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Feeding stacked array

3K views 13 replies 4 participants last post by  holl_ands 
#1 ·
Hi: Guys.

I just bought 3 Winegard HD-4400's and I want to stack 3 of them
in a vertical array.

I don't want to use 3 baluns & a 3 way splitter backwards as I
feel the loss of the three baluns and the splitter would defeat
the purpose of trying to achieve gain in the stacked array.

Would I be better off using 300 ohm twin lead to connect the
top & bottom antennas to the center one then use one balun
at the center antenna feed point or add sections of aluminum
feed line so there is one continues feed line for all three
antennas then just use 1 balun on the center antenna as the feed point.

I'm not sure if the continues feed line would throw off the impedance
from the 300 ohm at the center.

Any help would be greatly appreciated.


Bruce.
 
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#3 · (Edited)
Well, according to the Views number, 158 people have looked at your thread, so it has been seen.

I, for one, am trying to figure out why you want to do it that way, so it is difficult for me to give a useful answer.

Combining three is unusual; there isn't much gain over two, but just as much trouble as going from two to four, arranged as a quad with 2 over 2.

It seems you want to improve your DXing, but you haven't told us what you are now using for an antenna system.

TV DXing depends a lot on signal enhancement from Tropospheric propagation, which is much greater than the gain increase you are planning. I'm not convinced that it will make a lot of difference in your results, but I understand your desire to try it to see if it will.

Your method will have the same balun loss, whether it is for each of the three before combining, or for all three after combining.

The combining method can have losses for either way. There will be standing waves on a wire harness at certain frequencies that create loss. If you use 300 ohm twin lead or 450 ohm ladder line it will have increased losses when wet. If you use a splitter in reverse as a combiner, it will have an internal loss of about 0.5 dB under ideal conditions, but can be much greater if the incoming wave front is non-uniform and each antenna receives a different amount of signal strength.

Calaveras uses low-loss half-wave coaxial baluns for his two 91XGs. That type of balun is designed for a specific frequency, and doesn't perform quite as well away from its design frequency, but the UHF band is getting smaller every day. His background is in ham DXing, where the bandwidth requirements are a little less severe. Here is a test report by tripelo of using a half-wave coaxial balun for a VHF antenna.

The computer simulations by K6STI of coax baluns promise low loss for UHF:





The method that Calaveras uses for combining is also frequency sensitive, but seems to perform well. He combines the two equal lengths of coax from his antennas together in parallel, giving an impedance of 37.5 ohms. This is converted back to 75 ohms by a 1/4 wave matching section of 50 ohm coax.

http://www.aa6g.org/DTV/index.html

http://www.aa6g.org/DTV/ABD/Antenna_Block_Diagram.html



IIRC, the Kathrein-Scala DRV VHF-High panel transmitting antenna uses a 1/4 wave air dielectric section as a matching transformer:

 
#4 ·
feeding stacked array

Thanks for your reply.

I'm using a 8 bay now that has two adjustable 4 bays.
I'm in east York near Woodbine & Cosburn & my setup is on
a tower up 45'.

My previous setup was two CM4221HD's in a vertical stack
aiming at Buffalo & I was getting all Buffalo stations and Ion
from Batavia. I took these down because all the screws were
rusted & I have not had time to repair them.

I ended up with the 8 bay I mentioned above and even though
one side points at Batavia & one at Buffalo I can't get Ion anymore
except on inversion days.

So I bought the 3 HD-4400's with the hope of getting all Buffalo
and Ion with the new stack.

My thinking behind stacking 3 was not so much for gain but
for a large capture area.

I understand that it's not usual to stack 3 antennas and
it is usually 2, 4 or 8

So in your opinion am I better off with three baluns and a three
way splitter or feeding the top & bottom 4 bay using 18 gauge
300 ohm twin lead fed to the center antenna then 1 balun.

Any thoughts ?

Thanks.
 
#5 ·
As you found out, an 8-bay aimed in two directions will not get you ION. An 8-bay with both panels aimed at ION or two 4-bays aimed at ION will.

