Excellent link with antenna comparisons with spectrum analyzer

[EscapeVelocity]

I'd invite comment on that concept!

I think that in extreme fringe, knife edge refraction, and 2 edge situations, that a large active area can help keep a stable viewable signal.

However in general. Unequal illumination degrades gain, when combined. Similar to why N-Bays all use the same length driven elements. The combine better electrically and reinforce each other at the same points.

The Kosmic SuperQuad has the advantage of a smaller area that needs to be illuminated equally and not only avoids the pitfalls of unequal illumination, but is more likely to be totally engulfed in a UHF "hot zone," or "sweet spot" as I like to call it.

Note: I havent discussed multipath.

One of the things about horizontal ganged 8 bays, is that the Vertical beamwidth is not reduced (albeit the horizontal is cut in half as compared to a 4 Bay)...that vertical beamwidth is greater than a long yagi like the xg91, so that on axis aiming in the vertical is less critical. No need to tilt.

Ill note that the HDTVexpert, Pete Putman, placed a VHF antenna elevated to a hilltop ridge, to focus the axis of a yagi, and ALSO moved off axis of the as the crow flies heading of the transmitter by several degrees. This may hint at other phenomenon, that make a larger physical capture area of multiple driven elements (like the 8 bay) desirable for stable fringe reception with edge reflection circumstance.

Hope that makes sense.

EV

 

[EscapeVelocity]

In other words, if the UHF hot zone is moving about (not stable), then the larger active face of the antenna becomes beneficial.

 

[tballister]

"sweet spot"

Yes, your comments make sense, and your intuitions are good data points.

I would like to submit that:
[a] A "sweet spot" can and does move about on a plane orthogonal to the LOS, both diurnally and annually as a function of environmental changes.
There are an infinite number of comb filters that can result naturally due to reflections.

The focus of my theory and curiosity regarding N-Bays versus Yagis is predominantly on the multipath prone scenario.

 

[FOX TV]

Sorry it has taken me so long to re-visit this post. I was not saying that an uncalibrated instrument is unreliable as long as it shows a difference between two signals. If non critical, or other types of tests not requiring a calibrated instrument are being performed, then an uncalibrated instrument is fine.

And that is not to say that I have not used other methods as well. I bought a Magnavox converter box just because it has a numerical readout of signal strength, and the signal display will stay on the screen until you cancel it.

It also seems to give an initial peak signal level that is always higher than average levels, which may be the RF levels before the AGC circuitry kicks in.This is only speculation, but it seems to be true for any signal I tune to, or any antenna it is hooked to. In any case, when using it for this purpose, I log the peak and average numbers for each test and form a data chart in excel to compare antennas the poor boy way.

If pure signal gain is what you are measuring, then this will still give accurate data to a certain degree when comparing one signal to another, or in my case one antenna to another. The actual signal level is not important, but the difference between them is, as this will show a level of difference, which can then be a measure of an antennas performance.

I don't own any high end test gear myself, but I do have access to high end gear to use whenever I need it. If I find a deal on uncalibrated gear, I would buy it myself.

Here is a question for discussion. Since most TV broadcast antennas use downward mechanical, and or electrical beam tilt, is there any advantage to mounting a straight boom Yagi with a little upward or downward tilt since one element in front of the other kind of "Blinds" the one behind it from full RF exposure to a certain extent when in the true horizontal plain?

Just a random theory question to see if anyone has any opinions on this subject.

 

[EscapeVelocity]

Sometimes intuition is all one has. LOL! Thanks for the kind words, regarding my ramblings.

FoxTV has an interesting querry.

 

[Tower Guy]

Here is a question for discussion. Since most TV broadcast antennas use downward mechanical, and or electrical beam tilt, is there any advantage to mounting a straight boom Yagi with a little upward or downward tilt since one element in front of the other kind of "Blinds" the one behind it from full RF exposure to a certain extent when in the true horizontal plain?

Tilting an antenna is advantageous in some situations. Your suggestion of elements "blinding" each other is not the reason. Elements interact with each other due to mutual coupling. The effect is what causes pattern shaping and hence gain.

