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Discussion Starter #1
I plan to build one of the K6STI yagi's that Holl_ands describes here: http://imageevent.com/holl_ands/yagis/k6sti I would prefer to build it with straight dipole rather than with a folded dipole. The folded dipole has the advantage where one can simply use a conventional 300 ohm to 75 phm matching balun whereas the conventional dipole requires a 1:1 balun. I can't find any commercial sources for a 1:1 balun and I don't know how to build one. What is the penalty for using a 4:1 balun with a dipole feed? I'm looking to acquire four high vhf stations (7, 9, 10, 13) with this antenna. Thanks!
 

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K6STI's 5-Element Hi-VHF Yagi used a "stick dipole" and was swept back (vice "straight").
Hopefully you weren't considering a "mod" without the swept back driver....

The "Stick Dipole" version typically has SWR of about 1.6, rising to 1.9 on Ch13.

If you use a 4:1 Balun instead of 1:1, the SWR jumps to Excessive 4.3 on Ch7, up to 3.7 on Ch12 and
no less than about 2.7 on the other channels....so it isn't a good idea.....it would be better to simply
connect the 75-ohm coax directly to the antenna terminals (perhaps adding some Ferrite Cores), and
learn to live with the likelihood that the antenna pattern will be somewhat skewed, the excellent F/B Ratio
somewhat reduced and will be more susceptible to common mode (man-made) noise pickup.

Coax 1:1 Balun Instructions:
http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf_feed.html
http://www.iw5edi.com/ham-radio/files/I0QM_BALUN.PDF
http://www.ham-radio.com/k6sti/balun.htm
Note that some might be 75-ohms, whereas Amateur Radio uses 50-ohms.

They can probably be purchased as a special order (call first, these are for UHF):
http://www.atechfabrication.com/products/coax_baluns.htm
http://www.atechfabrication.com/tests/01-19-02_jbx21cd_tests.htm

Perhaps you want to reconsider using the Folded Dipole variant I optimized explicitly
so the readily available 4:1 (300:75-ohm) Balun Transformers could be used.
 

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Discussion Starter #4
Holl_ands, thanks for your reply. Apologies for not being completely clear, yes, I am aware that the K6STI uses a swept back dipole and was planning to follow your drawings. After further thinking, I've come up with a solution for constructing a swept, folded dipole with my available tools. My initial thoughts yesterday were too convoluted/complicated to do but I think my current thought will work and I'm looking forward to giving the folded, swept dipole version a go! Thanks for the links on DIY 1:1 baluns and possible commercial sources. Wow, commercial ones are expensive... Hey good deal though, I can convince the Mrs. that I need a couple new tools and I will then be able to do my project without buying commercially and saving $'s... A win-win! ;) Top notch info as always Holl_ands!

old sparks, thanks for the link. I'll check it out later this evening!

Best,

Rick
 

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Discussion Starter #6
Holl_ands,

I've decided to build two K6STI antennas. The first will use 10 guage copper wire for the elements and folded dipole. This antenna will be mounted in my attic. The second is for my son and would likely be an outdoor mount with 1/4 inch elements + folded dipole. Your K6STI model utilized 3/8 inch material. How do I rescale the folded diplole plans utilizing my smaller wire or tubing? Thanks!

Wildwillie, interesting little trick with two baluns! I'll play with that approach as well. I like quick and easy methods! :) Thanks!

Rick
 

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Discussion Starter #8
Good points 300ohm. I wasn't really thinking critically about potential losses or the actual construction process...
 

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If the feedpoint impedance is close to 75 ohms, you could dispense with a balun and use a split dipole with little effect on performance...

http://www.rfham.com/g8mbi/g8mbi/432.htm
I couldn't find info re "split dipole" in that webpage or photos....

Perhaps you meant the Asymmetric Dipole:
http://www.wa5vjb.com/references/CheapYagi4HDTV.pdf
Not sure how well that would work with the Swept Dipole.....

Copying the general shape of the above UHF Asymmetric Dipole to Hi-VHF Band,
I tried several variations of Element Separation and SOURCE Gap size looking for
a good compromise....I never found one for 75-ohm impedance....cuz it's clearly
a 300-ohm antenna!!!! See updates & 4nec2 files in the 8-Element K7MEM Yagi folder:
http://imageevent.com/holl_ands/yagis/hivhf
 

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Holl_ands,

I've decided to build two K6STI antennas. The first will use 10 guage copper wire for the elements and folded dipole. This antenna will be mounted in my attic. The second is for my son and would likely be an outdoor mount with 1/4 inch elements + folded dipole. Your K6STI model utilized 3/8 inch material. How do I rescale the folded diplole plans utilizing my smaller wire or tubing? Thanks!

