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tripelo 20-Sep-2014 3:14 PM

Pete, that's a nice image of your vertically stacked HDB8-X antennas.


Originally Posted by Pete Higgins (Post 46532)
... When I added a second HDB8-X to my TV tower array most channels signal strength improved 1-2 dB...

... but at least one actually appeared to get weaker?


There can be certain locations in elevation (height) where an antenna with an aperture that is small in the vertical dimension provides more signal strength than a larger antenna.

Could be that two (not-in-phase) signal fronts simultaneously arrive at different heights within the vertically spaced array (upper and lower portions of the antennas).

Maybe, analogous to the diagram posted earlier under the heading:

Stacked Antennas: Multipath & Diversity Gain?

The post, linked above, contains a drawing of stacked antennas and shows a direct wavefront (D) arriving at the lower antenna with a multipath wavefront (M=-D) arriving at the upper antenna. The signal summation would be less than the direct (D) alone.

A following post will show some measured signal variation versus height, likely the result of multipath.


tripelo 20-Sep-2014 4:14 PM

UHF Signal Strength vs Antenna Height (AGL)
1 Attachment(s)
This post is a continuation of:

UHF Signal Strength vs Antenna Height (AGL)

In the previous post linked above; the signal strength of 27 DTV channels was measured as a small antenna (Terk HDTVi) was raised in height.

The signal strength of the individual 27 measured DFW stations varied considerably, from ~-35dBm to about -90 dBm. Due to the wide range of strength, it might not be very instructive to plot all signals on the same graph.

The following graph shows three of the weaker signals strength plotted versus antenna height.

This graph illustrates what can be seen throughout all the measured signals (strong and weak).

That is:

1. Some channels show maxima and minima in signal strength as antenna elevation is changed.
2. The general trend is increasing signal strength with increases in antenna height.

Channel 50 shows the most distinct pattern of maxima and minima. This is evidence of signal layering; the constructive and destructive summation of two or more wavefronts.

Channel 51, although close in frequency, shows increasing signal strength with small perturbations. The two stations (channels 50 and 51), yield different responses at the receiving antenna, this could be related to differences in path geometry.

The transmitting antennas of these two stations are located on different towers and at different tower heights*. Incidentally, both are low power=15kW.

*Such transmitter/antenna info can be found at the FCC site, or more conveniently at

Channel frequency does matter. The signal wavelength is inversely related to frequency and constructive/destructive combinations are a matter of signal phase (which is wavelength related). Signal strength differences at the receiving antenna are a result of the combination of frequency and path geometry.

In a different environment, it is likely that the above trends continue, but a specific frequency (channel) may or may not respond with observable maxima and minima.

The response of channel 27 is somewhat typical of the measured channels (total number=27).


Pete Higgins 20-Sep-2014 6:58 PM


As always, very interesting information. I am guessing that the channel I lost by moving my UHF antenna up to 40’ is a phenomena similar to your channel 50 between 18’ & 22’. The ~ 6 dB weaker signal probably didn’t stop demodulation at your location but with my already weak 1 & 2 edge signals 6 dB is a real killer.

You are much better equipped to perform meaningful analysis with your Sencore SLM 1456 than I am using my software defined radios. My SDR’s have ~50 dB dynamic range and I believe the Sencore has ~114 dB (-98 to +16 dBm) range. It also appears to support direct readout in dBm and all I can do is compare relative signal strength differences in dB. But then I expect your Sencore cost you slightly more than the $8 to $9 that each of my SDR tuners cost me. These little devils plug into a USB port on a computer and tune from 24 MHz to 1.7+ GHz with good sensitivity. In addition to their limited Spectrum Analyzer capabilities I use a free program called SDR Sharp, under XP & Windows 7, to listen to commercial FM, tune the Ham bands (including all the local repeaters), VHF & UHF air and NOAA weather broadcasts. I even downloaded a plugin that allows me to start them scanning and stops on an active channel. I posted some information about them here:

When I posted my dual HDB8-X picture I couldn’t remember how you told me to make it show up on this site. If you have time, can you post that information again?

rabbit73 21-Sep-2014 3:47 AM

1 Attachment(s)
The clue is in the text box. Just read the text box in a post as if you were going to quote it. You used the link icon for a URL instead of just adding the BB code to the attachment URL:

Pete Higgins 21-Sep-2014 7:44 AM


Hi rabbit –long time again!

I put the image tags ([IMG] [/IMG]) on the link to my picture in post 80, expecting them to cause the picture to display in the post.

