Some Antenna Photos and Tests

[tripelo]

(continued from previous post)

Part 4. The HDTVi Antenna as a Reference

Another way of comparing gain of the antennas is to directly compare received signal levels to that of the HDTVi antenna. The HDTVi antenna was selected as a reference antenna mainly because it was thought that it’s gain would be relatively flat across the UHF band. Also it was thought that the SWR of the HDTVi would be relatively good (close to 1). Part of the reason for the above assumptions is that Log Periodic antennas can be designed to meet the criterion: flat gain and favorable SWR. The physical appearance of the HDTVI suggest as much.

A 4NEC2 analysis of the HDTVi tends to support the assumptions of flat gain and favorable SWR.

Below are images of the results of 4NEC2 analysis




Top Image shows Gain in dBi (recall dBi=dBd+2.15), Lower image shows SWR


The above simulation SWR results agree favorably with the actual measurements. If the HDTVi gain were to be measured, likely it would also agree. If that were the case, than a direct comparison of the higher gain antennas with the HDTVi might provide insight.

Below is an image that shows the other four antennas compared directly with the HDTVi antenna:






The above image illustrates the tendency of increasing gain of the four higher-gain antennas with increasingly higher frequencies.

As can be seen in this image and a preceding image, there is a discontinuity at channel 20 and channel 31. These two channels have lower signal strength than most of the others and also have been observed to exhibit multipath effects. The effects that were related to multipath were observed as part of a previous field test performed in May 2012, some results shown in a previous post (Ch 20 & 31 data were not presented).

UHF Signal Strength vs Antenna Height (AGL)

Omitting those points (Ch 20 & 31), the overall data might look smoother, but they were included for completeness.

The following image represents a best-fit linear approximation to the measured data and could illustrate some trends. Caution when interpreting this linear approximation; for a specific channel, it might be better to refer back to the raw data averages (shown earlier).




Both Yagis show increasing gain with increasing frequency (channel number). The 8-bay bow tie, or panel array, antennas show a flatter response with some increased gain with increasing frequency.

CM4228 and DB8

Notable is the approximately 2 dB difference in gain between the CM4228 and the DB8. This difference could partially be explained by the increased SWR of the DB8 (about SWR=4 at mid band), an SWR of 4 could cause approximately 2 dB loss. Some of the signal strength difference and transmission line matching could be attributed to baluns. The DB8 balun was supplied by the manufacturer (Antennas Direct). The CM4228 balun was a homebrew hand-wound ferrite-core balun with a measured loss of less than one dB.

91XG and HDB91x

The measured signal level difference between the gains of the two Yagi style antennas is less pronounced. The HDB91x shows a clear advantage at the top end of the band, however this indication is partially an artifact of the linear approximation. Note the raw data average show signal levels from the two antennas to be fairly close near the upper end of the band. The reason for the overall differences is unknown, but could partially be related to a better impedance match to the transmission line. The S11 measurement of the HDB91x reveals a better impedance match starting at ~500 MHz (Ch 19) and upwards. It is known that the 91XG has an ~microstrip balun that in some ways resembles a usual half-wave loop (but is markedly different). The factory-supplied balun of the HDB91x has not been inspected.

After a first set of data on the 91XG was collected, it was thought something might be amiss. The 91XG balun housing was opened and inspected; a nut that holds the driven element to the balun seemed to not be tight. The nut was tightened and a second set of data was collected. The results of the second 91XG data set were close to that of the first set.

Overall, the appearance of the 91XG and the HDB91x seems similar. But, close inspection shows most component parts of the HDB91x are in some way different than the 91XG. There are measurable physical and electrical differences that may account for some of the difference in performance.

Test Location and Data Variability

Even though the test site is LOS to the DFW stations, some multipath has been observed with received signal on some channels. This is probably a result of the first Fresnel zone not being clear.

A repeat of each measurement probably helped reduce effects of brief transitory multipath effects. The individual data sets were fairly consistent. Measurements using the HDTVi at both beginning-of-test and end-of-test somewhat verified data consistency. The HDTVi final-measurements were about 0.2 dB lower in signal strength than the beginning-measurements. A scaled correction factor could have been incorporated into the data but was deemed unnecessary.

.

[tripelo]

A series of antenna tests conducted on July 1, 2016.