In my opinion tilting an antenna has three effects.

1. When there is nearby terrain the signal is actually arriving at an angle from the top of the nearby hill.
2. Tilting the antenna lowers the antenna noise temperature ever so slightly.
3. When tilted upwards the horizontal beamwidth of the antenna at the horizon is reduced. If there is multipath, but extra signal strength, the result could be a better S/N than the signal from a horizontally aimed antenna.

That said, if you want tilting, there is a far better way to get elevated signals from an antenna. An antenna a few feet off the ground reflects such that the main beam is tilted upwards. The key is to find an open foreground for the antenna and determine the optimum height for your specific location.

Such ground gain is as much as 6 db over flat ground and can be even higher if slopes and swales happen to be an advantageous shape.

In one case I helped a friend get over-the-mountain reception using his steel pool deck as an artificial ground. The antenna was mounted on a fence post 3' above the deck.

Here's another example of antennas located unusually close to the ground:
Updates From The Field :: Out to the DTV Fringe
In this case there is a downslope in front of the antennas and a mountain about a half mile away.

 

[Piggie]

I did read with interest your description of the stacked Y10-7-13's. Its a respectable piece of empirical work and I appreciate the contribution. Gives me a little pause about my own plans for my modified unit. I've been pondering spatial diversity a lot lately. I never really needed the gain; I needed better directivity and that was the original motivation.

I am sure it was a typo thinking of your own modifications (very impressive aluminum work, exceptional) but just to correct for correctness sake I have Winegard YA-1713 stacked vertically.

However, (I've begun to theorize) multiple units may perhaps have a small advantage. The following is a comment I left at the highdefforum related to UHF reception.
---------------------------------------------------
I've often pondered the notion that where reflections are a significant contributor to quality (or lack of), the 8-Bay designs in general might provide better long term performance. There is a little bit of a concept of spatial diversity involved. A Yagi design, like the 91XG, concentrates energy along a single axis. On the other hand, an N-Bay design aggregates signals received along N different axes. At UHF wavelengths the separations between active elements approaches several wavelengths. It is easily imaginable that if you could independently observe the waveform levels and quality (I, mean "flatness"; absence of serrations), of each contributing element, there is some non-zero probability that at one point in time one element might produce the most optimal signal, while at other times some completely different element would be the better contributor.
---------------------------------------------------
I'd invite comment on that concept! (Note that it ignores level, focusing instead on flatness).

T, here is something to ponder at the same time. If you have not read here or on AVS my trials and tribulations with wind driven multipath. Seems lately just as I think I understand something I find evidence that could make me wrong again!

I had up an old CM 4221A about 20 ft pointing from my house to Gainesville. Everything was fine until the first spring. Analog was still on the air so that helped troubleshoot my problem. I was having a lot of break ups. I then researched and found between reading and looking at one of the stations, WOGX, that used the same antenna to transmit analog and digital what was going on. Wind driven rapid multipath. So I had a second 4221A and read everything I could about combining them. I had already tried a vertical stack which was an abysmal failure. I had an old Radio Shack U-75R I had taken off a tower 15 years ago. I put it up for a temp antenna while I played with the 4221As. Low and behold it was windy the next few days and no rapid multipath fading. Totally an accidental finding. Now. Why was the 4221A so bad at multipath? Because the bays are all stacked vertical, thus giving about the same azimuth aperture as a single bay? Or (here is what you posted just got me thinking) was it the spacial diversity that was picking up signals out of phase? I think it was the former still but worth a ponder.

By the way I also appreciated your anecdote about the VSWR meter of your youth! Its that necessity being the mother of invention thing. I've been there!

Best Regards,
t

In this danged economy I am finding myself there again! "Doing more and more of what I can with what I got." (intentional grammar misuse).

Good to have met you.

Piggie )

 

[Piggie]

I think he was saying that he stacked 2 Winegard 6000's and they perform better than a single FM-6..

Yeah, after I posted that later it hit me, dumb pig!

I have no idea which is better. There are advantages to stacking and advantages to a longer boom.

 

[EscapeVelocity]

I was reading this article in TVTechnology.