What I ended up with was the link displayed with IMG tags (see midway in post 80).

Since we can’t store pictures on this site in a photo album and use the bb code to display them I need to relearn how to get them to display. tripelo told me how some time ago, and it worked, but I lost the recipe. It looks like the example you posted worked so I’m not sure what I’m missing?

1. Add the graphic as an attachment to the post
2. Paste the graphic's link into the post
3. Add the image tags ([IMG] & [/IMG] to each end of the link
4. Highlight the complete link including the image tags and click on the "Wrap [quote] tags around selected text" icon. See above, it works!

Intuitively obvious to even a casual observer?

Thanks rabbit.

rabbit73 21-Sep-2014 7:55 PM

1 Attachment(s)
You're welcome.

You don't need to wrap quotes in step 4. Just leave the attachment showing at the end of the post and add BB code to attachment URL in text box. However, your way does eliminate the duplication of image in post and image in attachment. I never thought of that option. The quote box around the image does look nice.

In some forums, if the attachment isn't used at the end of the post, the link to the attachment is no longer active. So, it's best to do it like tripelo does, using just the BB code for the URL and leaving the attachment in place. As a bonus, you get an image counter by the attachment link that tells you how many times the image was shown, and anyone can download your image just by clicking on the attachment link.

The only reason I mentioned quote is to see what another person has done in his text box so that his attachment would show in post. Just click on the Quote button at the lower right corner of his post. This is what tripelo's text box would look like:

Intuitively obvious to even a casual observer?
It wasn't obvious to me until I learned how to do it. HA!:)

tripelo 24-Jan-2015 3:14 PM

Compare Several Preamplifiers OTA-DFW
5 Attachment(s)
Signal measurements from several preamplifiers were compared, while receiving selected UHF channels and one VHF channel in the Dallas Fort Worth area.

Preamps Compared (shown in image below):

- Channel Master 7777 (new version)
- Channel Master 7778 (new version)
- Channel Master 7777 (old version)
- Channel Master 7778 (old version)
- Winegard LNA-200
- Antennas Direct PA-18

All 7 preamps performed overall within ~0.5 dB of each other when comparing the measured MER* (modulation error ratio). The RCA TVPRAMP1R showed best UHF MER performance at 30.2 dB. MER is essentially the post-detection S/N ratio in dB. (MER performance graph below)

As expected, there was considerable difference in UHF output signal strength (approximately 9 dB variation). Both the CM7777 versions (old and new) had highest UHF output signal strength. (Output signal strength graph below)

Note: Previous measurements, made a day earlier, with these preamplifiers indicate that the results are somewhat repeatable. As the data in the graphs indicate, the precision of measurements can be made to within 0.1 dB, but mainly due to the propagation path, the accuracy is somewhat less. The rank order of the preamp’s measured performance was unchanged from one day to next.

*Modulation Error Ratio (MER)

Measured MER can be a good relative indicator of the overall system noise figure. Within limits, a system with a lower noise figure will have a higher MER. There is nearly 1:1 correspondence between noise figure and MER; a 1dB improvement in noise figure will provide about 1dB improvement in MER.

Other factors affect system MER, for example if two preamps had the same noise figure and one had higher gain, then likely the system MER would be highest for the preamp with highest gain. With typical preamps and system losses, there could be about 0.1 dB MER increase for about 1-2 dB of additional gain.

More to be posted, some tomorrow:

- Test Procedure & Configuration
- MER Performance per Channel
- Signal Output Level per Channel
- VHF Channel 8 Performance (MER & Signal Strength)
- Other Preamps

ADTech 24-Jan-2015 4:20 PM

Since all the MER readings were with 0.5 dB, it's pretty much a statistical dead heat. No one would ever notice any difference under typical customer reception conditions. If you use a wider scale on your bar graph say from 15 to 35, you'd have a very gradually sloped line, almost flat, around the 30 dB line.

Pete Higgins 24-Jan-2015 7:38 PM


Based on manufactures specs. & cost my mind tells me that my $70.00 LNA-200 should perform better than my $23.00 RCA’s but when I swap them back & forth I can’t tell any difference (signal strength & SNR). The one difference I’ve repeatedly noticed is that the RCA’s seem more immune to overload from my strong FM & TV stations when the array is pointed at them.

