Building The Stealth Hawk TV Antenna (See Post #304)

[ota_canuck]

mclapp;

SWR (less than 2:1) across the UHF frequencies and a raw gain of between 6 - 10 dbi

That sounds very interesting for my area, but how directional? So far my observations say that small simple antennas have wider beamwidths but lower gain, and wacky SWR values. Low-gain/High-SWR VHF doesn't seem to be an issue for my current analog VHF market area. But I really need wide Beamwidth to capture the scattered market. [see my tvfool in OP] Ganging antennas in various directions, can and usually does open the door to potential multipath problems.

Question? Is SWR a result of feed gap, phase line gap, element lengths. Is there any way to calculate the anticipated SWR. Though nec2 modeling is quite valueable as a tool of benchmark measurements, I'm still not convinced that nec2 gives us the whole picture.

[ota_canuck]

OK! This is the updated design showing re-tuned angles, gaps and leg lengths. The cheriex diamond design specifies 40°-45° angles, so I revised the diamond shape to reflect that standard. The rest of the angles are derived based on 30° angles for symmetry. The gap at the top [between the elements] ended up being only 1" between the diamonds elements. That revised gap seems to work just fine, so I left the 7" leg length as it was originally.

symmetry[patterned for self-similarity] eg: 0°/30°/60°/90°/150°/180° angles

The lower splayed legs on the bottom elements were adjusted to acheive the optimum real-life reception of VHF and analog. The bends were revised to reflect on the diamond [invisible air space] theory and to keep the angles within the 30° symmetry of the top half of the antenna.





Instead of testing it on the 20ft shop mast, I climbed the tower at my house and installed it as my one and only antenna on my 35ft tower. Awsome rock solid signal strengths across the board from Toronto, Buffalo, Erie, Hamilton and even ION from Batavia. Batavia at 80+ miles though is likely just the usual the nighly event as the air cools down at dusk.

[mclapp]

The better SWR is due to the center whisker, it's as directional as a 4 bay but going to smaller dimensions will give a wider beam with less gain. This is all without a reflector so it's bi-directional as well. If you want good gain with a wide beam I've found and IDRICK field tested a swept rearward flat reflector version of a 4 bay which widened the forward beam a lot. I have some polar plots of that antenna as predicted by nec2.

[ota_canuck]

mclapp;
Do you have a sketch of this 2bay? I'll give it a real-life try here as a bi-directional...

[Marbles_00]

ota, I was thinking of building this with 10AWG wire this weekend. I was going to start by following your first drawing, but noticed that it has since changed to the flat aluminum drawing. Any chance of re-uploading the 10AWG wire drawing again. I already have some framing up in the attic for a VHF 2-bay bowtie, but I figure I give this a whirl and see what happens.

Thanks

[ota_canuck]

Marbles_00,

I think this was the original pic from the OP. You can build to any of the posted sketches based on leg lengths and gap distances and the angles should fall into place. Any would work with #10 or flatwire material, I found the flat material easier for straightness, bends and mounting.

I would recommend using the most recent updated [tuned] dimensions, however any of the sketch designs provide fun projects to work with. It's quite the beast.

Check out holl_ands previous posted link for nec2 info.
http://imageevent.com/holl_ands/chireix/stealthhawk

[Marbles_00]

So with your newest design, the feed points are about 1" apart? Also your drawing is a little confusing in that if you follow an imaginary line from your upper diamonds to the lower VHF horizontal lines, it would appear that your bends would cross over the other element (looking at your drawings the horizontal lines appear to be about 1" below the feed points). At the feed points, do you bend the elements downwards about 1" (keeping a 1" spacing between the elements) then bend outwards at 90 degrees (hence using 2 screws per side of the box to hold everything in place?

[mclapp]

For the widest beam width with reasonable gain and SWR make a 2 bay with 9 1/2" whiskers and a 9" bay spacing and an added 8" long whisker coming straight out of the center of each bowtie. A 1 1/4" phase line spacing and a 4 1/2" whisker spread is fine, with this set-up I ran the models with #10 wire but #8 to #12 should be fine.

Larger sized ones (over 11") cover UHF and VHF-hi with more net gain and better overall SWR but the beam width will be narrower.

[holl_ands]

THREE different Stealth Hawk designs analyzed....so far....and a couple more yet to do:
http://imageevent.com/holl_ands/chireix/stealthhawk
The one currently labeled "ota_canuck" presumed Top: 7-in at 30-deg and Bottom: 8-in at 22-deg,
which was my first guestimate for dimensions. I need to rerun with 21-deg Top angle per first dwgs.
And, of course, the latest version(s) just revealed....& gotta revise the 4nec2 file for that top gap....

If you navigate to the MultiBay page, you'll also find UHF & Hi-VHF 2-Bay's and you can also
navigate to a comparison of simple Bowties: Six-Whisker mentioned above is best.

I attempted to model the 3/4-in Metal Foil Tape with it's Round wire "equivalent",
in accordance with VK6DJ's formula: http://vk5dj.mountgambier.org/Yagi/Yagi.html
which was derived here: http://fermi.la.asu.edu/w9cf/article...orm/index.html
The equivalent diameter was only weakly affected by the tape thickness....BUT,
4nec2 wasn't "converging??" AGT with the resulting big, fat elements....so I had to
back off the Equivalent Radius from 0.19-in to 0.1-in (didn't try any slightly larger....).

