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Discussion Starter · #1 ·
Okay, I'm a contributor here and I rarely ask questions. But when I do get an answer I always follow up.

I would like to know if anybody has tried to change the dimensions on a 91xg to make it geared more towards the repack?

My good friend here rabbit 73 it's already told me that everything needs to go up in size and dimension 29% I think. So that in itself could make it easier. I imagine I would just get some new boom stock, copy the other antennas increasing the spacing of the X Elements by 29% that would be the easy part. But how would I do the X elements? Does anybody have any suggestions on how to space them or how to make them?

Also, it seems when I look at a lot of the other hacks the reflectors don't really seem to respond much to increasing the size. But I would be more than willing to change those also.

ANYBODY ?

I had great luck with my 163 XG by basically doubling the size of the Boom and increasing the number of X elements with the same spacing all the way to the front of the antenna. But, I am looking to build something that Peaks before 608 MHz

Below is a picture of a stock 91XG versus the 163 XG:

3119234
 

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"A lot" would be a understatement ! And if it would really make a difference and to what degree? Wouldn't it would make more sense to 'stack' a 2nd antenna?
 

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Discussion Starter · #4 · (Edited)
"A lot" would be a understatement ! And if it would really make a difference and to what degree? Wouldn't it would make more sense to 'stack' a 2nd antenna?
Yes if you were looking for the extra power absolutely. And I've done it. I've paired two 91XGs.. with excellent results.

3121052


But I was thinking more along the lines of a more efficient single antenna setup. In other words taking all the antennas receiving capability and strengthen it in the channels that are actually used nowadays. In other words getting the antenna to peak at a much lower RF frequency.

For example, if the 91 XG is capable of 18dBi at Channel 51, I would like to shift the peak to be 18dBi at channel 36. As it is right now, the 91XG is much weaker at channel 36.

Essentially the 91XG's not quite the antenna that it was since the higher channels that are not being used anymore.
 

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Discussion Starter · #5 ·
Weld or rivet extensions like holl_ands suggested-
UHF 91XG - Wider Bowties, Better Ch14-51
It would be a lot of work for a little gain.
Good idea. But the problem is Holl_ands simulation is based on 14 thru 51. However, now we are dealing with 14 through 36. So, it would be nice to have a new simulation.

Hence, the reason for my post. I'm waiting for rabbit to chime in here. I think he gave me the magic formula before. If everything is expanded by a certain percentage then it should be good if it's based on the original 91XG
 

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Good idea. But the problem is Holl_ands simulation is based on 14 thru 51. However, now we are dealing with 14 through 36. So, it would be nice to have a new simulation.

Hence, the reason for my post. I'm waiting for rabbit to chime in here. I think he gave me the magic formula before. If everything is expanded by a certain percentage then it should be good if it's based on the original 91XG
holl_ands modeled some 4 and 6 bay types for the new repack. I'm not sure if he still models anymore, I haven't seen him post here for a long time. Not many people model antennas, it's not a skill one could pick up in an afternoon.
NEW UHF Free-Form 4-Bay + FlatScreenRefl
NEW UHF Free-Form 6-Bay (NU-FF6) + VDAR
 

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Good idea. But the problem is Holl_ands simulation is based on 14 thru 51. However, now we are dealing with 14 through 36. So, it would be nice to have a new simulation.

Hence, the reason for my post. I'm waiting for rabbit to chime in here. I think he gave me the magic formula before. If everything is expanded by a certain percentage then it should be good if it's based on the original 91XG
AD 91XG Gain vs Freq2_1.jpg


The original 91XG antenna was designed for the 14 to 69 UHF TV Band. Usually, the design frequency for a design is the midpoint of the desired band.

14-69 Band
470 to 800 MHz, midpoint = 635 MHz

14-51 Band
470 to 692 MHz, midpoint = 581 MHz
635/581 = 1.09
The dimensions for the change from the 14-69 band design to the 14-51 band would have to be increased 9%.

14-36 Band
470 to 602 MHz, midpoint = 536 MHz
635/536 = 1.18
The dimensions for the change from the 14-69 band design to the 14-36 band would have to be increased 18%. I would probably go to 20% if my important channels were at the low end of UHF.

These calculation are only a first approximation. More exact dimensions would require computer modeling, which I don't do; it's too tedious for me. I would rather do proof of performance measurements.
 
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Discussion Starter · #8 ·
View attachment 3120761

The original 91XG antenna was designed for the 14 to 69 UHF TV Band. Usually, the design frequency for a design is the midpoint of the desired band.

14-69 Band
470 to 800 MHz, midpoint = 635 MHz

14-51 Band
470 to 692 MHz, midpoint = 581 MHz
635/581 = 1.09
The dimensions for the change from the 14-69 band design to the 14-51 band would have to be increased 9%.