I’ve only found two amplifier designs that don’t show this overload issue, the RCA’s and my various PCT drop amplifiers (MA-B1015-1A, PCT-MA2-4P & PCT MA2-M). The drop amps don’t have as much gain but my SNR’s hardly change when I trade between the RCA’s & the PCT’s.

It would be really interesting to compare the dynamic ranges & strong signal handling characteristics of the various offerings.

As always, great work!

tripelo 25-Jan-2015 4:07 PM

Thank you ADtech and Pete. Your comments are appreciated.


Originally Posted by ADTech (Post 48947)
Since all the MER readings were with 0.5 dB, it's pretty much a statistical dead heat…

That is a possibility.


…No one would ever notice any difference under typical customer reception conditions…
Most would probably agree.


…If you use a wider scale on your bar graph say from 15 to 35, you'd have a very gradually sloped line, almost flat, around the 30 dB line.



Originally Posted by Pete Higgins (Post 48949)
...found two amplifier designs that don’t show this overload issue, the RCA’s and my various PCT drop amplifiers (MA-B1015-1A, PCT-MA2-4P & PCT MA2-M)...

Interesting experience with preamps and drop amplifiers.

As both you and ADtech seem to understand and appreciate, in many cases, the dynamic range of a preamp is a strong factor contributing to overall satisfactory performance.


…interesting to compare the dynamic ranges & strong signal handling characteristics of the various offerings…

tripelo 25-Jan-2015 5:04 PM

Compare Preamplifiers - Test Procedure & Configuration
4 Attachment(s)
Referring to Preamp Comparison data posted earlier.

Comparison method; signals and path from antenna to measurement receiver:

1. Received on Radio Shack all-band antenna (similar to VU-90) at approximately 29 feet AGL, thru 50 feet commercial grade RG-6 cable

2. Attenuated by 40 dB; two JFW serial step-attenuators total 37 dB followed by 12” RG6, then one Holland 3 dB FAM attenuator.

3. Amplified by preamp-under-test with its power inserter and power supply

(If available, preamp FM trap set to ‘In’, If dual inputs, set to ‘Combined’)

4. Attenuated 3 dB; Output thru 4” RG-59 and Holland 3dB FAM attenuator

5. Received from preamp and 3dB attenuation, measured on Sencore SLM1456CM

Image depicting Test configuration shown below:

The Sencore 1456CM executed a channel scan program wherein Modulation Error Ratio (MER) and Signal Level were measured and recorded while receiving DFW stations. Each test consisted of SML1456CM channel scan that measured signal MER and Strength for each of the following channels: 8, 9, 14, 23, 29, 36, 41, and 48. The scan was repeated three times (total three scan test time ~ 5 minutes). Then, the next preamplifier was tested, and sequence repeated until all 7 preamps had been examined.

Channel 9 data not planned to be included in the posted data. This is mainly because channel 9 is weaker than channel 8, to reliably receive it with all preamps would have required input attenuator adjustment during the tests.

The time from beginning-to-end for the complete test operation was about 75 minutes. This total time included, as a data integrity check, some replication of the first two scans at the end. Also included some other tests not reported.

The test data was downloaded from the Sencore 1456CM to a desktop computer for analysis and formatting.

The distance to the stations is ~28-30 miles, the path line-of-sight. Prior and post measurements indicated the signals were relatively steady during the test time. A clear stable-weather day was chosen to perform the tests. The signals are strong at this location, thus the need for 40 dB attenuation prior to the preamps. The objective was to lower signal strength such that the stations had only moderate S/N ratios within measurement range. (High signal strength can cause maximum S/N indication (max MER=36 dB) on the SLM1456CM.

The RF environment in DFW consists of dense signals. Virtually every usable UHF channel is occupied, only 2 VHF stations (Channels 8 & 9). The FM band has nearly every available channel occupied (many with full-power). Other strong signals include NOAA weather radio ~ 4 miles away.

The Radio Shack antenna responds to most of the above listed signals.

The test setup somewhat emulates conventional TV reception. The 40 dB of attenuation to preamp input reduces all signals, including FM and other potential interference sources. This reduction in signal strength could correspond to fringe area reception. The receiver is represented by the SLM1456CM and a transmission line loss between preamp and receiver being represented by a 3dB attenuator (similar to ~50’ RG-6 at UHF).


Reference charts below: TVfool & FMfool charts for the preamp compare test location.

More preamp test data to be posted later.


Pete Higgins 29-Jan-2015 5:46 AM

Fascinating read –well done!

