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Old 21-Sep-2020, 6:43 PM   #61
rabbit73
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Using an Attenuator to Measure the Strength of a Digital TV Signal, Part 2

Using an Attenuator to Measure the Strength of a Digital TV Signal, Part 2

I ordered a Mediasonic HW-150PVR and a Toner 0-20 dB Variable Attenuator to see how they would do at estimating the strength of a TV signal.

I had previously used a 0-10 dB step attenuator, but they are expensive; I was lucky to find a good used one. I'm trying to find the least expensive equipment to do the job.

I had also used an Antennas Direct 0-20 dB Variable Attenuator, but they have been discontinued along with their FM filter.









Since the AD ATT-1 was no longer available for others to buy, I made a calibration chart for the Toner attenuator:



I don't know why the factory pasted the label on upside down.

I put a black mark on the knob and on the housing to be able to count the number of turns as I rotated the shaft counterclockwise with a screwdriver to increase the attenuation to dropout.



This was the setup:

Code:


                    Fixed       Variable                      SDR
Ant > 25 ft RG6 > Attenuator > Attenuator > 4-Way Splitter > HW-150
                    20 dB    at 1-1/2 turns                   SLM*
                                  5 dB                   75 ohm terminator
* SLM is Sadelco DisplayMax 800 Signal Level Meter


To find the signal strength of Channel 20, I added the attenuator setting to the average tuner dropout point of -85 dBm:
-85dBm + 25 dB = -60 dBm

To confirm the method, my Sadelco DisplayMax Signal level meter reading was -11.9 dBmV = -60.7 dBm

(Since I used a 4-Way splitter, the signal before the splitter was 7 dB stronger. I put a 75 Ohm terminator on the unused port because without it, the outputs weren't equal.)

This method works best on UHF. On VHF, where the noise level is higher, dropout will be higher than at -85 dBm.



The Thermal Noise Floor for a 6 MHz wide TV signal is about -106 dBm.

The Noise Figure for the average tuner is 6 dB:
-106 dBm + 6 dB = -100 dBm

To that we must add 15 dB to allow for the minimum required SNR for the TV signal:
-100 dBm + 15 dB = -85 dBm

However, if the ambient noise is stronger than the internal noise of the tuner, the tuner Noise will be buried in the ambient noise.
Attached Images
File Type: png Toner Attenuator Calib Curve3.png (66.3 KB, 276 views)
File Type: jpg Toner 0-20 dB Variable Attenuator and 20 dB Fixed2.jpg (70.5 KB, 268 views)
File Type: jpg MediasonicHW-150PVR9-19-2020CH20SNR15.JPG (137.2 KB, 344 views)
File Type: png Minimum Indoor Signal Levels_5.png (81.2 KB, 264 views)
File Type: png AD ATT-1 Calib2.png (112.5 KB, 292 views)
File Type: jpg Variable T-Pad Attenuator.jpg (75.5 KB, 269 views)
File Type: jpg Piher Variable Attenuators2.jpg (105.7 KB, 266 views)
File Type: jpg T-Pad Attenuator Values5.jpg (59.5 KB, 259 views)
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Last edited by rabbit73; 22-Sep-2020 at 6:54 PM.
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Old 26-Sep-2020, 11:01 PM   #62
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Review of Toner 0-20 dB Variable Attenuator

I ordered two of the Toner attenuators in case one was defective. One of them had a coax connector with insulation that hadn't been completely removed, making it difficult to insert the center conductor of the RG6 coax.





I removed the excess insulation with a drill bit held in my hand; the coax was then able to be properly inserted.

I still haven't figured out why they would paste the label on upside down.



It's important to check the female coax connectors to see if they make good contact with the center conductor of the coax:







Attached Images
File Type: jpg Toner Attenuator Connector OK.jpg (195.0 KB, 206 views)
File Type: jpg Toner Attenuator Connector NG.jpg (220.2 KB, 204 views)
File Type: jpg Toner Attenuator Label Inverted.jpg (183.5 KB, 203 views)
File Type: jpg Testing Coax Connectors.jpg (115.0 KB, 211 views)
File Type: jpg TestingCoaxTip3_1.jpg (93.7 KB, 196 views)
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Last edited by rabbit73; 26-Sep-2020 at 11:44 PM.
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Old 17-Oct-2020, 2:14 AM   #63
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Quote:
Originally Posted by tripelo View Post

Would like your thoughts on dynamic range of Airspy vs. SDRplay RSP1.

Thanks
I did some additional tests with the SDRplay RSP1A and the RSP Spectrum Analyser (British spelling) software.

