Excellent link with antenna comparisons with spectrum analyzer

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#61
I understand the points you have made, and appreciate the time you took to make them, and the eloquence in doing so, however...

...that doesnt square with 9 dB gain on VHF High for the Channel Master 4228HD.


But I say when you start talking about antennas that are subharmonics of a 1/4 dipole, you either are matching it even to some degree to the feedline or the signal is just strong enough it works.
 
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tballister

DTVUSAForum Member, , , Webmaster of: Antenna Hack
#62
Let's Be Careful

...that doesnt square with 9 dB gain on VHF High for the Channel Master 4228HD.
Still feels to me that we really ought to remain cautious about the CM4228HD VHF observations.

Just doesn't' feel like anyone here has yet offered any deep and credible analysis of the physics and mechanisms of the design that could contribute to such VHF performance. Unsubstantiated remarks like "it has better nuts and bolts" really don't contribute to credibility.

I'd love to see someone provide the deep analysis that can explain why the nuts and bolts might be better.

For example, there have been comments about the continuous versus split reflector, and let's be clear its role is reflection. None of the energy landing on it goes directly to the transmission line. The reflected energy must be captured by the active element(s) and additive. I mean, is the reflector the main reason for the VHF performance? If so, why does having a continuous reflector have such a favorable impact on active elements whose optimal wavelengths are 1 to 2 octaves above VHF? Does the particular horizontal collinear arrangement of the active elements have anything to do with it? Does the horizontal spacing between the left and right active elements have anything to do with?

Sorry, its just that my gut tells me the answer may not such a simple one..... Dunno...
 
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EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#63
I think its pretty well established that the 4228A and the 4228HD have some useful gain on VHF High, whether its 9 dB or 6 dB or varies between 3 and 8 dB depending on the channel.

The reflector is pretty basic. Its a little long for a 1/2 wave at VHF High, but still in good area. The newer 4228HD has nicer larger diameter vertical rods which perhaps broadband a bit better than the fencing of the 4228A. However who knows exactly how that fencing acts, it could be broadbanding even better acting as a fat dipole. That is a question worth contemplating, especially after we get an A/B spectrum analysis of it.

Yeah, I was getting the sense that Piggie was saying that the screen or horizontal rods on the reflector are coupling with the feedlines or 75 ohm tranmission line directly (through space). Dunno about that one. It's not out of the realm of possibility...but?
 

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#64
Ill also say that the CM 4228A and HD reflector is in the area of a double wave at mid band of new UHF television spectrum. And 3/2 waves at the extreme low end of the UHF television spectrum....just under Ch. 14....which is good, because the bowtie design is known to drop off in gain there...the reinforcement at those frequencies is welcome.
 
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Piggie

Super Moderator
#65
Still feels to me that we really ought to remain cautious about the CM4228HD VHF observations.............

Sorry, its just that my gut tells me the answer may not such a simple one..... Dunno...
This is the same level of "doubt" I have about the antenna. Not the doubt that the antenna can receive VHF but how it receives VHF. It will be interesting to see it tested.

My gut feeling says there is little difference in a screen behind it to a rods behind it.

That even though that screen was designed as a UHF reflector, it's also very close to a half wave at hi VHF. This means it will resonant along it's horizontal axis at hi VHF. This part I have little to no doubt. As can be seen from the directors and reflector of a pure yagi, a 1/2 dipole doesn't have to be split in the middle to resonant.

Now how exactly is it coupled into the feed line (not directly like feed line shield pick up) is the question. Is it coupled into the bow tie whiskers? Is it coupled into the feed lines between the whiskers (though they run vertically which makes them not as likely to me). Or is it a combination of the whiskers and the feedlines? Even without measuring the feedlines, that is also the form of an Inverted L dipole. IF and only if (supposing without measuring) then the whisker and part of the feed lines could form close to a 1/2 wave that was not feed in the middle but more toward a 1/3-2/3 point. That would not be the best feed point but would seem better than end feeding a 1/8 wave of metal. And even if the vertical feedlines didn't do a single bit of coupling of the signal they do provide the needed resonance. Now this all supposes that the vertical feedlines even add in this way. Purely a guess, not based in any fact.

T, you may have the books I no longer have or can't remember but is there a dipole style antenna that is a whole wavelength that is feed at the 1/4-3/4 wavelength point? I seem one in my mind from a long time ago reading, but can't remember the overall length, or the impedenace but I seem to remember seeing a dipole being feed off center for some reason.
 