14-36 Band
470 to 602 MHz, midpoint = 536 MHz
635/536 = 1.18
The dimensions for the change from the 14-69 band design to the 14-36 band would have to be increased 18%. I would probably go to 20% if my important channels were at the low end of UHF.
Thank you Rabbit.
18% doesn't seem too bad. I feel like your suggestion of going a little larger would be better. And to be honest, it would almost make sense to make one maybe 18% larger and the other incrementally larger for each market that I receive. If you look at the Chicago Spectrum it is heavy to the left side of the graph so most of the channels other than RF 33, 34, 36 are down below RF 27. Incidentally RF, 33, 34, and 35 are pretty powerful stations. I think I would try to build one geared towards the lower channels in the Chicago Market so maybe go 20% or more larger.

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And my Milwaukee Spectrum it shifted more to the right:

3120968


Anyways that would be an interesting idea how to build a repack antenna for one market and a different repack antenna for another Market..

So, working with a round number like 20%, it seems like the math would be easy. So you would space the X elements apart 20% more down the boom. You would also put them across from each other 20% more. Then you would have to make the ex elements 20% larger
 

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So, working with a round number like 20%, it seems like the math would be easy. So you would space the X elements apart 20% more down the boom. You would also put them across from each other 20% more. Then you would have to make the ex elements 20% larger.
Yes, according to what holl_ands taught me, everything is increased the same amount. I would think the director size, director spacing, and reflector spacing would be the most important.
 

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But the problem is Holl_ands simulation is based on 14 thru 51. However, now we are dealing with 14 through 36. So, it would be nice to have a new simulation.
At the time, holl_ands was primarily interested in 14-51, but he tried a longer extension for the directors to give a MAX at 626 MHz; the curve looks pretty good even for 14-36:

91XG Wider Bowties  MAX 626_1.jpg


91XG Bowties for 14-51_1.jpg


If I read his modeling correctly, holl_ands did not change the spacing because the boom length was not increased. That makes the mod a lot easier, but I wonder how much more gain would have been available if the spacing had also been increased.

91XG Side View Orig_1.jpg


UHF 91XG Corner-FD-Yagi

91XG Wider Bowties MAX 626 Side View_1.jpg


UHF 91XG - Wider Bowties, Better Ch14-51
 

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I plotted the three 91XG model outputs and holl_ands extended director modification so it's easier to compare.

Turns out that the mod is better for the repack band than it was for the pre-repack band. Gain peaks just above channel 36 and there is a constant 2+ dB improvement across the repack band. Too bad it would be such a tedious mod..... 88 tips to add per antenna. It would be nice to buy a kit with new directors and just pop out the old ones and snap in the new ones. Anyone able to stamp out X-directors?

3121240
 

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I plotted the three 91XG model outputs and holl_ands extended director modification so it's easier to compare.

Turns out that the mod is better for the repack band than it was for the pre-repack band. Gain peaks just above channel 36 and there is a constant 2+ dB improvement across the repack band. Too bad it would be such a tedious mod..... 88 tips to add per antenna. It would be nice to buy a kit with new directors and just pop out the old ones and snap in the new ones. Anyone able to stamp out X-directors?

View attachment 3121240
I'd buy a couple sets if someone did that.
 

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Discussion Starter · #13 · (Edited)
At the time, holl_ands was primarily interested in 14-51, but he tried a longer extension for the directors to give a MAX at 626 MHz; the curve looks pretty good even for 14-36:

View attachment 3121199

View attachment 3121201

If I read his modeling correctly, holl_ands did not change the spacing because the boom length was not increased. That makes the mod a lot easier, but I wonder how much more gain would have been available if the spacing had also been increased.

View attachment 3121214

View attachment 3121215
I plotted the three 91XG model outputs and holl_ands extended director modification so it's easier to compare.

Turns out that the mod is better for the repack band than it was for the pre-repack band. Gain peaks just above channel 36 and there is a constant 2+ dB improvement across the repack band. Too bad it would be such a tedious mod..... 88 tips to add per antenna. It would be nice to buy a kit with new directors and just pop out the old ones and snap in the new ones. Anyone able to stamp out X-directors?

View attachment 3121240
Rabbit and Calaveras,

As always, your work is very thorough. Thank you for sharing your graph and your experience. Today I started the planning of the project. I also talked with Jeff Kitz about it and he felt that 33.3% was the correct figure for expanding the measurements.

However I've also heard 18 to 20%. So today, I scanned in a 91 XG director. Then I used my printer output to scale up and printed each size.

I was amazed how much larger they got at 33% but then again that is a third larger. That size change would be impressive.

3121273


I wish I had a way of stamping them. I'll have to look into that. I was thinking about either making either side directors into one piece without the plastic brackets that AD uses.

And then using those phenolic or the HDPE blocks that you used on your LPDA Calaveras. The directors are going to be the most difficult part of course. If I go to this much trouble I'm probably going to need to start with a brand new boom also.
 