Objective: Compare performance of selected VHF antennas while receiving DFW TV channels

Antennas:

1. Antennas Direct C5
2. Televes Band III Yagi

Antennas Direct C5: Driven element is a loop design (contains two loops, inner and outer) with reflector screen. The balun is ferrite core design.

Televes Band III Yagi: Folded-dipole driven element with reflector and five directors. The balun is sealed in metal can.

Images of antennas shown below:

Antennas Direct ClearStream 5 (C5)




Televes Yagi B-III





Test Location:

Garland, TX, ~30 miles line-of-sight to DFW stations, both VHF channels (8 and 9) located on same transmitting tower.

Test location TVfool info posted earlier:

Compare Preamplifiers - Test Procedure & Configuration

Test Equipment:

Same as previous*, except:

1. Sencore SLM1456CM instead of Sencore SLM1456
2. 3dB attenuator at input of Sencore SLM1456CM
(From antenna perspective, improves coax cable match to 75 Ohms)


*Previous similar test: Compare Received Signal Strength of 5 UHF Antennas

Test Procedure

1. Calibrate VNWA, with the 40’coax cable (refers measurements to antenna/balun connection).

Starting with Antennas Direct ClearStream 5 antenna:

2. Mount antenna w/coax and ferrites to mast, raise center of antenna to 25 feet
3. Align antenna for peak signal on Channel 8
4. With 1456CM scan DFW VHF stations (Channels 8 & 9) for signal strength and MER.
(repeat scan 9 times while in this configuration, record times).
5. Take at least 2 Photos
6. Turn antenna ~90 degrees, disconnect coax from 1456, connect to VNWA
7. Measure S11 (input impedance, SWR/Return Loss) for the band
8. Lower Mast remove antenna
9. Mount Televes Yagi B-III, repeat steps 2-8
10. Then again, for each antenna, repeat steps 2-9


The test location and setup was similar to that described in previous posts:

Televes DAT-75 vs. Antennas Direct 91XG


Credit:

Thanks to Stuart Sweet of Solid signal for providing the Televes Yagi B-III.

(To be continued)

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[tripelo]

Part 2. Images of the Two Antennas at Test site

The following are images of the two antennas at the test site (antenna center height=25 feet).

Image below shows Antennas Direct ClearStream 5 and Televes Yagi B-III.





(To be continued).

.

[tripelo]

Part 3 (continued from previous post)

Attached are Graphs depicting a summary of data analysis.

1. Average received power levels, (Image below):

An average 9 measurements of each channel (8 & 9) for each antenna (ClearStream 5 and Televes Yagi BIII).





2. Average Relative Gain per Channel (image below)

A nine-scan average of both antennas.





3. Average SWR 5 Antennas (image below)

SWR was derived from measured S11 (Return Loss). The curves represent the
smoothed scan averages for each antenna.





For mismatch Loss vs. SWR, see previous post*.

*Received Signal Strength and Data Analysis


No mismatch loss for SWR equal to 1, a SWR of 2 results in a mismatch loss of about 0.5 dB.


========== Comments ====================

The Antennas Direct ClearStream 5 (C5) and Televes Yagi BIII provided nearly the same average signal strength for Channel 8 and 9, with an approximately 0.1 dB advantage for the Televes antenna. Since 0.1 dB is near the measurement accuracy limits of the test configuration, the difference may not be very significant.

The per channel data shows that the Televes antenna provides a higher signal on channel 9 (by about 0.2 dB) relative to the Antennas Direct antenna. This could be a result normally expected of Yagi type antennas, that is the gain increasing with increasing frequencies, up to the design limit.

Test Location and Data Variability

Even though the test site is LOS to the DFW stations, some multipath has been observed at UHF with received signal on some channels. However, effects of multipath have not been explored at VHF frequencies, and there have been no symptoms to suggest it might be prevalent.

Several repeats of each measurement probably helped reduce any effects of brief transitory signal variability. The individual data sets were fairly consistent. Measurements using the Antennas Direct ClearStream 5 at both beginning-of-test and end-of-test somewhat verified data consistency. The Antennas Direct ClearStream 5 final-measurements averages were within a few hundredths of a dB of the signal strength of the beginning-measurements average.



(To be continued)

.