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tballister

DTVUSAForum Member, , , Webmaster of: Antenna Hack
#66
That even though that screen was designed as a UHF reflector, it's also very close to a half wave at hi VHF. This means it will resonant along it's horizontal axis at hi VHF.
I can't argue with your point that the length of the reflector is close to 1/2 wave resonance. Guess I'm just not sure that its resonant frequency, by itself, is the predominant factor to consider. Isn't it important how much energy at VHF frequencies is reflected off the reflector and captured by the active elements.

Here, are some mechanicals for you to ponder:




Note that 195 MHz and 584 MHz are the median frequencies of the Hi-VHF and (new) UHF bands, respectively. Note also that the horizontal spacing between center points of the active elements is 20.0", while wavelength at the median UHF frequency of 584 MHz is 20.2".

Just thought it might provoke a thought or two... :playball:

Regarding off center fed dipoles, I just went to google and entered:
off center fed dipole
... and got an eyeful.
 
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EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#67
When you get the Kosmic SuperQuad, you can cut the Bowties down to 9 and then 8 inches while reducing the interbowtie feedline spacing and see if that increases gain on VHF High, with a spectrum analyzer.

My testing has shown little difference. The Bowties being broadband elements and 8 to 10 inches being very close to 1/4 waves at VHF High. The difference being a shift from 1/4 wave at the center of the VHF HIGH band to the extreme low side of VHF HIGH band....which is a better configuration.

But the spectrum analyzer is more precise as a measuring instrument than anything I have here.

9" to 10" is a sweet spot in that regard, however my design also is concerned with UHF performance, and the 10" whisker length is beneficial for the extreme low end of UHF, which drop off dramatically in these 8" and especially the 6.75" bowtie elements of the 8 bays and 4 bays, that came before it.

The CM 4221HD increases gain at the extreme low end with its 24" wide reflector....which is a full wave at those frequencies.

The Kosmic SuperQuad uses a 28" reflector which also has reinforcing effect as a full wave wave reflector at the very low end of UHF....though not as tuned as the 24". But that is because I am also concerned with VHF High.

A 36" ish reflector is nice because its 3/2 waves at the extreme low end of UHF, and 2 waves at mid UHF television band. And is also 1/2 wave at the lower end of VHF High. Its better to err to the low side of a band, as its better to have a slightly long element than a slightly short element. Which can be seen with dipole plots off resonance....of the bowties for example. Which is the reason for their gain plunges at the low end of UHF.

So 9.5" whiskers with a 36" reflector is a good place to be. 10" with a 28" is a compromise off of that, but it is also a good place to be. 10" cheats toward the low end of both UHF full wave and VHF High 1/4 wave, and 28" is high end of VHF High (center band with wire lengthening effects), and is full waveish at extreme low end of UHF, plus 3/2 waves at mid UHF band.

Given the choice of $60 shipping for the 36" screen, and $20 for the 28" screen, the choice was clear.

That is my story and Im sticking to it.
 
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EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#68
The old discontinued American made CM 4228A should already be in T.Ballister's hands. The Kosmic SuperQuad should be there next Tues.

Shouldnt be long now!
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#69
EV,
Yes, he should have it by now and I think he tried to zap me a note to say he received it, but my PM box was full. I'm also anxious to see the results of his antenna shoot-out.
Jim
 

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#70
T.Ballister, I was reading some expanded information down at the bottom of this page at AntennaHacks.com that seems to be new.


Youve missed a couple of things.

First off, the diameter of the whiskers on the DB8 which increase end lengthening effects, and also the smaller width elements of the Winegard...which has the effect of decreasing the bandwidth of the design (meaning that swr and impedence rise at some points higher and lower to the resonant frequency closer to that resonance frequency.
 

tballister

DTVUSAForum Member, , , Webmaster of: Antenna Hack
#71
8-Bay Shootout - Revisited for VHF

Hello All.
I've posted an update that includes VHF results for the (nominally UHF) antennas covered in the previous 8-Bay shootout; just in case you're interested.
Best Regards,
t
 

tballister

DTVUSAForum Member, , , Webmaster of: Antenna Hack
#72
Youve missed a couple of things.