My "best so far" Stealth Hawk design: I look forward to comparing to the new ones.
Top: 9.5-in at 45-deg angle, Bottom: 9.5-in at 30-deg down angle, TopGap=0.75-in.
2-3 dBi FORWARD Gain up thru Ch51, with SWR < 3, although Gain drops off on Ch14-17
(Land Mobile in our area). Add half-an-inch to favor low channels at expense of top end.
Gain is slightly higher in the "dog-bones", which are well controlled except Ch14-17.
Hi-VHF is a Flat 1.6-1.7 dBi Gain FORWARD with so-so SWR of 7.5 dropping to 4.5.

Numerous other variations were tried....most resulted in higher SWR & Gain drop on Ch7 or 13.
I was surprised "stagger tuning" didn't push coverage across entire New UHF Band,
but perhaps some other combination of different element lengths will eventually succeed....
And BTW, there is lots of interaction between the Top & Bottom Elements...they are
NOT performing as independent UHF and Hi-VHF sections....

[holl_ands]

In diagram in Post #51 above, it seems inconsistent to have a 2-in Top Gap and
a 3-in Feedpoint Gap if all of the Top Elements are the same length and angle.....
Something's gotta give....perhaps the angle of the upper most elements...

[IDRick]

Questions for Holl_ands and mclapp,

Holl_ands, Excellent job on the modeling! I'd like to give the Stealth Hawk a try at my location. Which version do you recommend? My tvfool is here: http://www.tvfool.com/?option=com_wr...9c6bf7c105b276 The stations in green and ch 47 are the important ones. A mclapp 10x9.5 with a flat reflector works very well at this location with swept back elements and aiming at 225 degrees. This would be a good comparison of a high gain, narrower beam antenna versus a low gain, very wide beam antenna.

mclapp, thanks for mentioning your new mods on the two bay. I'll give it a try. Should the 8" center whisker be on the same plane as the 9.5" main whiskers? For construction, I was planning on cutting a 8-3/8" long piece and creating a loop at one end. It'll provide a flat surface for mounting to my carlon strap but would end up ~1/8 inch in front of the main whisker. Probably not an issue, but thought I should ask! What is the expected difference in beamwidth between conventional 9.5x9 two bay versus 9.5x9 2 bay plus center whisker? Does a reflector alter the beamwidth?

Thanks,
Rick

[ota_canuck]

Originally Posted by holl_ands

In diagram in Post #51 above, it seems inconsistent to have a 2-in Top Gap and
a 3-in Feedpoint Gap if all of the Top Elements are the same length and angle.....
Something's gotta give....perhaps the angle of the upper most elements...

In post #47 diag. you'll see that the gap is 1" and the elements are fixed onto the sides of a 2" PVC box using 3/4" screws.Those mounting screws protrude 1/2" inside the box's cavity. Inside the box is a CM balun connected to those element mounting screws within the box. The distance between the balun connection/screws inside the box are only about 1" apart.


Top element section: I can not pick up WQLN 50.1RF very well with larger legs on the top section & I lose VHF when I try to move the lower legs downward or try to shorten the lower legs. The squatty diamond [either 7" or 8" legs] still gives me the best real-life performance at the higher end.

Bottom element section: Various changes I've made in the angles or lengths or the lower legs seem to have a direct effect on VHF only. I agree that upper and lower section must interact, however it seems that the upper & lower sections possibly interact rather as directors/reflectors to one another.

Being that this is a very low gain antenna at best, would it be presumed that all my gain is due to my AP8780 preamp? I have never tried this antenna without a preamp.

[300ohm]

Being that this is a very low gain antenna at best, would it be presumed that all my gain is due to my AP8780 preamp? I have never tried this antenna without a preamp.

The preamp makes up for cable, splitter and weak TV tuner gain losses. So if you have a weak link in the above, it will provide "apparent" antenna gain.

[ota_canuck]

Holl_ands,

Top: 9.5-in at 45-deg angle, Bottom: 9.5-in at 30-deg down angle, TopGap=0.75-in.

I modified my 10" [#10wire] antenna to the above dimensions and it worked out to have a 1/2" gap at the top. So, I used a small diameter 2" length of rubber tubing at the tip to maintain a 3/4" gap. All was pretty good as expected, however 49.1 was weak and 50.1 was a no show. So, it appears that the gain may be great, but the gain drops off too soon to get a lock on UHF49-51. Also, I could not get VHF12.1 at all, so I did what I did with the lower elements on my post#47 sketch and VHF12.1 came in strong and UHF 49.1 & 50.1 also came in at about 50% signal strength. Pretty much all of the other channels come in at 90-100% before and after I bent the lower legs upward.

Note: This is the same result I found using the dimensions in the #47 sketch.

[ota_canuck]

Clarification to a PM I recieved about post #47.

The design shown in post#47 gave me the best overall results that included VHF 12.1 and gave me better reception at my highest UHF frequency at 50.1. Overall, the design in #47 gives the widest freq range for both UHF and VHF.

Design from post#47 gave me the best analog range from channel VHF 5 up to UHF 69.

The VHF trick seems to be in raising the lower splayed legs so that they are bending straight out from the mast with the ends flipping slightly upwards. In my specific location the top diamond seems to work fine with either the 7"legs or the 9.5" legs.

Here's a pic of what I have mounted on my 35ft tower.