[tripelo]

Part 4 (continued from previous post)

4NEC2 Analysis and Discussion

Antenna simulation software (4NEC2) could be useful in comparing antennas. Generally, software analysis is not a substitute for field tests. Choices must be made of available approximations that are necessarily incorporated in software models. Without some field measurement data to guide the design of a simulation model, rarely does a simulation model account for all factors in an antenna design.

In cases where field data is limited, but available enough to help confirm the models and if one can be fairly sure the software model something close to reality, then software analysis can provide insight.

The physical dimensions of both antennas; the ClearStream 5 and the Televes Yagi B-III were measured and software models created in 4NEC2.

The gain values shown in the following 4NEC2 results do not include balun loss or mismatch loss.

Below are images of the results of 4NEC2 analysis

Top Image, Gain in dBi, Lower image SWR

(Recall dBi=dBd+2.15)





The above simulation SWR results agree favorably with the actual measurements.

If the Antennas Direct ClearStream 5 gain were to be measured, likely it would also agree.

Below is an image that shows the Televes Yagi B-III antenna gain and SWR:







The shape of the Televes SWR pattern above above is in reasonable agreement with actual measurements (see previous post). Overall, the SWR is lower than measured. No explanation has been found. It could be that the software model does not account for some factor that could alter the SWR values. Another possibility is that the balun (not modeled) could contribute to the SWR difference. For example; In some cases LC components are added to a balun to improve response at band edges at the expense of impedance match at center band.

The above image illustrates the tendency of increasing gain for higher frequencies, often seen in Yagi-type antennas.

These simulation results are also in fair agreement with data present at the two manufacturers websites:


Antennas Direct ClearStream 5

Gain and SWR (simulations of both, plus measured values of SWR at 10 ft AGL),

Located in this document:

https://www.antennasdirect.com/cmss_...with%20uhf.pdf


Televes Yagi B-III (Gain),

Located in this document:

https://www.televes.com/sites/defaul...ennas_en_0.pdf


Azimuth Beam Width

The following images show the computed azimuth (horizontal) antenna patterns of the two antennas at three frequencies representing lower end of the band (174 MHz), mid band (194 MHz), and upper end of the band (216 MHz).






Note the C5 has a somewhat broader beamwidth, especially evident at the upper end of the band, than the Televes antenna.








The difference in gain at 216MHz is mainly related to increasing gain from the Televes B-III. The Televes B-III is advertised to cover up to 230MHz, so it is expected the gain would be higher at the upper end of the band.


(To be continued)

.

[tripelo]

Part 5 (continued from previous post)

Specifications, Measurement, Analysis and Discussion

In an effort to reconcile the various gain and SWR numbers available for these two antennas, the following chart was composed.

For each of the two antennas and each channel (8 and 9), the gain and SWR are presented. The sources for the gain and SWR are listed in the left column.

The manufacturers numbers are derived from the technical documents found in the links provided in a previous post. Antennas Direct lists gain figures as dBi, very likely the Televes numbers are also in dBi.

The 4NEC2 results are from simulations described in the immediately previous post. The gain numbers (dBi) listed here are calculated to include mismatch loss due to SWR.

The DFW Measure numbers are a summary of actual measurements at 25 ft from a location in Garland TX (described in a previous post). The units of values listed as gain are dB, either positive or negative relative to that averages of the two antennas (per channel).





Summary and Discussion

These two antennas are of different design, but for the channels tested yielded relatively close gain and SWR numbers.

The Antennas Direct antenna consists of driven element loops (inner and outer) with a screen reflector. The loop is made of relatively thick conductor that helps to broaden frequency response (flattens SWR). This design is in some ways similar to two stacked dipoles (top element fed at ends instead of center fed). This stacking effect tends to compress vertical beam width, thus for a given amount of total gain, allows for a slightly wider horizontal beam width. This C5 antenna being a relatively broadband design tends to have a flat gain and SWR response across the entire VHF high band.

The Televes antenna is a conventional Yagi design with folded dipole driven element. This design tends to have increasing gain with increasing frequency, up to the design limit. According to Televes literature the design limit is 230 MHz (the upper channel in the European band). Conventional Yagi designs tend to have a flattened oval shaped aperture (area over which signal is gathered). This flattened oval-shaped aperture shape means more of the antenna gain is a result of horizontal beam becoming narrow. So, one would expect the beam width of the Televes antenna to become more narrow at higher frequencies (where the gain is highest).

