DIY Antenna Field Test Results by mclapp: GH1 vs C2 vs M4 - Canadian TV, Computing and Home Theatre Forums
 

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Old 2010-03-28, 11:16 PM   #1
mclapp
 
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Arrow DIY Antenna Field Test Results by mclapp: GH1 vs C2 vs M4

The antennas tested were a G-H gen1 with a split screen reflector, a Centipede-2 and (3) sizes of M4's


The centipede shown in the center had #6 copper elements which measure 23 1/2" wide and 27" tall and uses a 36" wide x 40" tall reflector with additional upper and lower shelf reflectors making a total reflector area of 36" x 56" of 2"x 4" wire screen.

The G-H shown on the left uses elements that measure a max of 23" wide x 30 1/2" tall it uses a split screen reflector that measures 29" wide x 36" tall with a 1" gap running vertically up the middle it's made of 1" x 1" wire mesh. The feed gap is 1 5/8" and the diamond parts of the elements measure 7".

The (3) M4's shown on the right were sized with 10" whiskers x 9 1/2" bay spacing, 9 1/2" whiskers x 9" bay spacing and 9" whiskers x 8 1/2" bay spacing. The average size is 19" wide x 31 1/2" tall, the reflector used for all sizes was a 40" wide x 40" tall curved reflector made from 2" x 4" wire screen.

All readings were taken on the same afternoon back to back; all antennas except the G-H were tested twice in different order to verify the readings. I broke a screw on the G-H feed point which didn't allow me to test it twice but the readings were pretty consistent with other tests I've done in the past as compared to the M4's.The conditions were down a little from usual but they were steady all afternoon which made for good testing conditions.

A Channel Master CM0064 amp was used on all antennas while testing to allow me to get the maximum amount of stations. Some antennas may respond slightly different if not using an amp. The amp can help mask some high SWR conditions by limiting the high SWR to a short piece of antenna lead between the antenna and amp.

All antennas were about 20ft above ground in the exact same location in an open field with no obstructions and were carefully rotated on all channels to find the best reception. Antenna height and surrounding terrain can affect antenna performance so your mileage may vary.

All antennas are pretty close on UHF, the fringe stations that one antenna could get over another may not even be receivable on certain days. On VHF-hi the M4's were clearly better; I'm not able to test VHF-HI NARODS on my G-H due to the mesh screen reflector.




Additional graphs with the DT250 readings can be found here
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Old 2010-03-28, 11:31 PM   #2
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Thanks for your efforts to put this all together, mclapp - awesome work! Do you have a TVFool report to cross-reference your test results with?

So, anyone have any coulda/woulda/shoulda responses or suggestions? Fire away (nicely) folks! Just remember, these are field tests on real stations, not full band gain charts using a reference signal.
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Old 2010-03-29, 07:33 AM   #3
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Quote:
I'm not able to test VHF-HI NARODS on my G-H due to the mesh screen reflector.
Im surprised you got any VHF-Hi readings with the SBGH without NARODs. Your channels 7 and 8 must be very strong and I assume must be watchable on all the antennas.
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Old 2010-03-29, 09:24 AM   #4
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Yes, the signals on 7 and 8 are strong I can get them with just the twin lead sticking out of the amp I should have included the TVfool report at the test site.


Also to reinforce what stampeder commented on these are actual TV signals from the stations listed on the TVfool chart this is in no way a gain plot strictly a comparision of over the air signals as received by my equipment.

To clarify the spectrum analyzer readings, the B&K 2630 analyzer reads in dbm units with -107 dbm being the minimum. The noise floor is about 7 dbm above the minimum or -100 dbm and thats where the chart starts. So 7 on the chart is -100 dbm, if you want the dbm reading add the number in the bar chart to -107. I figured for most people it would be less confusing to present the data in positive numbers. I took the readings from flat portion of the ASTC wave form except the analog stations ch's 10 & 20 I took the peak of the audio wave form as my reading.
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Old 2010-03-29, 11:53 AM   #5
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Thanks mclapp, when I saw the readings for VHF-HI channels 7 and 8 with these UHF antennas I figured you must be almost underneath those stations!