First off, the diameter of the whiskers on the DB8 which increase end lengthening effects, and also the smaller width elements of the Winegard...which has the effect of decreasing the bandwidth of the design (meaning that swr and impedence rise at some points higher and lower to the resonant frequency closer to that resonance frequency.
EV: Sorry its taken so long to re-visit this.

I just went out and measured the diameters of the CM4228HD and DB8 "whiskers". I don't have a caliper, but with aid of a magnifying glass and my trusty old Engineering Ruler's 50 Units per (1") division scale, I observe 8/50'ths, or about 0.16" inch for both. I note that #6 AWG wire is 0.16202" diameter and thus both antennas appear to me to be using #6 AWG aluminum wire. So I don't understand what DB8 diameter aspect you are thinking of.

Also, I didn't quite understand the second part of this relative to the "width" of the Winegard elements. Unlike the CM4228HD and DB8, the Winegard 8800HD does not use wire. Instead it uses relatively flat rectangular elements, maybe about 20 gauge thickness aluminum, with (what I would call) a width of about 1", and are as I indicate in my table, a length of about 7 1/4 inches "long".

As such, the overall dipole length of the Winegard dipoles falls in between the CM4228HD's (longest) and DB8's (shortest). The primary effect of this, I would think, would be that the peak response of the Winegard would fall in between that of the CM4228HD and DB8, (all other factors being equal, which of course, they aren't).

However, the greater factor associated with the Winegard, at least to me, is the obvious difference that it is not a "fan dipole" ("bowtie") design. Rather its a two dimensional collinear array of common dipoles. From what I've read, the most significant aspect of that should be less bandwidth than that of the triangular fan/bowtie.

So, is the "width" you are discussing the "width" as I've defined it above, or something else?

If it is, then certainly I can accept that the characteristic impedance of the flat rectangular elements would be something quite different than that of triangular elements formed from wire. But, certainly it is the job of the balun/combiner design to be optimally matching characteristic antenna impedance to that of the transmission line, and thus optimally maximizing power transfer, right?

So after I pondered that I realized I still don't understand the point being made...

Sorry...
 

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#73
I just went out and measured the diameters of the CM4228HD and DB8 "whiskers". I don't have a caliper, but with aid of a magnifying glass and my trusty old Engineering Ruler's 50 Units per (1") division scale, I observe 8/50'ths, or about 0.16" inch for both. I note that #6 AWG wire is 0.16202" diameter and thus both antennas appear to me to be using #6 AWG aluminum wire. So I don't understand what DB8 diameter aspect you are thinking of.
I dont have any experience with the HD version. I was talking about the old American made discontinued Channel Master N Bays.

As an aside, both the DB2 that I have and the old CM 4221A that I have, both use step downs in diameter for the feedlines. I bet this step down in wire gauge increases impedence.

Didnt know that the Winegard elements were shorter than 8"....although I have a 4400HD here.

However, the greater factor associated with the Winegard, at least to me, is the obvious difference that it is not a "fan dipole" ("bowtie") design. Rather its a two dimensional collinear array of common dipoles. From what I've read, the most significant aspect of that should be less bandwidth than that of the triangular fan/bowtie.
I concur.

If it is, then certainly I can accept that the characteristic impedance of the flat rectangular elements would be something quite different than that of triangular elements formed from wire. But, certainly it is the job of the balun/combiner design to be optimally matching characteristic antenna impedance to that of the transmission line, and thus optimally maximizing power transfer, right?
Indeed.

So after I pondered that I realized I still don't understand the point being made...
I dont remember what point I was trying to make either, and cant muster the energy to dig back into it.

No worries.
 

tballister

DTVUSAForum Member, , , Webmaster of: Antenna Hack
#74
Hi All.

Its been a while since I've visited this thread, but just thought I would post a small news update here.

At my Antenna Hacks page I've published results of an implementation
of Ken Nist's recommendations for improving the CM4228HD harness.

Just in case you're interested...

t
 
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tballister

DTVUSAForum Member, , , Webmaster of: Antenna Hack
#75
Been a long time gone...

Hello all, looks like this thread has gone dead. But I revisit after all this time to announce I put myself back into the field last week to test the Stellar Labs 30-2476 Hi-VHF antenna against a stock Y10-7-13 I've preserved.

The Y10-7-13 being no longer manufactured I was curious as to whether the 30-2476 is a viable alternative.

I think it is. You'll find the results on my Antenna Comparisons page. http://www.antennahacks.com/

Best Regards to all,
t
 
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