Unfortunately, most of the links to the Rhombics are dead, and after the death of its creator, cebik is now behind subscription fee walls. Which just goes to show you, save the data that you want to reference in the future, as it may or possibly even probably disappear, move, etc, in the future.

Crazy Enough to Build Your Own TV Antenna?

by Doug Lung, 01.25.2006

FCC DTV planning factors are based on antenna gains of 4 dB, 6 dB and 10 dB for low-VHF (Channels 2-6), high-VHF (7-13) and UHF (14-69) respectively. Kerry Cozad of Dielectric measured the Channel Master Model 4228 eight-bay bowtie-with-screen UHF antenna and measured gains of approximately 3 dB, 9 dB and 15 dB for low-VHF, high-VHF and UHF. This UHF-only antenna exceeds the planning factor gain at both high-VHF and UHF!

What do you make of those claims?

Note: That is the old discontinued CM 4228A.

 

[tballister]

Tilting an antenna ...

I can offer my own personal experience that for stations east of me an upward tilt of about 13 degrees yielded a 5-6 point increase in digital "quality" as displayed on various DTV's. The antenna height is about 5950 feet, but the signals arrive via a pretty good knife edge at about 10000 feet. The actual angle involved is about 5.4 degrees. But I found that roughly 7 degrees of additional upward tilt improved things noticeably. The same wound up true for stations coming from the north for which there is not a signicant height difference. For that antenna is was something around 7 degrees that helped, but it was only 2-3 points, although repeatable.

My theory is simply that the upward tilt puts ground reflections just a little bit off the LOS. I note also that the 91XG's mounting plate is intentionally designed such that the boom can be angled upwards at any angle up to maybe about 20 degrees.

Such ground gain is as much as 6 db over flat ground ...

Yeah! That's cool. And another way to get a ready-made reflector is to leverage your metal roof - if you have one. Where I am they are quite common; the snow slides of 'em pretty well. I received a C5 VHF loop antenna from AntennasDirect for testing and popped it onto a 20' pole just to quickly see what I could see. At first I was at the bottom edge of the roof so there was no metal underneath the LOS. But then I moved along the rear edge of it, such that there was now some surface area of the roof below the antenna, somewhat emulating the bottom half of a corner reflector. I could find a sweet spot about 5-6' or so above this surface where there was more than a 10 dB increase in level!

The games we play ....

 

[tballister]

What do you make of those claims? ... the old discontinued CM 4228A.

I'll wait for my comparison (it'll stop snowing any day now...). I'll only say that to claim 9 dB of gain for Hi-VHF implies the UHF centered bowtie design is nearly as efficient at Hi-VHF (~1.5 octaves below UHF center) as a Y10-7-13 or YA1713, both 10 element Yagis with about 10-12 dB gain across the Hi-VHF band.

Hmmm.....

 

[tballister]

RE: WOGX and the 4221A vs U-75R

T, here is something to ponder ...

This is the kind of story that makes me wish I had a 3000 mile long piece of coax.

It would have been enlightening to have had the opportunity to do a comparison of those two installations; to have been able to compare not only signal levels but also to have stared at the two waveforms on the spectrum analyzer for a while. The questions are how flat was the waveform for each antenna, and to what degree do any observable serrations vary over time?

I could not find any vertical or horizontal polar pattern specs for either antenna. I have to say this is one thing I appreciate about the Winegard Company. What ever they've got, they've got, and they're not afraid to publish real engineering data.

I went to Winegard to find comparables to see what their characteristic polar patterns were. Their HD4400 (a 4-Bay BowTie), as a comparable to the 4221A, shows a horizontal 3 dB beamwidth of 46 degrees at Ch 50 (close to WOGX's 51). Their HD9075 (a Yagi), as a comparable to the U-75R, shows a beamwidth of 32 degrees.

So this may have been a factor; maybe some reflections creeping in an out of the aperture. But the HD9075 also shows about 3dB higher gain than the HD4400. This could also have been a factor; maybe just enough additional margin to keep you above the cliff.

If we had some real data for the 4221 and U-75R maybe we could draw some conclusions. Sigh.

