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Nice hardware hack - I wonder if we have another member with a 91XG who can replicate/confirm/deny your results for us?
 

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I'll try wider mesh as 300ohm suggests tomorrow but so far the extra mesh is working very well testing the last few hours under varying weather conditions.
 

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I tried 40" wide and it did not seem to improve things. Mind you todays weather is overcast with a low ceiling and light showers. The extra mesh definitely helps. I am not seeing the large gains today but still enough to inspire me to climb my tower yet again to add the mesh to my 91XG.
 

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Extension is possible.

As with any Yagi design, extension is always theoretically possible. The limit always comes down to physical feasibility. In theory, you can pick up about .4dB of gain per additional director. So theoretically, seven additional directors buys you .4 x 7, or 2.8dB of gain; just about double the signal power.

BUT: I'm pretty sure that just inserting an additional "middle" boom is NOT going to be productive. In pure Yagi designs the spacings of additional directors increases as you move forward from the active element. You can see this as you look at the spacings of the 91XG. The 1st directors (nearest to the folded dipole, the active element near the screen) are just over 1" while those at the front are about 6.5" between them.

While optimal designs for pure single frequency Yagis tend to have director spacings that increase continuously from director to director, the 91XG like many broad band designs has spacings that sometimes remain constant and/or increase in qauntum steps. For example, in inches, here are the spacings between the 91XG directors: 1.35, 1.55, 1.75, 1.8, 2.12, 2.35, 2.35, 2.5, 3.2, 4.4, 4.5, 5.15, 5.1, 5.5, 5.5, 5.5, 5.5, 5.5, 5.5, 6.3, and 4.7.

From my little bit of empirical knowledge modifying Yagis with the help of a Spectrum Analyzer with integral Tracking Generator, I know that in broad band designs small adjustments in spacings can favor one end or the spectrum or the other. Keeping in mind that successful UHF reception means the ability to receive with as flat a gain as possible over almost an octave, I suspect that such irregular spacings help with bandwidth optimization, as opposed to optimizing gain at a single (center) frequency.

Bottom line: adding gain through extension is definitely possible through additional directors at the front end. I suspect the spacing will approach 7 to 7.5". If that's true then at 7 additional directors required for 2.8dB, at just a 7" spacing, then 7 x 7 = 49, thus requring at minimum about 50" of additional boom. But 5 or 6 additional directors might get you 2.0 to 2.4 dB, which is pretty close to what you would get for ganging two antennas (if all your choices of interconnection and combiner are optimal), and might be physically manageable.

I would think the length support can be accommodated with the concept earlier in this thread, a support wire above the boom. I've used that technique myself on longer designs. A small vertical 3/8" support rod mounted with "L" brackets on the boom near the mast, about 6" high, and a support wire running from stem to stern. I've just used a #8 eye bolt, nut, and washer to attach wire to boom. There will be no significant electrical effect from the wire, which like the boom will be perpendicular to the electrical wave, nor from the minimal hardware.

However, my biggest worry would be that if your directional requirements are such that the prevailing winds are lateral (from the side), and strong, the single mast clamp may be insufficient to withstand the resulting torque (the boom could turn on the mast). I would consider drilling holes in the boom itself for an additional U-Bolt and knurled edge bracket.

I'm intrigued a little about this, myself being subject to intervening terrain and knife edge effect reception. I think in the next month I might be able to experiment with an extension and will publish the results...
 

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So theoretically, seven additional directors buys you .4 x 7, or 2.8dB of gain; just about double the signal power.
It doesnt really work that way, unfortunately. The first few directors add the most additional gain, then with additional directors the additional gain tapers off to the point of marginal utility.
Also the directors on the 91XG are sized for the high channels, above channel 51.
 

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Hmmm... seems contrary to the literature

Just want to say that in any of the literature I've read about Yagis I've never heard mentioned the concept of deteriorating electrical utility; just ever increasing spacings and therefore deteriorating physical feasibility.
- Modern Antenna Desgin, Thomas A. Milligan, 2nd Ed.
- Antenna Engineering Handbook, Johnson & Jask, 2nd Ed.
- ARRL Antenna Book, 21st Ed.