The problem with a 3-way splitter is that it is weird. There are two types; one consists of two 2-way splitters in cascade. The ports are labeled 3.5 dB, 7 dB, and 7 dB, so you don't want to use that. The other type has equal outputs, so it might not be suitable for combining. If you are set on combining three, ask holl_ands about a harness; he is working on a harness project now. Personally, I don't care to use one; they are too tedious and it certainly didn't work very well with the original 4228. But holl_ands is convinced that they can perform well if properly designed and constructed.
My thinking behind stacking 3 was not so much for gain but for a large capture area.
A large capture area can help if the wave front presented to the array is uniform, but with 2Edge signals the wave front is rarely uniform because the rays are scattered which forms a non-uniform field.
http://www.hdtvprimer.com/ANTENNAS/siting.html

scroll down to Skyline Multi-path and Non-uniform fields
 
#6 · (Edited)
I did a tvfool report for Woodbine & Cosburn since I can't find your report. The 35 ft report shows WPXJ ION as Tropo with a NM of -23.1 dB. It is impossible to receive a signal with a NM lower than -15 dB because it will be buried in the thermal noise floor at -106 dBm. If you are able to receive it, either the report is wrong or the signal has been enhanced by Tropospheric Propagation as in your case.
TV Fool

and at 70 ft
TV Fool

I tried to do a coverage map for ION with the interactive map browser, but it would not show until I raised the antenna to 70 ft





Note that only 80 kW of 455 kW ERP is sent to your antenna because of the very directional transmitting antenna.
http://www.rabbitears.info/tvq.php?request=items&facid=2325
 
#7 ·
Thanks again for your reply & all
your additional research.

I guess I can stick to just stacking
2 of the Winegard HD-4400's and
2 baluns with a two way splitter
backwards to feed my tv.

I will also be using a Channel Master
7775 uhf preamp at the output of
the 2 way splitter as I want to
eventually feed 3 TV's and tell Roger's
to stick their cable.

I wish someone would invent a mast
mounted preamp with two 75 ohm
inputs for uhf so we could avoid the
use of two way splitters backwards.

I do have a Channel Master with two
inputs but one is vhf & the other uhf.

There is a switch inside to combine
both inputs but I suspect that the vhf
input will be designed to amplify vhf
only so I guess feeding the two
uhf antennas into this amp won't work.

I guess I will stick with the uhf only 7775

I do also have a Winegard 2 input mast
mount antenna combiner. I don't know if these
are a better option than a two way splitter
backwards or are the same thing.
Being that they are plastic I don't know if these
are more subject to interference than a
all metal 2 way backwards wired splitter.


Thanks again.


Bruce.
 
#8 · (Edited)
I guess I can stick to just stacking 2 of the Winegard HD-4400's and 2 baluns with a two way splitter backwards to feed my tv.
That might work, since the two 4221HDs worked.
I will also be using a Channel Master 7775 uhf preamp at the output of the 2 way splitter as I want to eventually feed TV's and tell Roger's to stick their cable.
Ah, yes; I remember the 7775----a good UHF preamp. There is a chance that it will be overloaded by your strong local signals. Even a partial overload can create spurious signals within the preamp from IMD (Intermodulation Distortion) that can wipe out the weak signals that you need. The strong signals stay OK, but the spurious signals raise the noise floor and reduce the SNR of the weak signals to below the minimum required 15 dB.

Your estimated report for reference:
TV Fool

If you click on Pending in your report, you will see two channels that are stronger than CICA. CBLT is now at the top with a NM of 66.3 dB. If you have an antenna with 12 dB gain and it is aimed at CBLT, that brings you up to 78.3 dB NM, which is overload territory.



Interpreting Noise Margin in the TV Fool Report
http://www.aa6g.org/DTV/Reception/tvfool_nm.html

Looking at it from the standpoint of signal power, CBLT has a signal power of -24.6 dBm.

CBLT -24.6 dBm = 24.4 dBmV.
Max input of 7775 is 25 dBmV
CBLT 24.4 dBmV + 12 dB Ant = 36.4 dBmV; preamp overload if the antenna is aimed in that direction



CBLT -24.6 dBm + 12 dB Ant + 16 dB preamp = +3.4 dBm; tuner overload

ATSC Recommended Practice:
Receiver Performance Guidelines

Document A/74:2010, 7 April 2010

RECEIVER PERFORMANCE GUIDELINES

5.1 Sensitivity

A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from –83 dBm to –5 dBm for both the VHF and UHF bands
.

5.2 Multi-Signal Overload

The DTV receiver should accommodate more than one undesired, high-level, NSTC or DTV
signal at its input, received from transmission facilities that are in close proximity to one another. For purposes of this guideline, it should be assumed that multiple signals, each approaching –8 dBm, will exist at the input of the receiver.
 
#9 · (Edited)
I wish someone would invent a mast mounted preamp with two 75 ohm inputs for uhf so we could avoid the use of two way splitters backwards.
That would be nice, but it doesn't seem possible. It would have the same problem as when you combine two antennas aimed in different directions with a splitter in reverse. When the same signals from each antenna reach the combining point, they would interfere with each other if they are not in phase (arrive at the same instant). IOW, it doesn't always work; you just have to try it.

