DigiWave Hinged 8 Bay Bowtie

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#2
The bottom model doesnt seem to have equidistant interbowtie spacing. The middle section with the center feed point is greater than the outer.

Furthermore the upper one seems to have larger surface area whisker elements.

Although the lower one seems to have wider reflector screens. (Assuming the whisker lengths are the same, probably 8 inches long).
 

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#6
You are exactly right Jim. I do like the flexibility it provides, for experimentation in that regard. If I was designing an 8 Bay antenna for Antennas Direct to replace the DB8, it would integrate this feature.

You could also use it with a A/B switch as well, each half on its own line.
 

EscapeVelocity

Moderator, , Webmaster of EV's Antenna Blog
#7
Here is a pic of an installation of one of them. The antenna to the left is an old discontinued CM 4228A.

Just to note, many people gang 4 Bays together facing different directions. Its pretty common.

 
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IDRick

DTVUSA Member
#8
EV,
Wouldn't the competing halves of each of those antennas tend to cancel their benefits? I think its use would be in very special situations such as explained half-way down the page on the following link. What say you?
Jim

The Two-Antenna Trick (outdoor version)

Stacking multiple antennas
Interesting, look at the first example on the primer link (Balt & DC with 50 degree separation). Ken suggests mounting two CM4221's with a 23 inch spacing (mast to mast). Note, that the inside edge of the reflectors would almost be touching with a 23 inch spacing. Also note, that the spacing becomes closer as the separation between the two lobes increases. If we assume 70 degree separation, the two antennas should be spaced 18 inches apart, mast to mast. We can't do it as the reflectors would overlap and the whiskers almost touch.
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#9
IDRick,
Good point, but he does mention earlier in the text something regarding "practical antennas" so I suspect his model is a simple teaching example. On the other hand, if end-mounted Yagis were used instead of screen antennas, it works!
Jim
---------------------------------
Interesting, look at the first example on the primer link (Balt & DC with 50 degree separation). Ken suggests mounting two CM4221's with a 23 inch spacing (mast to mast). Note, that the inside edge of the reflectors would almost be touching with a 23 inch spacing. Also note, that the spacing becomes closer as the separation between the two lobes increases. If we assume 70 degree separation, the two antennas should be spaced 18 inches apart, mast to mast. We can't do it as the reflectors would overlap and the whiskers almost touch.
 

IDRick

DTVUSA Member
#10
Maybe you can explain something to me about Ken's example 1. He states: "At channel 40, the antenna will have two equal main lobes 25 degrees from the centerline. The de-rating curves below show that the 25-degree aiming error is costing 3 dB, so the performance is no better than a 4-bay pointed at one city." As I view the diagram, the two lobes are square on for the towers. Why does he subtract 3 dB for being 25 degrees off axis? Also, can you explain how to reverse polarity on a 4-bay?

Thanks!

Rick
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#11
IDRick,

I think the answer to your question is on the tutorial page I posted above. Stacking multiple antennas

Although his example refers to a splitter combining two antennas, its basically the same thing because the two antennas EV pictured above are each essentially two different antennas on a single mast receiving two seperate energy sources sent to a single load. Here's part of a quote:

"1. one quarter of the input power is reflected back towards the input,

2. one quarter of the input power is diverted to the other input, and

3. one half of the input power is forwarded to the intended load
. (there's your 3dB loss result)

These numbers assume the splitter is 100% efficient, and that the antennas point in different directions."

No clue about how to reverse polarity on a bay antenna, except to build a mirror image or perhaps install it upside down!

Jim
 
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Piggie

Super Moderator
#12
First he explains to reverse the polarity of the second antenna on a CM4221A you just reverse the leads from the balun. I have not tried yet (waiting till spring) to stack two old 4221As side by side (which may solve my NBC if WESH-24 ever goes on the air). I have stacked them vertically on the same mast, using identical baluns and it was very obvious when I had one reversed, the reception went to nada.

But if you read below the derating curve you see some Ken wrote.

"The above graph is just a modified presentation of the radiation diagram for the 4221."

He is explaining here in my mind the because the two antennas are facing the same direction he is not seeing quite the same thing that happens when you combine them in different directions in phase and combine them. Then you simply suffer a 3 db loss in the combiner due to reflected waves not a having an equal and opposite signal to cancel like you do when you stack in exactly the same direction.