It seems that it is necessary to properly set the Reference level of the RSP Spectrum Analyser. If you set it too high, part or all of the signal will be buried in the noise.

The RSP Spectrum Analyser tries to simulate what a real spectrum analyzer does. You have to be careful with the Display scaling settings to get a correct representation of SNR. Manual:
https://www.sdrplay.com/docs/RSP-Spe...lyser-V1.1.pdf

Setting RSP Spectrum Analyser Reference Level

Quote:
Reference Level

A common error is a complaint that signals with a 40-50 dB SNR as displayed on SDRuno, shows a signal barely breaking through the noise floor on the analyser. In almost all cases, AGC is enabled on SDRuno, and the LNA gain reduction slider set at a low gain reduction value resulting in a lower noise floor. IF gain reduction will be whatever the AGC system sets it at.

Conversely, the analyser is usually set at a high reference level of somewhere between -30 to+10 dBm, has a very high noise floor and sees little or no signal above the noise. This is usually caused by the user attempting to use the reference level control to position the trace within the display area, instead of using the range and offset controls provided for this purpose.

The reference level should initially be set to around -80 dBm, while ensuring the ADC is not overloaded and spurious signals are not generated. This will give maximum sensitivity and a lower noise floor. Increasing the reference level towards 0 dBm will result in an increasing noise floor, and depending on signal level, may result a lower S/N ratio. Use the range and offset controls to scale and position the display as required.
An example:









Testing for spurs:





Then I decided to just wrap the plastic SDR enclosure in foil and turn On the Clock spur removal



CONCLUSIONS:

It is important to properly set the Reference level to display the correct SNR.

The RSP1A has a plastic enclosure that is letting strong signals through. I had hoped that the inside of the plastic enclosure had been sprayed with a conductive paint that would act like a shield, but apparently that wasn't done.



The aluminum foil that I put on the outside of the enclosure, and contacting the ground of the SMA antenna input connector, acted as a substitute shield.



The RSP2 looks like it has a conductive coating inside:

Attached Images
File Type: jpg Ref level -30 dBm3_1.jpg (135.7 KB, 125 views)
File Type: jpg Ref level -40 dBm3_1.jpg (136.1 KB, 124 views)
File Type: jpg Ref level -50 dBm3_1.jpg (138.0 KB, 128 views)
File Type: jpg Ref level -60 dBm3_1.jpg (137.3 KB, 122 views)
File Type: jpg Ref level -60 dBm 75ohmR3_1.jpg (149.6 KB, 126 views)
File Type: jpg Ref level -60 dBm 75ohmFoil3_1.jpg (125.4 KB, 127 views)
File Type: jpg Ref level -60 dBm 75ohmFoilEnclSpurRemovOn3_1.jpg (130.1 KB, 136 views)
File Type: jpg RSP1A-Inside3.jpg (131.5 KB, 125 views)
File Type: jpg RSP2-Inside3.jpg (147.2 KB, 123 views)
File Type: jpg RSP1AfoilShield.jpg (107.4 KB, 119 views)
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Last edited by rabbit73; 3-Nov-2020 at 1:28 AM.
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Old 26-Oct-2020, 3:42 PM   #64
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Quote:
Originally Posted by tripelo View Post
Have noticed that (spurs) in the low cost RTL devices. Most of the time not a problem if one is aware of their presence.
I found another scanner that can be used with the RTL-SDR.COM V3 dongle.
http://rfexplorer.com/touchstone/

The resolution isn't too good and there is no way to adjust the gain of the dongle. It is necessary to insert an attenuator before the dongle to obtain the correct representation of SNR.

But, the Touchstone software does cover more than one channel and you can see changes in real time. With the RTLSDR Scanner, you don't see the scan until it is completed.





The amount of attenuation must be found by trial-and-error. It helps if you have a TV that will tell you the SNR of a known signal.

The Toner attenuator shown above could be used.



My amateur Videos

I didn't use a preamp for this scan. The channel 16 signal (first strong signal on the left) measured -6.3 dBmV (-55.1 dBm) with my Sadelco DisplayMax 800 signal level meter:
https://www.dropbox.com/s/no068tnzs0...45-43.mp4?dl=0

https://www.dropbox.com/s/no068tnzs0...45-43.mp4?dl=1

When I used a 17 dB preamp (CM7777HD Amplify, low gain setting), more attenuation before the dongle was needed.
https://www.dropbox.com/s/18g3b36dmv...27-48.mp4?dl=0

https://www.dropbox.com/s/18g3b36dmv...27-48.mp4?dl=1

I use VLC Media Player.