I'm struck by how close these three different high quality designs perform in such a real world setting, leaving aside beamwidth and stackability.
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Old 2010-03-29, 12:56 PM   #6
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Thanks McLapp - these have got be the best/ most useful posts I've ever read when it comes down to comparing all these DIY antennas! Nice to see real world results for these antennas, especially considering I've made most of them (and gotten similar but less scientific results). Now if only we had a commercial antenna in there to benchmark/ cross refererence against.... Well done!
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Old 2010-03-29, 05:13 PM   #7
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All I have of any note is an old CM4221 and I have tested it against some of these antennas.

Here's that graph, don't compare the two graphs because the signals on each day were different and some of the stations have changed channels since then. The G-H is the same antenna as in the DIY test and the 4 bay is very similar to the 10x9 1/2 in the DIY test. I really wish I did have some big gun commercial antennas to compare like a xg 91 or cm 4228, I keep my eyes open for them all the time.
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Old 2010-03-29, 06:45 PM   #8
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Mclapp,

Nice work, one thing I did way back was post some spec an screen shots in a pic album here...I have a round from inside the basement, and another round with the antenna placed on the roof as normal. That was from Buffalo and lookin at just one crude antenna I built. I see ur in Binghamton area??? Fair to say Pretty rough terrain down there? More so than the Buffalo area...

I was noting the pilot peak, as at the time I was under the impression the pilot is used for initial sync, etc. But it probably makes more sense to use that "flat portion" as you say, since that'll average things out.

Was the 50 ohm spectrum analyzer matched to the 75 ohm system at all? Was there a Receiver in line at the time also?

Would be interesting to see if ya can build a directional coupler similar to mine, so when ur monitoring with a Spec An
the 50 Ohm Spec An isn't loading down as much. I can use this inline with a receiver, while monitoring with the spec an as well.

This seems to work OK for me here, just tested this 75 ohm directional coupler over the weekend.
Not that my Spec An is very accurate or anything. One of these days I'll bring a newer one home for a weekend.

http://www.digitalhome.ca/forum/show...postcount=1010

Edit: BTW, I was looking at the Mini-Circuits site the other day, looking for some data on the one I have, and seems it's either a "special",
or since been discontinued. But, they still have plenty of designs of various sizes that could easily be put together inside a similar
Pomona or Bud type project Box. Prices I saw were anywhere from 5 or six bux apiece to twenty sumthin bux apiece.
Within hobbyist range. Digi-Key didn't seem to have Mini-Circuits parts, but they do have various similar sized Directional
Couplers from other mfrs within similar price range.

Another use for such a D.C. may be to use as a return loss bridge, along with a Spec An with Tracking Generator,
i.e - in a poor man's Antenna Sweep Test setup...
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Old 2010-03-29, 08:11 PM   #9
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A test I've been contemplating, is to use the USB ATSC Tuner and TsReader to log it's Signal Quality and C/N reports of various stations periodically for long term trending over time.
Like, use it's control server support to periodically do an html export of each channel, once or twice an hour for a day or two. Then parse the results later, for import in to Excel. That would give a somewhat real world measure of how signals are fading, etc. over time.

Was hackin away a little bit over the weekend with Perl, to do some parsing.
I'm not a programmer at all, but it's gettin there, seems doable at least.
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Old 2010-03-29, 11:02 PM   #10
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The analyzer has a 50 ohm input, I have a mini circuits 50 to 75 ohm pad but it has a 6 db insertion loss so hooking the 75 ohm line to the 50 ohm input results in less loss. The amp is a buffer so that doesn't throw any impedance mis-match back to the antenna in test at all. I split the signal after the amp power supply 2 ways one for the analyzer and one for the Converter box. I have a cheap directional coupler that I wanted to try to use for some return loss stuff but the tracking generator on this unit doesn't have enough output to do the job for that. I think I will need a better directional coupler to do return loss.