But I accept that every design is unique, and may have its peculiarities, and my own knowledge is a fraction of what there is to know.

So I'll rely on empirical results. I'll conduct the experiment, and publish the result as objectively as I can, positive or negative.

Thanks for your input!
 

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Well, you can experiment with the yagi calculators. (they scale the director spacing better than commercial antennas do, as the commercial uhf antennas generally use a constant director spacing after the first director) As you go up in gain, it takes more and more directors to get that elusive extra db.

As far as the 91XG goes, http://www.antennasdirect.com/91XG_HDTV_Antenna.html 16.7 db gain with a 93 inch boom vs the 43XG, http://www.antennasdirect.com/43XG_digital_antenna.html, 15.7 db gain with a 50 inch boom. The difference looks to be the additional 43 inch director boom. With that scaling, I think you would be lucky to get .5 - .7 db additional gain with another additional 43 inch boom on it. (.5 - .7 db additional gain is really hard to notice)

A better bet would be to gang two 93XGs (or 43XGs) for a possible 2.5 - 3 dbi increase in gain.
 

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Making it more durable

goforit said:
So for those with new 91XGs, is it recommended to seal all fasteners/joints with a lock-tite substance- to increase durability/longevity?
Yes , I recommend doing a few things to increase durability and longetivity . I just had to take the antenna down to replace the rotor that wasn`t giving me a full rotation ( quite old ). BTW the Source is clearing them out for $50. Great deal !!! GeoStar commented on how his directors are coming loose . I questioned myself on that when i was assembling the XG , one of the elements fell off due to it was bent where it snaps into the plastic holder from the factory . Thanks to 300 ohms idea of using adhesive for the directors , i used pcv adhesive by applying it to the topside of the director letting it run down between the director and plastic holder filling the gap , should hold alot better . I also didn`t like how the rubber seals on each side of the balun sealed so i used black silicone to make sure that was sealed up good .

As for hoopitup , thanks for the suggestion of moving the antenna up & down on the mast . I moved it up 2" and lost a few bars on signal strenght and moved it down afew inches and lost alot . Guess i hit the nail on the head the first time around. Awsome feedback on this thread . It`s much appreciated for all those contributing !!!
 

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91XG vs 43XG specs

Uh, while published specs are by no means always authoritive, taking them at face value what I see it:
http://www.antennasdirect.com/91XG_HDTV_Antenna.html 16.7 db
http://www.antennasdirect.com/43XG_digital_antenna.html, 15.7 dbi
Note: "db" infers dBd or dB(dipole), while "dbi" stands for dB Isotropic. To convert dBe to dB subtract 2.15 (or 2.16 depending on the literature). But if just truncated to 2.1 and subtracting from 15.7 (dBi) for the 43XG means its gain in dB is 13.6.
Then, 16.7dB - 13.6dB = 3.1 dB difference.

The 91XG has 22 directors, the 43XG has 17. that's 5 additional directors, and at .4 * 5 that ought to be just 2.0 dB.

What? Something for nothing?

I don't think so. Look at the "Net Gain" curve for the 91XG at http://www.hdtvprimer.com/ANTENNAS/comparing.html. While this is a modelled response curve it does illustrate a common characteristic of most broad band designs: mainly the advertised "Max Gain" figures are typically representative of just one point in the spectrum. The **average** gain of the 91XG is probably around 13 dB across the band, and the 43XG will be a few dB less.

Tom
 

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Sorry about the following typos in my previous post. I don't know why but the "Edit" button has disappeared for me so I can't correct the post.
I meant:
"... what I see is...", and
"... To convert dBi to dB ...",
:-|
 

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Antenna Direct has numerous examples of typos in their marketing literature. The 16.7 db figure has to be dbi, which corresponds closely with http://www.hdtvprimer.com/ANTENNAS/comparing.html

Im sure they would have loved to post 16.7 + 2.15 = 18.85 dbi figures if it were true, heh.
that's 5 additional directors, and at .4 * 5 that ought to be just 2.0 dB.
I have no idea where you got the .4 figure from.
- ARRL Antenna Book, 21st Ed.
But if you have that book handy, look in Chapter 11 - HF Yagi Arrays, page 11-10, figure 10. It will show you the marginal gain drop off curve of more directors and longer boom length. For example, you need to add 9 directors and 4 1/2 wavelengths of boom to go from 18 dbi to 20 dbi with a theoretical optimal yagi.
 