Since much of the data presented in the above preceding chart is in agreement.

The above chart for these two antennas tends to indicate:

1. Antennas Direct and Televes gain figures are consistent with each other.

2. Manufacturers gain numbers are consistent with 4NEC2 simulation results.

3. Antennas Direct SWR values are consistent with actual measurements (at 25 ft. AGL).

4. Computer simulations combined with field measurements can be used to characterize antenna performance, in some cases.

.

[tripelo]

Antenna tests conducted on June 15, 2017.

Objective: Compare performance of selected VHF antennas while receiving DFW TV channels.

Antennas:

1. Terk HDTVi (as reference)
2. Antennas Direct DB8e
2. Channel Master 4228HD

Terk HDTVi antenna measured as reference (see previous posts).

Both Antennas Direct DB8e and Channel Master 4228HD are panel type antennas, also known as 8-bay antennas.

Main differences between the two panel antennas are feed system design and reflector backplane.

Images of antennas shown below:

Antennas Direct DB8e



Channel Master 4228HD (below)




Test Location:

Garland, TX, ~30 miles line-of-sight to DFW stations.

Test location TVfool info posted earlier:

Compare Preamplifiers - Test Procedure & Configuration

Test Equipment and Test Procedure:

For details of test equipment & procedure, see previous similar tests:

Compare Received Signal Strength of 5 UHF Antennas

Compare: Antennas Direct C5 & Televes Yagi B-III


Credit:

Thanks to forum members ADtech and re_nelson for providing the Antennas Direct DB8e antenna.


(To be continued)

.

[tripelo]

(Continued from previous post)

Part 2. Images of Three Antennas at Test site

The following are images of the two antennas at the test site (antenna center height=25 feet).

Image below shows the reference antenna: Terk HDTVi





The image below shows both the Antennas Direct DB8e (left) and the Channel Master 4228HD (right).



(To be continued).

.

[tripelo]

(continued from previous post)

Part 3. Data Collection & Analysis

The amount of data collected represents:

Number of UHF channels: 23
Measurements per channel
DB8e4 and 4228HD: 9
HDTVi: 12

Total (individual channel measurements): 23 x 9 x 2 + 23 x 12 x1= 690

Attached are Graphs depicting a summary of data analyses.

1. Average received power levels, (Image below):

The image below represents an average of 23 UHF channels, nine measurements per UHF channel for each antenna (4228HD and DB8e), 12 measurements per channel for the HDTVi.





The image below shows the received signal strength per UHF channel for the three antennas. Note signal strength is shown in dBmV (dB referenced to milliVolts in a 75 Ohms system. To convert to dBm (milliWatts) subtract 48.75. from the dBmV numbers.





The image below shows the received signal strength of the DB8e and 4228HD antennas in dB, relative to the HDTVi antenna. The HDTVi signal strength is referenced to zero on the vertical dB scale.





The image below shows a linear best-fit approximation of the received signal strength of the DB8e and 4228HD antennas in dB, relative to the HDTVi antenna.





The following image shows SWR for the three antennas, it was derived from S11 (Return Loss), measured at 25 feet AGL.

The fuzziness of the lines in the SWR graph results from simultaneously receiving the broadcast stations in DFW with the Vector Network Analyzer. The reception of these strong signals caused some interference with the live impedance measurements. The antennas were not turned away from the stations as was done in previous measurements





All three antennas have respectively good values of SWR. No mismatch loss for SWR equal to 1, a SWR of 2 results in a mismatch loss of about 0.5 dB.

Antennas Direct Technical data sheet indicates the SWR is less than 2 from 470 to 698 MHZ. The SWR measurement shown above for the DB8e is fairly well in agreement with Antenna Direct published specifications. Published SWR specifications for the 4228HD are not available.


For mismatch Loss vs. SWR, see previous post*.

*Received Signal Strength and Data Analysis


========== Comments ====================

The Antennas Direct DB8 provides highest overall signal strength, with an advantage over the 4228HD at high channels approaching 4-5 dB. TheCM4228HD performs well for channels below about channel 23, with an advantage of more than 1 dB at the lowest end of the band.