People would be surprised how many db a TV signal can change in a days time I know around here east of Binghamton the signal traveling through the hills can vary a lot. It makes OTA antenna measurement a challenge, I have to do a lot of quick back to back tests.
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Old 2010-03-30, 01:49 AM   #11
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Thanks mclapp… I’ve always liked your ‘actual’ testing, and I love the graphs (see below).


Prior to seeing these graphs, I have relied purely on my cognitive abilities to juggle the words and numbers from your various posts. In fact, I was under the impression that the M4 9-1/2 x 9 would be the best bet for channels 9 & 11 (future digital channel assignments for those of us near Toronto and Hamilton). Based on the graph that’s obviously not the case, at least for channel 11.

On your VHF Hi graph the antennas do a switcheroo as they pass through the vacant channel 9. Is it a simple matter to interpolate how the M4 9-1/2 x 9 / M4 9 x 8-1/2 would perform on channel 9?
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Old 2010-03-30, 08:04 AM   #12
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Quote:
People would be surprised how many db a TV signal can change in a days time I know around here east of Binghamton the signal traveling through the hills can vary a lot. It makes OTA antenna measurement a challenge, I have to do a lot of quick back to back tests.
A VERY important point to keep in mind when looking at prediction data from any source. The signal value you see on TVFool, the FCC's sites, and probably anyplace else is qualified with (50,50) - it's the signal value that is met OR exceeded 50% of the time at 50% of locations. The first 50 captures the behavior that mclapp described. The second 50 captures local terrain effects that don't show up in even the best terrain database, and reflects the varaitions found when measurements were taken by the FCC long ago.

The first 50 is why aiming for +10 to +15 for NM is key to avoiding dropouts. The second 50 leads to cursing at the propagation gods when you don't get the results you expect (and this is before attics are involved!)
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Old 2010-03-30, 08:32 AM   #13
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The F(50,50) and F(whatever) probability functions are generally used to set service contours based on the prescribed LRII algorithm. Service contours are generally used to define the limits of interference protection and, to a lesser extent, a (very) coarse prediction of service availability.

The signal power forecasts on TVFool, for example, are not based on the probability functions but are individually calculated for the radial from each station to the particular location "on the fly" whenever a request is made and take into account the specific terrain path the signal must pass over.
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Old 2010-03-30, 09:55 AM   #14
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Quote:
Originally Posted by chico2 View Post
On your VHF Hi graph the antennas do a switcheroo as they pass through the vacant channel 9. Is it a simple matter to interpolate how the M4 9-1/2 x 9 / M4 9 x 8-1/2 would perform on channel 9?
Most likely the 9 1/2 x 9 will do better on ch 9 and keep in mind that the ch 11 and 13 stations are 80 mi away so even though my efforts to do the testing quickly as possible and test each antenna more than once there could be some error especially when were talking 1 db. I will say though that the 9 x 8 1/2 did beat the 9 1/2 x 9 on ch's 11 and 13 every time I've tested them even on other occasions so I would feel pretty confident that for those channels the 9 x 8 1/2 would be the choice. If I were to put up an antenna for that site I would use the 9 x 8 1/2 based on the signal readings where my weakest channels are upper VHF-hi and UHF.
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Old 2010-03-30, 10:51 AM   #15
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Quote:
Im surprised you got any VHF-Hi readings with the SBGH without NARODs
I'm not. I mentioned before how my GH builds with a wide mesh reflector and no NARODs have received VHF-lo signals. Though they were analog, it still surprised me to pick up RF5 with picture. I've picked up RF5/6/9/11 and 13. I don't really consider picking up RF11 a big accomplishment just because the transmitter is close, and could be picked up with a paperclip, but getting the rest, are pretty amazing.

Oh and Mclapp, as usual, I bow to you, and your amazing work, and contributions. Incredible!
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