It’s obvious that you put a lot of work into this professional caliber test report. I wish more people would take the time to pictorially depict their test setups like you do. Clear, concise & helps answer questions generated while reading your reports.

I’d be really pleased if I had your signal environment. My strongest signal of interest is ABC, channel 7 @-87.0 & 2-edge to boot. Box Springs Mtn. 3.5 miles out my back door and in-line with my LA stations is home to numerous communication towers, a channel 26 PBS TV station, several FM broadcast stations, a 2-Meter repeater that just booms in on my TV antennas plus I also pickup four NOAA weather channels with my LA array.

One of the items I found to eliminate some of the unknowns in my testing was the inline Channel Plus LPF-750 Low-Pass Filter. It’s basically a 0 to 750 MHz band pass filter I use to remove the cellular signals emanating off Box Springs Mtn.

The only amplifier that I’m aware of that filters that portion of the spectrum is the new Antennas Direct Juice. ADTech did an excellent write-up on that amp here:


More preamp test data to be posted later.
Looking forward to your follow-on.

tripelo 30-Jan-2015 9:13 PM

Pete, thank you for your comments.


Originally Posted by Pete Higgins (Post 49103)
…I’d be really pleased if I had your signal environment…

Your signal situation does seem particularly tough.

Guess most people have a more difficult TVfool chart than this one in DFW.

This DFW location is good for UHF test purposes.


…One of the items I found to eliminate some of the unknowns in my testing was the inline Channel Plus LPF-750 Low-Pass Filter…
That's a good idea.

Filters are almost a must for detailed testing, and sometimes necessary for ordinary TV reception. Although, often it is difficult to purchase filters with desired frequency and attenuation characteristics.

On one occasion for the KY setup, built a custom band pass filter.


The only amplifier that I’m aware of that filters that portion of the spectrum is the new Antennas Direct Juice.
Thanks for the info, looks interesting.

tripelo 30-Jan-2015 10:00 PM

Compare Several Preamplifiers (cont.)
2 Attachment(s)
Referring to previous posts:

- Compare Several Preamplifiers OTA-DFW
- Compare Preamplifiers - Test Procedure & Configuration

As mentioned earlier, the two VHF channels in the DFW area are channels 8 and 9. Measurements of both channels were attempted during the channel scans. Because channel 9 signal strength is weaker than other channels in the scanned list, it could not be reliably decoded by some of the preamps without attenuator adjustment during the tests.

Below is a graph showing the averaged MER value for each of the 7 preamplifiers receiving channel 8. The values shown represent the average of 3 individual measurements. The range of MER for the preamps is nearly 3 dB. The AD PA-18 provided the highest MER (26.7 dB), with the new CM7778 second (26.6 dB).

The image below shows the output power level for each of the preamplifiers.

The highest output power is shown for the CM7777 versions:

- CM 7777 new (-54.6 dBm) and,
- CM 7777 old (-57.7 dBm)

Notably, the lower MER values for channel 8 originated in the preamplifiers with separate amplifier sections for VHF and UHF.

- CM7778 old
- CM7777 old
- WG LNA200*

*The WG LNA-200 has separate amplifiers even though it doesn’t have dual inputs.

A plausible explanation for lower channel 8 MER values could be;

The VHF-only sections have higher noise figures & generally lower gain than the VHF-UHF combination or UHF-only sections (single or dual amplifier versions). Investigations into the hardware design tend support this. Some factors specific to VHF could make this design choice seem logical to a manufacturer.


Acme 13-Mar-2015 12:07 PM

Some info please
Hello Pete,
Do you still have the RCA 10W707 user manual and hopefully some schematic too ? I have this rotor but do not have a rotor control box. I would like to make it computer controllable, so the schematic would come in hand. Thank you for any info you might get me.



Originally Posted by Pete Higgins (Post 36972)

No comparison. The Radio Shack 15-1220 is a 5-wire rotor that uses a balanced bridge for exact positioning. In the 50 years I’ve used it; it has never gone out of alignment. It also seems to produce more turning torque. The accompanying control box displays ‘N’ at the top & ‘S’ – ‘S’ at either end of rotation. Being an old flyer, that fits the way I think.

I also have an RCA 10W707S that is almost identical. Motor unit is exactly the same and control box looks identical except it displays ‘S’ at the top and ‘N’ – ‘N’ at either end of rotation. The one drawback is the directional calibration. Tic marks on both rotors are 4.5 deg. apart. When aiming towards a “True” heading, I can visualize the heading with respect to true north and align the knob pretty close. Degrees with respect to true south –not so much. (If I put the RCA in service, I’ll make stick-on labels for the cardinal headings (S-S, W, N, E) to cover the ones silkscreened on the control panel. Other than control panel screening & branding, they really are identical.)