If you want to be able to adjust the gain, you would need to purchase the Rational Waves RF SPECTRUM ANALYZER SOFTWARE:
http://rationalwaves.com/rationalwaves/

http://nutsaboutnets.com/supported-rf-analyzers/

But, I think it would be better to buy an SDRplay RSP1A and use the RSP Spectrum Analyser software.

When I used the 17 dB preamp with the SDRplay RSP1A and the RSP Spectrum Analyser software, I was able to adjust the Display Scale settings for a proper display:



The ADC is the Analog to Digital Converter in the SDR.



The channel 16 signal input to the RSP1A with the 17 dB preamp measured -35 dBm. I don't usually use the preamp; just wanted to see how the RSP Spectrum Analyser software would handle the stronger signal.

When the RSP1A is used with the SDRuno software, there is an overload warning if the gain is set too high. It did flash when the gain was set to MAX when using the 17 dB preamp. To avoid damage to the RSP1A, the input signal should be kept below 0 dBm (49 dBmV).

Attached Images
File Type: jpg Touchstone 10-25-2020 VHF-High_1.jpg (98.0 KB, 103 views)
File Type: jpg Touchstone 10-26-2020 UHF 10dBAttn_1.jpg (108.2 KB, 104 views)
File Type: jpg 11-1-2020 RSP SpecAnal-17dBamp_4.jpg (152.4 KB, 69 views)
File Type: jpg 11-1-2020 RSP SpecAnal-17dBampUHF_2.jpg (115.7 KB, 71 views)
File Type: jpg RSP1A Overload2.jpg (70.5 KB, 70 views)
File Type: jpg TV Minimum Signal3.jpg (111.7 KB, 18 views)
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Last edited by rabbit73; 7-Nov-2020 at 8:27 PM.
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Old 24-Nov-2020, 2:57 PM   #65
tripelo
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Export Data Airspy R2?

Hi rabbit73, or anyone familiar with Airspy.

Do you know if there is software for the Airspy R2 that exports logs of spectrum scans in .csv format (or something similar)?

Interested in software for Airspy R2 that can log and export amplitude vs frequency, similar to RTLSDR Scanner.

The purpose would be to save signal data sets (many scans) for later comparison using Excel.

Thanks.
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Old 24-Nov-2020, 4:11 PM   #66
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Are you familiar with rtl-sdr dot com? not sure I can post a url here -
https://www.rtl-sdr.com/big-list-rtl...rted-software/ has a large list of software.

I use with SDR# for tuning as well as Orbitron and WXtoimg to download satellite weather.

Also to check out signal strength from my TV antenna.

Rabbit started me on this journey. I am sure he will have other ideas for you.
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Old 24-Nov-2020, 4:52 PM   #67
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Quote:
Originally Posted by eclipsme View Post
Are you familiar with rtl-sdr dot com? not sure I can post a url here -
https://www.rtl-sdr.com/big-list-rtl...rted-software/ has a large list of software.
Nice list of software, thanks.

RTLSDR Scanner is shown in the list. Used it several years ago with the least costly version of RTL-SDR dongle. RTLSDR Scanner with its export feature, worked fine for the specific application.

Looking for something similar that will work with Airspy R2.

Not sure how much, if any, of the software in that list, will work with the Airspy R2.

.
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Old 24-Nov-2020, 4:57 PM   #68
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I have no direct experience running on Airspy , but as far as I know, it should. Why not give it a try and see what happens?
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Old 24-Nov-2020, 7:13 PM   #69
rabbit73
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Quote:
Originally Posted by tripelo View Post
Do you know if there is software for the Airspy R2 that exports logs of spectrum scans in .csv format (or something similar)?

Interested in software for Airspy R2 that can log and export amplitude vs frequency, similar to RTLSDR Scanner.
Sorry, none that I know of.
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Old 24-Nov-2020, 7:25 PM   #70
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Quote:
Originally Posted by eclipsme View Post
...Why not give it a try and see what happens?
Haven't tried it for a couple of reasons:

1. Do not yet possess Airspy R2.
2. Haven't seen on Internet where anyone has successfully used the combo.

The reason for Item #2 could be because the RTLSDR Scanner was designed to be used on the more common inexpensive RTLSDR devices. There appears to be considerable hardware differences between the lower cost devices and the Airspy.

Quote:
Originally Posted by rabbit73 View Post
Sorry, none that I know of.
Thanks, rabbit73.