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Thanks to 300 ohms idea of using adhesive for the directors , i used pcv adhesive by applying it to the topside of the director letting it run down between the director and plastic holder filling the gap , should hold alot better
I applied Plumbers Goop/Loctite Plumbing and Marine Adhesive on the loose/cracked reflectors on my Antennacraft corner reflector about a year ago. (It dries semi-hard unlike silicone sealant, and is also non conductive ) The exposed glue still looks good, with no signs of UV damage so far.
 

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Yeah, thinking about it some more, the 91XG reflector should be sized for 470 mhz already. Trying to shift the gain curve to the left with a larger reflector can only go so far. The only thing to do is to resize the yagi section for the narrower band. On the 91XG, thats no easy task.

On the otherhand, with my Antennacraft corner reflector, with its all straight tubing, all pop riveted construction, it would be much easier.
 

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FYI,

I doubled the reflector size on the 43XG (shorter 91XG) and it only helped slightly on the upper UHF digitals...
 

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Nice hardware hack - I wonder if we have another member with a 91XG who can replicate/confirm/deny your results for us?
I am interested in doing this but it is difficult to get to, perhaps this weekend.
 

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I'll be having my Crow medium rare, thank you...

300 Ohm: I stand humbly corrected. I'd forgotten about the relationship illustrated in that chart.

I've spent a lot of time extending a Y10-7-13 over the last few months, from its default 6 directors to 11 (adding 5 directors), as well as a planar reflector. Via A/B Spectrum Analyzer measurements I've achieved an increase of 2 to 3.5dB across the Hi-VHF band. The empirical rule of thumb that has evolved in my head has been an **AVERAGE** of about .4 dB per additional director.

However, while focusing in this narrow range of directors, and holding it in mind that that gain is proportional to number of directors, I lost sight of the reality that this proportionality is not linear, but rather logarithmic.

In addition to your reference there is a good verbal description beginning at the third paragraph after the illustration of the "Seven Element Yagi" at http://personal.ee.surrey.ac.uk/Personal/D.Jefferies/yagiuda.html.

Thanks for the reality check.
 

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And thank you for your classy response. :)

Being that this is an R&D forum I think most of us have had some 180° turns in our thinking at some point or other in here! :D
 

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I think most of us have had some 180° turns in our thinking
Yeah, around to 180, then to 90 then to 270 and then to 169 and still going, heh.


tballister, theres a free simple desktop yagi calculator (called Yagi Calculator, how imaginative, heh) that will show director by director gain.
http://vk5dj.mountgambier.org/Yagi/Yagi.html


The Y10-7-13 is a bit different, but it will show the general cumulative effects of what you should be getting.
 

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Stacked 91XGs produce disappointing results

With the cold weather approaching fast and tropo finished for the most part, I decided to finally try stacking my 91XGs atop my tower.

I spaced them 25" apart boom to boom. Two identical lengths of coax feed a reverse splitter which then feeds my RC 9260 amp. They are pointed in the exact same direction and the booms are perfectly parallel.

I actually had them sitting about 4 ft. off the ground in a tripod so I could adjust the spacing and direction before tightening everything down. Just for fun, I checked signal strength and I was surprised to see WGRZ (real 33) coming in around 70 - 75.

I then hauled the whole assembly up my tower which is about 30 ft. up. Signal strengths are for the most part no better than the single 91XG I had up before. Some are worse. WGRZ now is about 63 with the odd jump to 70 and then it goes to 0.

The only benefit is WUTV (real 14) now comes in at a very steady 64 where it used to dropout before.

I have to assume that the poor results are height related. I also assume the PCB baluns are phased the same. The single 91XG seems to offer the best results I am going to get.

I had hoped that this would be my final tower climb until the spring, but now I have to climb up again and take one antenna down or at least play with the height. This RF black magic is causing me to pull the last few remaining hairs out of my head! :confused:
 

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