If you look at Ken's loose graph vs the azimuth or overhead plot of the CM4221 Channel Master 4221 it looks like just what he says, another way to plot the CM4221 with db down vs degrees off axis. I picked a few points and that seems about what it is. Also if you do it be sure to take into account that not all the channels on the overhead plot hit the zero db line. But the loss graph has them all at zero db equally, so when subtracting in the overhead start from the maximum on axis gain of a particular channel
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#13
regarding reversing polarity of antennas

Piggie wrote:

First he explains to reverse the polarity of the second antenna on a CM4221A you just reverse the leads from the balun.
-----------------
Piggie, et al:

I didn't mention reversing the balun's leads because I have a variety of unmarked baluns that have neither a front or back side with specifically identifyable markings.

Getting nit-picky -- to actually verify the polarity of an enclosed 4:1 balun, wouldn't it require an RF signal and an oscilloscope that can 'see' that frequency?
Jim
 

Piggie

Super Moderator
#14
Piggie wrote:

First he explains to reverse the polarity of the second antenna on a CM4221A you just reverse the leads from the balun.
-----------------
Piggie, et al:

I didn't mention reversing the balun's leads because I have a variety of unmarked baluns that have neither a front or back side with specifically identifyable markings.

Getting nit-picky -- to actually verify the polarity of an enclosed 4:1 balun, wouldn't it require an RF signal and an oscilloscope that can 'see' that frequency?
Jim
Jim, I don't remember where I read it, but I am pretty sure it was on Ken's site.

It is mandatory to use identical baluns. Now we can argue that those with test equipment find variations even in the same make and model of baluns, of which I totally believe.

You can look at it as the least of two evils without test equipment to even consider ganging phased antennas without two identical baluns.

I didn't mention that I even reversed the phase of one of my YA-1713s early in testing due to it was cold Feb day and we worked too fast. On the Winegards with the built in baluns it's simple to get the phase correct or reverse it. My point was the performance was wild. Just rotating the antenna to check if it has the predicted lobes and nulls or is it suddenly a lot worse than a single antenna.

All I can say so far in the experiments I have done phasing two antennas, it was obvious when the phase was wrong.

Now that said, no, for example, using the CM balun, currently my favorite, there is a Made in China, that one can use as a start one side of the black barrel of the balun, though it could have been molded backwards.

But I can't say enough that in every case so far, it's been blindingly obvious when the phase was wrong.

But don't think for a minute I too didn't have my doubts about needing a test equipment solution to really know polarity.

Back in the day, there was a saying in the electronic tech profession, that if you can't trouble shoot something with only a Simpson 260 (Volt-Ohm-MilliAmp Meter) you were not a technician. I truly believe this and have amazed many an engineer that always had test equipment through school and work, what I could fix.

Another person I know that is my age and worked telco twisted pair, has as his signature "If you can't fix it with just a butt set, you're not a real telephone repair man. "

Even recently I blew his mind when I troubleshoot a faulty NID interface with my volt meter and a home made butt set. My DSL was sufffering extreme attenuation. So I wanted to disprove it was my internal wiring, because I had tested it and it was very low resistance without high resistance between the pairs in my house. Yest when I plugged my modem into the outside customer NID connection everything was fine! But hook up my house wiring and plug through it things went downhill. Well there is only one thing in the common NID between that plug and the house wiring. A very short piece of wire with a male RJ11 on one end and screw terminals on the other end. Luckly I had a 4 line NID at my house, and only actually use 2 of them. So I went in the company side, which exposes where you can remove individual demarcation piece and swapped with an unused one. Fixed the problem. No telephone test equipment. Only a DSL modem and a multimeter.
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#15
Piggie,

Like you, I'm also becoming fond of the CM (Chinese) baluns mainly because they don't use 'twin-lead' and once a project is being finalized, I can cut the individual lead-in wires "as short as possible". As you said, there's no guarantee any ('generic') balun didn't have its case molded backwards or reversed. My point too.

So back to my Q+: to prove an "over-the-counter" balun's polarity, wouldn't it take an RF source and an oscilloscope that can see that frequency? It seems to me, the lead-in line on the scopes' image would tell all.

My scope is a terrific old RCA probably limited to under 2 mHz. So ... can the polarity of baluns designed for VHF be determined by testing them far under their design frequencies using my stone knives and bearskins, say at 1.8 mHz?