Wish I could've seen 'em...

t

 

[EscapeVelocity]

Of the 4 Bays I have played with, the Winegard 4400 was the worst performer. I suspected that the HD 8800 was the least of the 8 Bays based on my 4 Bay experience and your tests show that.

I dont have the CM 4221HD, Antennas Direct DB4, or the AntennaCraft U-4000. I do have the CM 4220A and the DB2, and the CM 4220A is superior to the DB2, but not by much. Furthermore the DB2 is more compact, I prefer it for a small form antenna for indoors and patios and such. Then Id move up to a 4 Bay bypassing the 4220A altogether. The 4220A isnt a bad antenna as far as 2 bays go, but it just falls through the cracks of other design considerations.

Here is my analysis of forward gain of the 4 Bays, Ch. 14 -51, from highest to lowest.

Kosmic Antennas SuperQuad
AntennaCraft G1483 Hoverman
Channel Master 4221A
Winegard HD 4400

On VHF High, those above are in the same order, though the margin is much greater for the Kosmic.

 

[tballister]

I'll be having crow for dinner; medium well, thank you.

I'm really shocked at this, but after my last skeptical post about the CM4228's claimed 9 dB Hi-VHF gain, I had a "Boing. I should've had a V8!" moment.

I have both a CM4228HD and a Winegard HD7698P mounted 85' up in a Pine tree at the moment. The 4228 is about 4 degrees off the 7698's LOS, the later of which is optimally pointed for 7-13.

So I dragged the analyzer up to the porch to capture both signals, and here is the resulting comparison.

I have to say I'm chagrined. Moreover, I'm even more of a bing-bong because after sniffing around in a capture directory I found a similar set of captures done back on November 12, and they are quite similar.

Now, I've previously compared the Y10-7-13 to the HD7698P and the Y10-7-13 is just 1 or 2 dB hotter at the ends of the band (7 & 13); e.g., pretty close to the HD7698P.

So, while there's a dramatically poorer envelope from the CM4228HD (much more significant multipath impact, in conflict with my theories about N-Bays vs Yagis) the absolute levels are within a few dB of each other.

Two caveats. (1) The off-axis pointing of the CM4228HD may be having a slight effect on envelope flatness, but I would only expect a rather small change if re-pointed 4 degrees, not a dramatic one. (2) just to be up front, the amplifier in the CM42228 path has about 3 dB higher gain than the one in the 7698 path. But I corrected for that on the analyzer's level setting prior to capturing. I think any errors here are within a few dB.

The point is, there's not an order of magnitude of difference between the two. Rather, poorer wave shape aside, the levels are actually pretty close!

[Could you pass the salt & pepper please?]

 

[EscapeVelocity]

wow!



.

 

[EscapeVelocity]

I knew the VHF performance was good, but not that good.

Similarly on the Tune-A-Tenna, I went very conservative on the VHF High claims. But I dont know if the Tune-A-Tenna can beat that. It will be fascinating to see. Jim in Seattle says he will be shipping soon. Sounds like a good 3 way battle on UHF and VHF High.

Can you test VHF Low or FM?

 

[EscapeVelocity]

Yeah, the Channel Master 8 Bays and the Tune A Tenna dont have tight beamwidths on VHF High, they are basically acting like dipoles at VHF. The F/B ratio is pretty low too, though they do favor the forward direction.

 

[tballister]

VHF Low and FM

Can you test VHF Low or FM?

VHF Lo is not possible from the field here, unless I put up my own multi-channel transmitter.

FM: yes. Maybe as we get into spring/summer.

t

 

[EscapeVelocity]

A fellow in LA had success with the KST on FM....as well as VHF High....about 45 miles from Mt. Wilson where almost all the TV transmitters are in that DMA.

 

[tballister]

I just want to say that I'm going to remain a little suspicious of the CM4228 vs HD7698P comparison I posted earlier, at least for a little while. Another difference I've thought of is that the CM4228HD is about 6 feet higher than the 7698P.

Not until I get a chance to do a proper field test with all components, placement, orientation, and height carefully duplicated, such that the only variable is the unit under test, and do this against a real Y10-7-13 will I be comfortable.