The 4228HD appears to have reduced gain as the frequency increases. The reason for this phenomenon was not investigated in this effort. However, it may be related to the transmission line feed for individual driven elements in the 8-element array, and particularly the feed between the two 4-bay segments.

Others have explored this 4228HD gain effect:

HDTV primer: Designed an improved antenna transmission line feed.

Antennahacks: Implemented an improved transmission line feed system.


Test Location and Data Variability

The test site is LOS to the DFW stations, however effects of multipath have been observed at UHF with received signal on some channels

Several repeats of each measurement probably helped reduce any effects of brief transitory signal variability. The individual data sets were fairly consistent. Measurements with the HDTVi, at both beginning-of-test and end-of-test, somewhat verified data consistency.

(to be continued)

.

[tripelo]

(continued from previous post)

Part 4. Compare 6 antennas (DB8, DB8e, 4228, 4228HD, 91xg, HDB91x)

Having earlier measured four other high-gain UHF antennas relative to the HDTVi antenna, this could suggest that all six could be compared.

http://forum.tvfool.com/showpost.php...2&postcount=99

The main link between the two data sets is the measurements of the HDTVi antenna. The HDTVi was used as a reference antenna in both experiments. Realizing that errors can accumulate, a direct comparison of the two data sets might be somewhat useful.

Below is an image that shows the other four antennas compared directly with the HDTVi antenna:





The Yagi antennas (91xg and HDB91x) stand out as having higher gain as the channel number increase. The two Yagis appear to have the highest gain available at the top of the UHF band.

It appears that the older CM4228 (non-HD) is a fairly good match to the newer DB8e, both show a wideband response with relatively high gain at the lower channels.

If the above graph were accurate for every channel (which is doubtful), one could add the gain of the HDTVi to every antenna, to obtain a total gain figures. Previous calculations show the gain of the HDTVi to be about 6.5-7 dBi.

Caveat: Keep in mind that real antenna gain patterns are rarely (if ever) linear, and the above chart (although based on measured data) is an approximation.

.

[tripelo]

The tower shown in previous posts has been in use since 1971. It is located in Russell County, KY. The earlier posts showed some antennas that have been resident on the tower. Hopefully, a series of posts could show nearly all that antennas that have been on the tower and some rationale as to the purpose of those antennas.

In 1971, with the help of my younger brother, tower sections were individually lifted & assembled, then two antennas installed with a Channel Master rotator.

- Channel Master Crossfire CM-3600 VHF antenna
- Predecessor to CM-4228 UHF antenna (shown previously)

The image below shows the almost completed project of installing antennas just after the final tower section was placed. The 2nd image shows an excerpt from Channel Master literature describing the new 'Crossfire' antennas.






The next image shows the predecessor to what is now know as the old CM-4228 UHF antenna (previously posted).




The objective was to receive stations from 3 cities:

- WAVE-3 Louisville (~101 miles, across typical hill & forested areas)
- WHAS-11 Louisville (~101 miles, across typical hill & forested areas)

- WLEX-18 Lexington (~77 miles, across typical hill & forested areas)
- WKYT-27 Lexington (~74 miles, across typical hill & forested areas)

By rotating antenna, receive:

- WATE-6 Knoxville TN (~93 miles, mountains in between)
- WBIR-10 Knoxville TN (~93 miles, mountains in between)

For most of the stations above, allowing for a bit of 'snow', reception was satisfactory mornings and night. Reception could be unreliable during day.

With the antenna rotator, this might have been a good setup for DX’ing. Stations could almost regularly be seen from various cities in Indiana, Ohio, Tennessee, Virginia, and West Virginia.

Maybe later, more details of subsequent antennas.

.

[Pete Higgins]

tripelo,

Really interesting material. I always look forward to your posts. Hope the New Year finds you & yours in good health.

[davodavo]

Situation: 70-120 miles away from some very big signals (1 MW), but almost nothing less than 30 miles away. Everything is UHF except VHF 7 and 11.

Goal: Grab every signal I can; currently at about 75 virtual channels (but up to 100 are possible)

Baseline: Stacked AntennaCraft Yagis with WG7870 combiner CM7777 preamp. Due to wife's objections, these are at about 15' AGL (although we're near at the top of a hill, the signal has to pass through some treetops).

Hypothesis: Hotter antenna (think CM4251) would do the trick. But...why not go even bigger with the parabolic reflector? Acquired 10' C-band satellite dish, repurposed with bow-tie element.