The U-106 is a three wire rotor that uses a timing circuit for calibration. 3-wire rotors use AC synchronous motors that run at a predictable speed, and the control box simply runs the motor for the amount of time needed to turn the antenna from where the controller thinks it is to where the controller wants it to be. Over time, the position error grows.

After moving it to “home” or 00 deg., basically, you rotate it full clockwise (360 deg.), hit a button labeled “Initial” and it memorizes the time it takes to return to 00 or “home”. Unfortunately, it frequently requires recalibration. Since my signals come from four directions, it’s fairly easy to detect when it needs calibration because when I turn the array there is either nothing there or the signals are barely watchable. I probably have as much calibration mileage on the rotor as actual array turning mileage. Sometimes, when the calibration goes off, it won’t fully turn to a true 360 so to get it full clockwise you have to power off, power on with a “reset” turn some more and repeat the process until it visually has the array pointing north. I’ve since read that other folks are experiencing the same calibration issues with other brands of 3-wire rotors. I think AntennaCraft, Centronics, Channel Master, Magnavox, Philips, RCA, Stern and probably others sell this same design under their own labels.

I bought the U-106 because the literature said “Digital display indicates antenna position during operation” & “Pre-set to 12 TV/FM station directions for automatic antenna positioning”. What I didn’t pick up on was that the bearing readout was a 2-digit display. North is 00 or 36, east is 09, south is 18 and west is 27. My street is aligned with True North and my house sits squarely on the lot facing east so visually it’s easy to determine array alignment. Even after a fresh calibration, manually turning the rotor to display “18” can have me pointing somewhere between ~175 & ~185. Fortunately, if you “bump align” for max SNR and memorize the location to a pushbutton it seems to return to that location –at least until it goes out of calibration.

I’ve been tempted to put the RCA on the tower, but then I’d have the new U-106 just sitting in the garage gathering dust. It’s useable, but I couldn’t in good conscience sell it to anybody.

Hello Pete,
The TB-105 support bearing that you ordered is currently out of stock. The vendor will not have anything available until mid September.
Please let me know if you wish to keep your order open or cancel.
Thank You

Kept order open, so it looks like I don't have to go out in the 100 deg. heat (at least until September).

Found this ad in the Pitsburg Post-Gazette for Friday August 24th 1973. I think I bought mine about 10 years earlier for $29 or $39.

Pete Higgins 13-Mar-2015 4:46 PM


I have a service manual that covers the Alliance C225 & C225A rotor’s. As near as I can tell, the Radio Shack 15-1220 and RCA 10W707S circuitry is almost identical to these Alliance models.

Send me a PM with your email address and I’ll be happy to email you a copy.

MrMotivations 23-Jan-2016 10:31 PM

Homebrew 183Mhz
1 Attachment(s)
Since I mounted this on the eaves, around 40 + miles from Nashville signal sources, this design I targeted for 183 Mhz on the Vhf frequency. To my astonishment, it picks up all the available channels as well as my commercial Antennas Direct 91XG. We are happy with the Antennas Direct unidirectional, however. My little jobbie is not very robust. But it works surprisingly.

tripelo 29-Apr-2016 5:10 PM

Compare Received Signal Strength of 5 UHF Antennas
1 Attachment(s)
A series of antenna tests were conducted on April 14, 2016.

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


1. Antennas Direct 91xg
2. Solid Signal HDB91x
3. Channel Master CM-4228 (old version)
4. Antennas direct DB8 (non-e version)
5. Terk HDTVi (as reference)

Images of antennas at test location in a following post.

Test Location:

Garland, TX, ~30 miles line-of-sight to 27 DFW stations, located within +/-3 degrees azimuth.

Test location TVfool info posted earlier:

Compare Preamplifiers - Test Procedure & Configuration

Test Equipment:

1. Sencore SLM1456 – Signal Level and MER measurements
(Scans channels one-by-one, dwells a few seconds per channel to lock and obtain a signal average, total scan time for 27 channels ~6-7 minutes)
2. DG8SAQ Vector Network analyzer -VNWA
(Measure scattering parameter S11 (complex input impedance->return loss/SWR)
3. Belden Coaxial Cable 1694A (40 feet)
4. Ferrite Cores at Coax to balun/antenna connection (Qty=4)
5. Balun for CM4228 (homebrew, hand-wound ferrite core, measured loss <1 dB)
6. Push-Up Telescopic Mast, 25+ ft.