Televes makes two and three input amps that they claim can combine two UHF antennas, but I doubt that it will always work, which is the same when using a splitter in reverse with an 8-bay antenna with panels that can be aimed in different directions.
https://www.tapatalk.com/topic/38557-signal-group-forums/9293-solid-signals-hands-on-review-televes-mast-mounted-amplifiers

https://www.amazon.com/Televes-Three-Input-Mast-Mounted-Amplifier-Combiner/dp/B01F48DMMK
I do have a Channel Master with two inputs but one is vhf & the other uhf. There is a switch inside to combine both inputs but I suspect that the vhf input will be designed to amplify vhf only so I guess feeding the two uhf antennas into this amp won't work.
Correct

I do also have a Winegard 2 input mast mount antenna combiner. I don't know if these are a better option than a two way splitter backwards or are the same thing.
Pretty much the same.
Being that they are plastic I don't know if these are more subject to interference than a
all metal 2 way backwards wired splitter.
Yes, they are.

If you had a separate antenna just for WPXJ, there are single channel custom combiners that make it possible to combine the WPXJ signal with the rest. Tin Lee Electronics makes an expensive one and Jan Jenca makes an inexpensive one.

If the separate WPXJ antenna had a very narrow beamwidth, it might make your strong local signals weaker when aimed at WPXJ. This might allow you to use a medium gain preamp that is resistant to overload just for WPXJ.

The 91XG has a little more gain for channel 23 than the 4400.
http://www.hdtvprimer.com/antennas/comparing.html

scroll down to Net Gain for some common UHF-only antennas
The 91XG is curve M
The 4400 is curve H

Both have lower gain at the low end of UHF than at the high end. A yagi designed for 23 would have more gain.

 
#10 ·
CBLT -24.6 dBm + 12 dB Ant + 16 dB preamp = +3.4 dBm; tuner overload
Don't forget that those old specs were done using analog signal levels and were already peak power levels. Digital signals are measured using the average power in the occupied channel bandwidth. You have to add another 6-7 dB to the average digital signal levels to get to the peak value that must be used to calculate the risk of overload. Also, keep in mind that CM used the -46 dB cross modulation threshold,a practice that makes the number appear to better than it would be if compared to an amp tested at -60 dBc.
 
#12 · (Edited)
Don't forget that those old specs were done using analog signal levels and were already peak power levels. Digital signals are measured using the average power in the occupied channel bandwidth. You have to add another 6-7 dB to the average digital signal levels to get to the peak value that must be used to calculate the risk of overload.
CBLT is a new digital transmitter that has been added. I have no idea what tvfool uses, but I must assume that they are using the same method for all digital transmitters. If what you say is true, then the risk of overload is even greater, and the fact that there are more than two transmitters to cause IMD makes it even worse.

I have more confidence in the specs produced by the talented engineers in the NC plant that was closed by PCT International. The specs used now are based on marketing convenience. The max input for the 7777 has been "fudged" to make it look better than what it is. They state:
Gain 30 dB
Max input level 15 dBmV
Max output level 40 dBmV

But that doesn't compute; it should be
Max input level 10 dBmV

40 - 30 = 10

And the last 7777 that I received has a gain of only about 23 dB, making it less susceptible overload. They "forgot" to tell us about that.

What it boils down to is that it is necessary for Bruce to try it, since the 2Edge signals on a report are known to be less accurate than LOS signals. It is only a computer simulation, and I can only make an estimate using the numbers available, again assuming that a dBm on the report is the same as a dBm in ATSC A/74.

In his favor is the fact that he did, at times, receive ION.

DXING.Bruce seems determined to try something to improve his reception of ION. It was my hope that I could help him understand why it is so difficult to receive, and suggest some things to try that would have a better chance of success than trial-and-error.

What is your estimate of his chances for improved reception of ION?

Is there a better way that I have overlooked, other than having a friend close to the WPXJ transmitter stream ION to him over the internet with a Sling box?
 
#14 ·
W-G HD-4400 Model

FOUND IT....turns out I DID upload 4nec2 Results for W-G HD-4400 Model (posted long ago by Ken Nist on HDTVPrimer)....except it was filed as a 4-Bay DIPOLE, rather than a 4-Bay BOWTIE that I was erroneously looking for. So I guess that I (or anyone else) can Copy/Paste a second copy to form a Vertical Stack [with Variable Stacking Distance] and Copy/Paste my HVH [Holl_ands Vertical Harness] into that file and let nikiml's Optimizer find the "Best" Dimensions:
W-G HD-4400 4-Bay Dipole with Reflector
 
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