Photos, measurements, and lessons learned: when it stops raining.

[Stereocraig]

I've been working on a similar project, w/ a couple of 8 footers for the last couple of years.
I have the bowties and reflectors built, but will need to reconfigure the suspension conduits higher, or lower for focal point due to the bowtie placement conflicting w/ the same distance as where the old horn mount is.

Oh well, eventually.

[davodavo]

So, the first thing you have to realize is that the 10' C-band antennas are pretty solid, heavy and with a fair degree of wind resistance. As the reflecting surface is typically expanded metal with 2 mm perforations, this thing is going to present quite a wind load. Even after removing all the C-band electronics and unnecessary aiming motor, the parabolic and its mounting structure is probably 150 lbs...so this is not going up on a mast.



What it does get mounted on is 3" galvanized steel, and for obvious reasons that pipe needs to be at least 5' out of the ground. In my case, I went for 7 feet out of the ground...but that means 5' in the ground, surrounded by at least 300 lbs of concrete. Unless you have a really big augur, it's hard to do much more than that. Luckily, the ground drops off significantly from where the pole is, so it's effectively maybe 12' AGL.

Once the dish is mounted, configuring the antenna element is pretty straightforward. I live in an area with almost no signals in any direction, so I don't need a reflector behind the antenna element: it's just a matter of making the element (bow-tie) the right size and in the focus of the dish.



According to theory, the bow-tie is of infinite size...and I'd like to get down to to channel 7 so the wavelength is almost 2 meters. But the focus of the dish is probably less than a meter across, so there's not going to be any advantage to an bow-tie even that big. My first pass is with bow-tie wires 16" long. (I will experiment with solid bow tie and other variations in a week or so.) Through experimentation and measurement, I find the best distance for the bow-tie element and we're off to the races.



My main signal challenges are about 100 miles away, and my baseline for comparison metrics is a pair of yagis, with the bottom one about 12' AGL actual...but due to drop off of ground level effectively 17' AGL. According to the models, I shouldn't be getting much of anything...but with the stacked yagis and a nice preamp most of the time I get about 75 virtual channels (from about 20 transmitters). The new dish with a preamp gets nearly all the channels the same way, and the first pass of spectrum analysis (using the Nuts about Nets USB-based system on a PC) shows nearly identical signal strength. The dish is more directional than the Yagis (no surprise) but not as super-directional as I was worried about.

So now: how do I make the bow-tie outperform the Yagis? They are signficantly farther off the ground, which gives them a big gain advantage (I'm guessing 6 dB)...but the "collecting surface" of the dish should give the bow-tie even more than that.

The answers come next time I have time to mess with it.

[Stereocraig]

Originally Posted by davodavo

So, the first thing you have to realize is that the 10' C-band antennas are pretty solid, heavy and with a fair degree of wind resistance. As the reflecting surface is typically expanded metal with 2 mm perforations, this thing is going to present quite a wind load. Even after removing all the C-band electronics and unnecessary aiming motor, the parabolic and its mounting structure is probably 150 lbs...so this is not going up on a mast.

I have a pair of 45' towers each sunk in 35 60# bags of ready mix and various engine blocks and bicycle frames, so I'm confident I can go up at least 2 sections. I just need to fabricate clamps.

[davodavo]

Originally Posted by Stereocraig

I have a pair of 45' towers each sunk in 35 60# bags of ready mix and various engine blocks and bicycle frames, so I'm confident I can go up at least 2 sections. I just need to fabricate clamps.

Wow, well if you've got a 3000 lb+ foundation that's cool.
I'm not sure how you're going to get the dish very far up in the air--you'll need to set up some sort of winch system or use a very tall bucket. Just don't try it on a windy day--the dish becomes quite an unwieldy sail.

[rabbit73]

Originally Posted by davodavo

So now: how do I make the bow-tie outperform the Yagis?

Thanks for the interesting photos of your dish.

Bowties are full wave dipoles. Since they are a collinear pair of dipoles spaced very close together, they don't have a lot of gain over a halfwave dipole. I suggest you use a stacked pair of halfwave dipoles, one above the other, fed in phase, for smaller size and greater efficiency at the focal point. A vertical spacing of 5/8 wave at the design frequency would give max gain. Adding a reflector behind the dipoles would give additional gain.

http://www.w1ghz.org/antbook/chap6-2.pdf

[davodavo]

Originally Posted by rabbit73

Thanks for the interesting photos of your dish.