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I also talked with Jeff Kitz about it and he felt that 33.3% was the correct figure for expanding the measurements.

However I've also heard 18 to 20%. So today, I scanned in a 91 XG director. Then I used my printer output to scale up and printed each size.
I did some more calculations based on the director extensions as modeled by holl_ands.

91XG  Director Sizes with Extensions_1.jpg


It is important not to make the directors too big, or they will become reflectors and block the incoming signals.

Since this is an experiment, I can't give a guarantee that it will work.
 

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Discussion Starter · #15 · (Edited)
Rabbit,

Excellent, that's the information I was looking for. So I think I can use my X directors left over from the big accident. 🤔

So I can cut the lengths I need off of my directors and just rivet them to the new directors on my new 91XG. Hell, I have a spare just hanging in the garage... that's a great place to start off. I think that's how I'm going to do this. If this works out then I will play with spacing. What do you think about the first director?

So it is 131% overall increase in size? So in actuality I don't really need to make a whole new design director for now? I can just add pieces of the old directors onto the new antenna.

How do you feel about changing spacing on the director's lengthwise down the Boom? I've got plenty of Boom stock.
3121728


I'm looking forward to this project.
 

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How do you feel about changing spacing on the director's lengthwise down the Boom? I've got plenty of Boom stock.
Without modeling, it's like shooting in the dark, and I don't see the director distances from the reflector to make calculations for the location of 22 directors.

All I see is this:

91XG Side View Orig_1.jpg


UHF 91XG Corner-FD-Yagi

Maybe someone who knows modeling can generate the distances from the code given for the stock 91XG.
 

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Discussion Starter · #17 ·
Here's the first look at my new X element directors. I'm not perfectly happy with them but I think they will work. I've got to go buy some 3/32 aluminum rivets. The 1/8" rivets will work but I like the 3/32" better.

3121797



3121798


I even made a jig but I have to modify it because I thought I could just flip the X element bracket and put the other tips on but it didn't work that way so I've got to get a larger of piece of wood probably a 2 x 6 or 2 x 8 and change my stops then it's going to work well.


3121799


21 more to go...
 

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I even made a jig but I have to modify it because I thought I could just flip the X element bracket and put the other tips on but it didn't work that way so I've got to get a larger of piece of wood probably a 2 x 6 or 2 x 8 and change my stops then it's going to work well.

21 more to go...
I like the idea of the jig. It allows you to set up the distance from the centerline to the tips as done in the model.

The only thing that bothers me is that his drawing of the director in the model doesn't exactly match the actual director at the base.
 

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Discussion Starter · #19 · (Edited)
Rabbit,
You know what? If you looking at that photo that I took of the tape measure and the X element. I change the setting on my phone camera to zoom out and it makes it look like the tape measure is not centered in the middle if that makes sense.

But it really is 6.2". I added 1.5" to the stock X elements with my director extensions. I'm absolutely sure that I can make them just a tad bit longer in this case half a tenth of an inch. I'm not sure I've got enough X elements left from the two broken antennas, so I might have to cannibalize one of my good ones..

I will keep everybody informed on this project. Anybody who has any suggestions, join in ASAP!

Rabbit, thank you so much for taking interest in this project.
 

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Chiwaukee, Rabbit73, & others,

Your plans look fine.

Think it will work out well.

A couple of years ago did something similar, results were satisfactory. The antenna was an MCM 30-2370, it has thirty some elements, so it is longer than typical 'XG' type antennas. It was installed a on the tower in KY.

Used #12 stiff aluminum wire to extend directors 1.15 inches. According to 4NEC2, that director extension put the peak response around channel 32 or so. Some field tests showed about 2 dB gain across the repacked band compared to the extended antenna without director extensions.

For the 20-25 directors nearest driven element, left director spacing along the boom near stock. There is some small additional gain available by tinkering with director spacing near the front of the antenna.

In general, closer spacing lengthwise on the boom (as stock) affects the available gain only slightly, in some cases closer spacing modestly improves gain. Some of the main reasons for wider spacing is to reduce weight, wind load, and cost of materials.

Wider spacing (between directors, across boom) could further improve gain, however that requires more modification.

Have constructed a kit for that antenna to replace driven element with longer one, add three more director sets, and replace reflector with a modified version in a couple of weeks (when in KY, currently in Dallas).

Image below shows director extension.

Director Ext 1.15 inch.jpg


Image below shows portion of installed long antenna with director extension.

Long Ext Dir Installed.jpg





One other thought; The optimum length of directors for a given band of frequencies depends on boom length and obtained gain. IOW, if the boom is long (say much longer than stock), then the optimum director length will be slightly shorter than what might have been optimum for an otherwise stock antenna.

Appreciate all ideas discussed, they seem fine and interesting.

Chiwaukee, enjoy the modifications.

Currently, a bit distracted, can maybe write more detail at a later date.

.
 
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