightweight design required very careful balancing of both the quantity and vectors of loads. Most failures involved screw-eyes (fixed with thicker screw eyes) and rope-slip failures. The ABS frame itself has endured up to winds of up to 45 MPH without direct issue. This delicate balancing means that the failure of one part will result in the cascading failure of additional parts as their torsions and loads become unbalanced. Repairs must be done quickly to avoid further damage.

There are many channels whose signal strength should be high enough to acquire a lock but lack the S/N ratio to do so, apparently due to severe multipath-of both static and dynamic varieties which leave the signal quality in poor condition: By the time a signal reaches the antenna is may have traveled hundreds of miles among mountains, temperature inversions and buildings. Further increases in signal strength have diminishing returns in the case of multipath as interference scales up alongside desired signal, with a speculative exception of narrowing the beamwidth. There is also the issue that this antenna is....awfully large.

Increasing aperture size is unlikely to happen for practical reasons. There is a lot of promise in using or implementing a more advanced tuner, such as a diversity mobile tuner. Solving multipath would sharply improve performance.

The original plan was to have a single C1 at the end of the feed. When this was changed to four C1s the feed became heavier making the antenna front-heavy. This, combined with bouncing of the feed and ice buildup resulted in the steel rotor mast pipe slowly bending from stress.

In the future this could be addressed by switching back to a single C1, and compensating for the wider feed pattern by shortening the feed arm, which would reduce front-weight greatly. In compensation for needing a shorter focal point, the reflector curvature would have to be sharpened, resulting in a slightly increased reflector weight relative to aperture size which would be far compensated by the benefits of the redesigned feed.

80% strength and no tuner lock.
An extreme example of multipath gone awry from tropo cells.
(LNA distortion, co-channel and jamming ruled out)

Reflector shakes in wind but survives storms without breaking. Effective wind profile is relatively small because of the use of screen, rope and cylindrical pipe. This antenna will be kept over the winter to test its ice/snow handling. Dismantled due to mast pipe stress following sleet.
Bent mast pipe

Reception is affected dramatically by weather: Humid, calm, clear nights are best while nights preceding a storm are also good while dry and/or windy weather are most detrimental. This antenna can detect lightning as raw signal strength will momentarily spike on lower channels with each strike. Jokingly, 'passive weather radar' may be a better application than HDTV.

Amateur Radio
70cm (420MHz) feasibility is being investigated. Dismantled before testing could be done.

Sub-article is here.

The brands and products mentioned in this article are for reference and example only and do not constitute an endorsement, recommendation, or affiliation with any mentioned party. Though this information is intended to be useful and functional, it is offered AS-IS without warranty even for safety or fitness for any particular purpose.

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