Bowties are full wave dipoles. Since they are a collinear pair of dipoles spaced very close together, they don't have a lot of gain over a halfwave dipole. I suggest you use a stacked pair of halfwave dipoles, one above the other, fed in phase, for smaller size and greater efficiency at the focal point. A vertical spacing of 5/8 wave at the design frequency would give max gain. Adding a reflector behind the dipoles would give additional gain.

http://www.w1ghz.org/antbook/chap6-2.pdf

Thanks for the tips, and the reference to the antenna book! Seems like all of its characterizations are for transmission...but they should apply reflexively to reception.

I'll give the stacked dipoles a shot. In the "duh" department: won't I have an impedance mismatch when using stacked dipoles (I'm feeding a 300 ohm balun)?

Bowties are actually "supposed" to be solid (rather than "whiskers") for maximum gain...I'm going to be giving that a shot and will tell you the results.

However, given the special case of bowtie in the focus of a parabolic reflector I doubt that a reflector behind the bowtie (or dipoles) would help much: at that point, the waves are out of focus and bouncing off at a range of angles from 0 to about 120 degrees, causing a bunch of phase cancellations. Given I'm trying to use this from 170 to 700 MHz, I'm doubtful there's an optimum at all ;-) I don't have any issues with front/back ratio as there are no signal sources within 30 miles of me, and the landscape around me shields unwanted strays.

I'll give it a try and report back here.

[davodavo]

So with COVID-19 I have time on my hands at home and did a bunch of comparative measurements. Used Nuts About Nets spectrum analyzer to compare--the only variation in the configuration was the final antenna.

Note that the dish is mounted on a short pole (due to wind loading issues, couldn't go higher) but the ground level slopes off and the dish is effectively maybe 10 feet above ground level.

Also note that this would have to be called deep-fringe conditions--the nearest real station is 75 miles away and some of the stuff I'm shooting for is 125 miles away. Due to atmospherics, signal strength varies significantly during the day, so I always did my measurements within the same hour and comparable weather conditions.

Baseline was as shown in the photos above--very thin copper "whiskers". They are long because (1) I'm trying to get some VHF channels and (2) from what I read, making them longer doesn't significantly hurt the UHF.

Modified the whiskers to make solid triangular pieces. Within the variability of the signal/atmospheric effects, no measurable difference.

Put a reflector screen behind the active elements. No appreciable difference

Modified the whiskers and made "stacked" dipoles. Maybe 1 dB difference.

Took a Channel Master 4228 HD antenna and used that as the active antenna (facing in toward the dish). Depending on its location in the dish focal plane, got up to 5 dB of gain. The 4228 HD can be modded to improve its gain by another 3 dB (there are a couple of sites that describe how to do this)--I didn't make those mods, but I'll believe the claims.

But---the advantage of the parabolic, collecting a huge amount of signal, is fighting the disadvantage of "no altitude". And the measurements showed that it's a losing battle.

I took the Channel Master and raised it about 15 feed higher than the parabolic--and got more signal at every channel from the naked 8-bay bowtie at that altitude. Sigh.

Then I did a comparison of the Channel Master versus a pair of stacked broadband Yagis (AntennaCraft) and got surprising news: at almost every frequency, the Yagis beat the Channel Master. Obviously in VHF (actual channels 7, 8, and 11) there was no contest...but even in high UHF the Yagis were providing more signal, or were within a dB or two of the Channel Master.

So in this deep-fringe reception area, the lessons are pretty clear (even if somewhat counter-intuitive):

1. Altitude trumps everything else (duh)
2. If you need VHF channels, a Yagi or two is in your future
3. The 10-foot parabolics don't offer that much of a gain advantage at terrestrial TV frequencies (this was a surprise to me)
4. Modifying the active elements of the antenna didn't make as much of a difference as I'd expect
5. I'm glad I ordered the Channel Master via Amazon, as they allowed me to return it

Although this has turned out to be a wild-goose chase (I'm back with the Yagis I started with), hopefully my experience will persuade some of you to not go down this path unless you have a really strong tower to put your 10-foot, 100 pound parabolic on.