Jim
 

IDRick

DTVUSA Member
#16
Jim and Piggie,

Thanks for the discussion! Broadcast towers in my town are 70 degrees apart. I had been toying with the idea of trying to horizontally stack two mclapp DIY 4-bay antennas. Just for fun, not really needed at my location as my single 4-bay has worked well. But, I have two spares and thought it might be fun to try. Now I'm confused at a higher level and feel that it may more of a challenge than I had hoped... Sooo, I'll probably drop that idea a ways down the list of thing to do with antennas.

Thanks for your discussion and advice!

Best,

Rick
 
#17
Why does he subtract 3 dB for being 25 degrees off axis?

Thanks!

Rick

Because the actual angular deviation puts the reception angle off to the side of the antenna's normal reception lobe. While the graphic represents a lobe that points directly at the tower of interest in the illustration, that lobe is actually weaker (by 3 dB in his example) than the lobe would be if it were "straight ahead".

Also, can you explain how to reverse polarity on a 4-bay?
If the antenna has a 300-75 ohm balun, swap the leads. If it's a PCB balun that cannot be reversed, flip the antenna upside down.
 
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IDRick

DTVUSA Member
#18
Jim, Piggie, and Projectsho89,

This doesn't apply directly to the digiwave but it does apply to horizontal stacking. I'm hoping you gentlemen can clear up my confusion on this topic. This is a play thing to do with some spare antennas, not a necessity....

My situation.

I have stations in two general directions (200 and 270 degrees). I am using a mclapp 4-bay antenna with elements swept back 1 inch. The antenna orientation is set at 235 degrees. My derating curve for off axis aiming is shown in the attached file (actual data from my location). With aiming 35 degrees off axis, I'm losing 4 dB from each station with the swept back elements and would lose 5 dB if I used flat elements.

I receive the following stations (real ch#'s): 8, 15, 17, 23, 31, 36, and 47. We primarily watch ch 31 (200 degrees) and ch 36 (270 degrees). Ch 31 has a lower NM than ch 36 (41 vs 50dB). Horizontal spacing should probably be based on ch 30.

Questions:

Can I improve performance by using the horizontal stacking?

If so, how much gain improvement would there be?

What is the correct spacing between antennas (assuming ch 30 as primary)?

Projectsho gave the quick and dirty on how change polarity on one antenna. I understand I need equal length cable from each balun. How do I join the two feed lines? Use a common splitter?

Are there any special requirements for locating the baluns and combiner? My antennas are built to attach the balun on the rear side. The CM baluns actually extend beyond the reflector, so the combiner would be behind the reflector and the mast would be in the v between cables and combiner. The mast will be pvc for this experiment so probably not an issue.

Can I use the swept back technique on the whiskers or must they be flat for a horizontally stacked antenna?

My cost to do this little project is actually quite low. I already have two identical 4-bay mclapps with pvc spines. I can simply construct a pvc frame to set the spacing between antennas and to mount the reflector. Note, it would be a real pain to alter the spacing between masts once the frame is constructed.... I'm looking at less than $5 in new pvc pipe plus the combiner. If I can use a Perfect Vision 2-way splitter, I'm good to go for the combiner. I have three identical CM baluns looking for work. Could be a fun project, even more so if there is likely to be a significant gain (3 dB or more).

thanks for your assistance!

Best,

Rick
 

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Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#19
IDRick,

Here's a quick comeback because I have to leave in 5 minutes. You refer to your plan as horizontal stacking, but you could do it vertically stacked without having to build the supports.

It seems to me a second length of coax and an A-B switch would be preferred (less loss) unless there is a specific reason to establish a null somewhere between the two signal sources. Gotta go.
Jim
 

IDRick

DTVUSA Member
#20
Good point Jim! Clearly two antennas + a/b switch has the lowest losses. But, what is the second best option? A single 4-bay with swept back elements or two horizontally stacked 4-bays? Both provide a single downlead that does not require switching and enables recording OTA with a computer htpc.

I did say I was confused, right? :) I know that my current setup loses 4 dB with a single 4-bay. So, is the horizontally stacked better? As I understand Ken's comments, a properly constructed horizontal stack would give me +2.5 dB if both antennas pointed in the same direction. Since the side lobes are 35 degrees off axis with reversing polarity, I would then see +2.5 dB minus the 4 dB for off axis lobes or -1.5 dB. If my math is right, there is a benefit to horizontal stacking. I would lose 1.5 dB rather than 4 dB, correct?

Thanks!

Rick
 
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