.
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Old 24-Nov-2020, 8:30 PM   #71
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Quote:
Originally Posted by tripelo View Post

1. Do not yet possess Airspy R2.
2. Haven't seen on Internet where anyone has successfully used the combo.
The only software that I know of that will produce CSV output is for the RF Explorer SA and the V1.1 RSP Spectrum Analyser for the SDRplay RSP1A:



Attached Images
File Type: jpg RSP-SpectrumAnalyser-V1.1 Window2.jpg (131.3 KB, 20 views)
File Type: jpg RSP-SpectrumAnalyser-V1.1 CSV Window2.jpg (132.0 KB, 19 views)
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Last edited by rabbit73; 24-Nov-2020 at 8:32 PM.
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Old 24-Nov-2020, 9:18 PM   #72
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Quote:
Originally Posted by rabbit73 View Post
The only software that I know of that will produce CSV output is for the RF Explorer SA and the V1.1 RSP Spectrum Analyser for the SDRplay RSP1A:
Interesting, thanks for the info and images.

Seems the Airspy has such a good reputation, that there would be logging/export software available for it.

As we discussed earlier in this thread, interested in high dynamic range obtainable with either SDRplay RSP1 or Airspy R2.

Tend to favor the Airspy R2 over the RSP1 because it seems to display less spurs (indicating better dynamic range at least in spectral regions where spurs occur).

However, also would like to log and export data.

The specific high dynamic range application does not necessarily require logging, and the logging application does not necessarily require high dynamic range.

Therefore, an option could be to use Airspy R2 for high dynamic range tests and the low cost SDR dongle with RTLSDR Scanner software for logging.

.
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Old Yesterday, 1:56 AM   #73
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Quote:
Originally Posted by tripelo View Post
Therefore, an option could be to use Airspy R2 for high dynamic range tests and the low cost SDR dongle with RTLSDR Scanner software for logging.
I wonder if the Bandscan approach would be suitable for any of your needs.
https://rabbitears.info/tvdx/all_tuners
Map takes a while to load.

https://m.rabbitears.info/index.php?...10311B6D&tno=0

https://rabbitears.info/tvdx/one_tun...0311B6D/tuner0

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Last edited by rabbit73; Yesterday at 3:52 AM.
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Old Yesterday, 2:20 PM   #74
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Quote:
Originally Posted by rabbit73 View Post
I wonder if the Bandscan approach would be suitable for any of your needs...
Yes, thanks for your thoughts on this topic, that approach could be quite helpful.

Having capability for logging signal quality over longer periods of time (at least 24 hours) would help in evaluating the performance hardware design changes.

Think most contributors to RabbitEars Bandscan use SiliconDust HDHomeRun tuners.

Recently purchased one of those tuners for the purpose of recording signal strength and signal quality.

According to Trip at RabbitEars, seems the software that produces the web display is not set up to export raw data to a file format that could be conveniently imported to Excel.

A person who developed a Windows version of that software said it would take some time, but he could add the option to make the export.

.
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Old Yesterday, 3:02 PM   #75
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Quote:
Originally Posted by tripelo View Post
Having capability for logging signal quality over longer periods of time (at least 24 hours) would help in evaluating the performance hardware design changes.

Think most contributors to RabbitEars Bandscan use SiliconDust HDHomeRun tuners.
That is true. Here are Trip's instructions for setup:
Joining the Live Bandscan
https://www.rabbitears.info/static.p..._live_bandscan

It is possible to modify the graph for clarity:

Attached Images
File Type: jpg tripelo-Signal graph for KTVT modified_1.jpg (115.7 KB, 15 views)
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Last edited by rabbit73; Yesterday at 3:21 PM.
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Old Yesterday, 4:43 PM   #76
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Quote:
Originally Posted by rabbit73 View Post
...
It is possible to modify the graph for clarity:...
Yes, in many cases, the graphical images could be helpful.

Having the raw data to analyze, when looking for relatively small performance differences (say between two different antenna preamp combinations), could help to ascertain differences of less than one dB.

FYI:

Have designed, built, and tested several low-noise preamplifiers in combination with custom high gain antennas.

It is somewhat straightforward to evaluate relative noise figure of amplifiers, or the relative gain of antennas.

But more difficult to evaluate the performance of a particular preamplifier and antenna combination. The degree of impedance match or mismatch between amplifier and antenna can affect both noise figure and effective antenna gain.

Further complication is (in KY) receiving stations at 100 miles distant, there are large variations in signal levels vs time.

It is difficult to ascertain performance differences, especially when the performance difference may be a dB or even less.

.
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Old Yesterday, 5:25 PM   #77
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Quote:
Originally Posted by tripelo View Post
Having the raw data to analyze, when looking for relatively small performance differences (say between two different antenna preamp combinations), could help to ascertain differences of less than one dB.
It would be necessary to compare then two systems at the same instant because OTA signals constantly vary in strength. I have seen OTA signals vary as much as 10 dB in 30 minutes.