Test Procedure

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

Starting with Terk HDTVi 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 29 (~central angle to DFW stations)
4. With 1456 scan all DFW UHF stations for signal strength and MER (record times)
(repeat scan, while in this configuration)
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. For each antenna repeat steps 2 through 8, finishing with a repeat of HDTVi antenna.

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

Televes DAT-75 vs. Antennas Direct 91XG

UHF Signal Strength vs Antenna Height (AGL)

Compare Preamplifiers - Test Procedure & Configuration

Compared to previous tests, some differences in equipment, this test:

- No attenuator was used,
- Sencore SLM1456 instead of SLM1456CM
- Others, VNWA, etc.


Forum member re_nelson provided much appreciated assistance throughout test planning, test, and follow-up.

RE_nelson also:

- Provided most of the antennas
- Provided the Sencore SLM1456
- Aligned antenna, operated the SLM1456, performed scans, & collected data.

Image below: RE_Nelson at the antenna test station with his Sencore SLM1456.

The antenna mast is at far left behind the ladder and porch post. To aim the antenna, re_nelson observes SLM1456 and hand rotates antenna mast to maximize the signal strength indication.

(To be continued)


tripelo 29-Apr-2016 5:11 PM

Images of 5 Antennas at Test site
3 Attachment(s)
(continued from previous post)

Part 2. Images of 5 Antennas at Test site

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

Image below shows Terk HDTVi and CM4228 (old version)

Image below shows Solid Signal HDB91x and Antennas Direct 91XG.

Image below shows Antennas Direct DB8 (non-e).

(To be continued)


tripelo 29-Apr-2016 5:12 PM

Received Signal Strength Data Analysis
5 Attachment(s)
(continued from previous post)

Part 3. Received Signal Strength Data Analysis, Five UHF Antennas

Attached are four Graphs depicting a summary of data analysis.

1. Average Relative Gain, (Image below):

An average 2 scans of all channels for each antenna, relative to the HDTVi antenna.

2.Average Relative Gain per Channel (image below)

A two-scan average of all antennas except HDTVi relative to HDTVi
(included four scans, two at beginning and two at end).

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.

4. Mismatch Loss vs. SWR* (image below, see Note at end)

Table showing mismatch loss as a function of SWR.

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

The CM4228 had overall slightly higher signal levels, this was mainly due to better gain at lower channels.

The HDB91x delivered highest signal strength at mid and higher channels, and with exception at channel 20, 2nd highest at lower channels.

The 91xg pretty much tied with the HDB91x at very highest channels, with average levels at lower channels.

The DB8 was about average at lower channels similar to the two Yagis. At midband signal levels dropped and stayed lower thru upper band. The signal level loss at midband might be related to the SWR (approaching SWR=4, with nearly 2 dB mismatch loss).

Notes: SWR* & Mismatch Loss

SWR is a positive number representing how well the antenna impedance matches a transmission line. For Example: SWR=1 represents a perfect match, Antenna impedance transformed by balun or other means to 75 Ohms perfectly matches a 75 Ohm coaxial line. Greatly simplified a SWR of 2 could be that the antenna impedance is either 150 Ohms or 37.5 Ohms, either of which is mismatched by a factor of 2.

When there is an impedance mismatch at intersection of antenna and transmission line, there is a certain amount of signal that is rejected (cannot be transferred from the antenna into the transmission line). This loss is called 'mismatch loss'.

When modeling antennas in software (i.e. 4NEC2), to obtain the true gain (sometimes called 'net gain') the mismatch loss must be subtracted from the calculated gain.

When measuring antennas in field test or at an antenna range, the effect of SWR is incorporated in measured signal levels. So, no subtraction is required. One could add back in the mismatch loss and derive a gain-like number, this number would be the 'directivity' of an antenna.

In the case of the antennas above, if the mismatch losses were added back to the measured responses, then one would see that the 'directivity' of the DB8 might be nearer to the 'directivity' of the other antennas.

Derivation of SWR & Mismatch Loss

Scattering parameter S11 was measured with reference at antenna/balun terminals. S11 represents a complex (real and imaginary) reflection coefficient, which includes ‘return loss’.

From return loss (RL), both SWR and ‘Mismatch Loss’ (ML) can be can be calculated:

(To be continued)


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