Wildwillie, interesting little trick with two baluns! I'll play with that approach as well. I like quick and easy methods! :) Thanks!

Rick
Going from 3/8 to 1/2-in Copper Tubing wasn't much of a difference wrt Gain & SWR, although F/B & F/R Ratios
degraded a couple dB at 174 MHz due to the falling curve. I could make this go away by shifting the entire
frequency response curve downward by a 0.5-percent increase in ALL Yagi dimensions. SWR was a bit better
in the second decimal using 1.5-in rather than 1.0-in center-to-center separation beween Folded Dipole Elements.

So "optimum" for 1/4-in Copper is found by multiplying all dimensions by Factor = 1.005. If you can measure that close....

I was surprised how good the AWG10 version performed using Factor =1.01 and element separation = 1.0-in.
Gain, F/B & F/R Ratio degradation was less than 0.2 dB (negligible).
SWR was also very good: under 1.5, then climbing to 1.9 on Ch13.
 

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Discussion Starter #11
Awesome, thanks for modeling the two construction materials holl_ands! :) Should be a fun project! Greatly appreciate your assistance!

Best,

Rick
 

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Discussion Starter #12
holl_ands,

I do have a follow up question... According to K7MEM's site, location of the folded dipole relative to the elements has little impact on antenna performance. Is that statement true for the K6STI antenna as well? Your drawing shows the elements and the bottom portion of the folded dipole to be parallel (similar to the top mounting option here: http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf_feed.html). With my planned method of construction, the elements would be be approximately centered between the top and bottom of the folded dipole (bottom mounting option here: http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf_feed.html)

Thanks!
 

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According to K7MEM's site, location of the folded dipole relative to the elements has little impact on antenna performance.
Your question has me slightly confused.
Make that - relative to the "parasitic" elements. :) And where specifically does K7MEM say that ?
On the Z axis maybe here ?
The Spacing between the horizontal sections can vary rather widely. This nicely accomodates do-it-yourselfers, but also give you some latitude for varying boom diameters and mounting methods.
 

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Discussion Starter #14
Quoting K7MEM, vhf feed page,

Here again, you have several choices. Some of this depends on your choices for the antenna boom. Initially you have to decide on the orientation of the folded dipole feed in relation to the parasitic elements. The picture on the right shows three different orientations. On top, the folded dipole feed point is parallel to the parasitic elements. In the middle, the unbroken section of the folded dipole is parallel to the parasitic elements. An then on the bottom, the parasitic elements are parallel to the aperature between the feed point section and the unbroken section.

Neither of the choices has any particular advantage over the other, so you can choose the mounting method that best fits your application. The boom running through the folded dipole doesn't seem to have any significient effect of the performance of the overall antenna.

I understand the above paragraph to mean that there is little difference between folded dipole positions relative to the parasitic elements (ie parallel to top, bottom or middle of folded dipole). See K7MEM's picture on that page if my statement is not clear... In fact, I would prefer to have the boom run through the folded dipole...
 

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(ie parallel to top, bottom or middle of folded dipole).
Yep, hes just referring to the Z direction immediately above, below or in the middle of the boom for a folded dipole. In other words, if the directors or reflectors stay within the top and bottom height range of the folded dipole, the gain effect will be minimal. I think that probably would also apply to the K6STI antenna as well.
 

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Hi-VHF 5-Element Folded Dipole Yagi's (based on K6STI's) I analyzed so far had the bottom of the
Folded Dipole at same height (Z-Axis) as the passive elements. I just ran it again with the TOP of the
Folded Dipole at same height as passive elements....no difference.

If there was any difference, it would be very small differences in elevation gain...but since this
antenna has a 40-deg elevation beamwidth at 216 MHz, we shouldn't care. And to ANALYZE to see
if 4nec2 can display the difference would mean running at smaller than 1-deg elevation increments,
which I am disinclined to take the time to do and leave as an exercise for the 4nec2 student.....
 

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Discussion Starter #18
Holl_ands, thank you for modeling and comparing the effects of dipole location on antenna gain! Very helpful confirmation that I can build this K6STI antenna the way that works best for me without reducing antenna performance. What is the preferred method to describe this antenna? Do I simply refer to it as a K6-STI yagi? It seems to me that I should recognize your modeling efforts in adding the folded dipole which IMO, makes it an easier antenna to build.
 

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Since I had modified his design, K6STI requested that I NOT label it as a "K6STI Yagi"....
But since I did not do the original Yagi-Optimizer work, I still give credit to the originator....

Maybe "holl_ands modified K6STI Yagi" or simply "5-Element Swept Folded Dipole Yagi".
 

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Discussion Starter #20
Just received an email reply from Atechfabrication. They no longer sell 1:1 baluns. Which is fine, was just curious as to what they would charge...
 
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