I have more confidence in the results when using a constant strength transmitted test signal for a comparison.
Quote:
But more difficult to evaluate the performance of a particular preamplifier and antenna combination. The degree of impedance match or mismatch between amplifier and antenna can affect both noise figure and effective antenna gain.
Very true. Keeping the preamp close to the antenna will minimize the mismatch loss. When you compare the two systems, that will tell you which has the superior performance.

Low noise preamps can make a difference on UHF if the ambient noise is below -100 dBm, but they don't seem to make much difference as compared to average NF preamps on VHF because of the higher ambient noise level on VHF, especially VHF-Low.

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Last edited by rabbit73; Yesterday at 5:35 PM.
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Old Yesterday, 6:44 PM   #78
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Quote:
Originally Posted by rabbit73 View Post
It would be necessary to compare then two systems at the same instant because OTA signals constantly vary in strength...
That would be good if was practical. For accuracy, the two antennas would need to be located at the same physical place. In many cases at UHF, a separation of a foot or so, and the two antenna combos can receive different signals.

Quote:
...I have seen OTA signals vary as much as 10 dB in 30 minutes.
Yes, at 100 mile path have seen variation of 10-15 dB in a minute or so.

Quote:
...I have more confidence in the results when using a constant strength transmitted test signal for a comparison...
That can allow measurement of signal strength related to antenna gain.

But doesn't address realized noise figure, or realized signal quality.

Quote:
...Keeping the preamp close to the antenna will minimize the mismatch loss...
In general, short transmission line minimizes transmission line losses.

Transmission line losses contribute directly to signal loss & degrade overall system noise figure.

However, even with virtually no transmission line, mismatch loss between antenna and preamp may still be present.

Mismatch has two main effects:

1. Signal loss which is is a result of reflection and reduces realizable antenna gain.
2. Degrades realizable noise figure of preamp.

The noise figure of the first active device in a preamp is dependent on the active device seeing a specified termination impedance.

Low noise preamps rarely have an input impedance of *real 75 Ohms*.

An antenna rarely has an impedance of real 75 ohms.

A preamp only has a specified noise figure when loaded with a specified complex impedance.
Preamps are usually tested for noise figure when terminated with 75 Ohms real impedance.

In reality, impedances of antennas are not real 75 Ohms and preamps input impedances are not real 75 Ohms.

So when combined;

- The realized antenna gain is less than optimum and
- The realized noise figure of the preamp is not as predicted.


-----------------
*real 75 Ohms*
-----------------

rabbit73, mainly for others benefit, realizing you know this:

Impedances generally are complex numbers, consisting of a real part and an imaginary part.

Usually denoted a R+jX or R-jX.

Where R=real and X=reactance, either inductive (+) or capacitive (-).

For perfect 75 Ohm match, the real part must be 75 Ohms and the reactance or imaginary part must be zero Ohms

.
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Old Yesterday, 7:07 PM   #79
rabbit73
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Quote:
Originally Posted by tripelo View Post
That would be good if was practical. For accuracy, the two antennas would need to be located at the same physical place. In many cases at UHF, a separation of a foot or so, and the two antenna combos can receive different signals.
That is true because the wavefront is non-uniform. I have seen a difference in the scan of a channel when moving the antenna a foot horizontally.



But then, how is it possible for you to make a valid comparison of the performance of the two systems? Have the two antennas on a sliding carriage that moves each antenna into the same location, or what? Otherwise, how can you possibly get a comparison like you would on a lab grade antenna range? On a range, you would use a constant strength test signal and place each antenna, in turn, in the same location.

The closest I got to the ideal was to switch the two antenna locations, but I really wanted to be able to slide them sideways.



The white wall was just a background for the photo. The actual comparison was done at a location with OTA signals that were as constant as I could find. I used an A/B switch for a rapid comparison.

Attached Images
File Type: jpg CH42setup2.jpg (224.1 KB, 13 views)
File Type: jpg QEXimg1_1.jpg (168.2 KB, 2 views)
File Type: jpg Antenna Test Site5_2.jpg (78.0 KB, 11 views)
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Last edited by rabbit73; Yesterday at 8:16 PM.
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Old Yesterday, 7:13 PM   #80
eclipsme
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"But then, how is it possible to make a valid comparison of the performance of the two systems? Have the two antennas on a sliding carriage that moves each antenna into the same location? "

would splitting 1 feed to the 2 systems work? Then reverse the feeds to compensate for output differences of the splitter and average the results?

There *must* be something wrong with this idea (lol!) It is just too simple, and I am too ignorant!

But very interesting discussion!
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