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Loop with reflector for VHF-hi

52K views 52 replies 15 participants last post by  ExDilbert 
#1 ·
This is not really an original design, but just a build inspired by research and design by holl_ands, and a little borrowing from the Clearstream C5.





The 23" diameter loop is made of 1/4" copper tubing. The gap near the feed point is 3/4", while the attachment points are 1.5" apart, as this was the spacing on the plastic conduit clamp. The reflector is 2x1 fencing, 34" by 29", set back 13" from the loop. The bar of PVC up the middle is just for stability.

I tried a loop reflector, 5% bigger than the driven element, but the results were better with a continuous mesh, so I just went for it, resulting in something like the C5, albeit bigger. These loops are good performers in a smallish package for VHF-hi. For indoor builds, a cardboard box or styrofoam blocks of the right size and an aluminum foil reflector would probably do fine. For a lazy outdoor build, a milk crate might make a good frame. For this build, the cost of materials, including the balun was about $20. That PVC conduit at $0.99 for 10 feet has to be the best bargain in the hardware store! The fence was ~$2 per foot at 36" width. 10 feet of copper tubing is ~$7.

holl_ands' modeling predicts 6 to 7.5 dBi for a loop with loop reflector. The mesh seemed to do better, though I don't have a way to measure it. The C5 claims a peak gain of 8.4 dBi. In any case, I am finally able to pull in all 4 VHF-hi stations in my tricky location here in L.A. for the first time since June 12.
 
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#33 ·
Thinking more about it, since circular polarized broadcasts have a 1/4 wavelength (90 degrees) phase difference between the horizontal and vertical components, you will only get 1.5dB gain over an equivalent horizontally polarized antenna. To achieve the full 3dB of gain, you would need an make it into a spiral instead of a loop, but then the reflector would need to be a spiral (not sure how to make that). You would also need to know it the broadcast has a clockwise or counterclockwise twist.

This is getting off topic, but we may be better off building an antenna designed for circular polarized broadcasts, like the modified cubical quad design shown here (assuming Global doesn't change channels).
 
#34 ·
A top or bottom-fed Loop has MINIMAL response to Vertical Polarized signals....and
a side-fed Loop has MINIMAL response to Horizontal Polarized signals.

To see H, V & Total Gain responses, in 4nec2's PATTERN box, under "Far-field", enable
"Show Multi-pattern"...or just hit the "M" key.

===========================================================
RabbitEars tracks status of Circular ("C") and Elliptical ("E") Polarized Station (U.S. Only):
http://www.rabbitears.info/oddsandends.php?request=polarization&type=C
http://www.rabbitears.info/oddsandends.php?request=polarization&type=E

===========================================================
K6STI's analysis Circularly Polarized Quadix, Cubical Quad, Loop & Rabbit-Ears FM Antennas:
http://www.ham-radio.com/k6sti/
Can Rescale to Lo-VHF, Hi-VHF or UHF Bands....

===========================================================
Two alternative designs for Hi-VHF 5.5-Turn (103-in) Helical Antennas with FLAT (NOT Spiral)
Reflectors can be found here, if you are interested in DIY Circular Polarized antennas:
http://imageevent.com/holl_ands/circularpolarized
I included some design references if you want to do your own design (good luck....)

If you're interested in longer, higher Gain, narrower bandwidth alternatives,
try adding more turns to the above....
Or try to copy one of the fol. Wade Helical Antenna designs:
http://www.wade-antenna.com/Wade/WH14-69.pdf
 
#36 ·
That is what I was hoping. Now the scary part. How large would it need to be to receive channel 6.
Not sure where your channel 6 is in relation to you, but the RF6 that is close to me is 27 miles out...almost 10 miles closer than RF13 that I was gunning for, yet I can't get a whisper from it. Two things: 1) the antenna would most likely need to be a little larger, and 2) I think that RF6 CIII is transmitting at a pretty low power...much like all the VHF stations.

According to TVFool, I shouldn't really have much of an issue in getting 6, and it is almost in line with RF13. Oh well, doesn't really matter since I'm getting CIII from Toronto just fine on RF41.
 
#37 ·
I am not surprised you aren't able to receive RF6 with your loop as it has a centre frequency of 95 MHz compared to the centre frequency of 213 MHZ for RF13. I am assuming to tune it for RF6, it would need a diameter of about 6 feet (about 3X the diameter of your 23" loop). That would make the circumference close to 19 feet (an awful lot of copper tubing). Probably not feasible.
 
#38 ·
You could use AWG10 if you only need enough bandwidth for Ch6, which would be easier
to construct if it were a Square Loop:
http://imageevent.com/holl_ands/loops/vhfloop
Rescale Hi-VHF (16.75-in if AWG24) size LARGER by Factor=198/85=2.33 or 39-in per side.

If you were to use a Reflector Screen, the Separation distance would be 2.33 x 13in = 30.3-in,
with a Screen size of at least 2.33 x 28in = 65-in x 65-in. AWG10 or AWG12 wire could be
strung horizontally (only) between outer box supports at say 8-in separation between each rung.
No need to string vertical wires, other than you could conveniently move to the next rung at each end....
loops don't have any meaningful vertical polarization response.
http://imageevent.com/holl_ands/loops/hivhfsquarelooprefl
 
#39 ·
Hi all,

I also build this antenna. I build this for hi-VHF. But it is also catching UHF channels from Toronto and Buffalo as well. I installed it in my attic. No reflector yet. It is catching Barrie CKVR and Hamilton CHCH from opposite directions. I am getting 70% signal on both. But when I try to amplifi with Kiztech it reduces to 60%. I don't know why ?

Meridish
 
#40 ·
It is catching Barrie CKVR and Hamilton CHCH from opposite directions.
Yep, without a reflector, its bi-directional.

I am getting 70% signal on both. But when I try to amplifi with Kiztech it reduces to 60%. I don't know why ?
Possible overloading, or youre amplifying a multipath signal which is common in an attic.
 
#43 ·
So I built one of these and tried it out to see if it would out perform my stacked 2 * rca751's unfortunately it did not. Will try different placement etc and keep you posted.
 
#44 ·
I wouldn't have believed it

Thanks to all of you for your great work and pointers. I'm getting ready to dump my local cable provider, going back to strictly OTA. And of course I live in a basement apartment in a valley, 35 to 40 miles from the "local" towers. The apartment complex rules say I can't put anything outside...so it has to be something indoors. Nothing I bought retail would work. Worked great for the UHF signals, but couldn't pull in the two local VHF-high channels. After reading though this thread, I went down to the local Menards, picked up some PVC and 1/4 inch copper tubing and built a quick and dirty free-standing version of this antenna. No reflector, just the loop. Hooked it up to my existing 24db amp, and the results were nothing short of amazing. Where I had no VHF signal before, I was pulling in the farthest station at 38% signal and 85% quality. Very usable. The nearer VHF station was 68% signal and 100% quality. Even pulled several of the UHF stations with this. So a small UHF loop to pull in the missing stations, and I'm there! All this in my basement apartment, thick concrete walls and all. I'm well and truly impressed.
 
#46 ·
Once again thanks for the pointers

300ohm, wanted to thank you and leave an update. After looking around town for the UVSJ's, I opted to order them in from Solid Signal. Radio Shack was the only place I could find them in town, and they wanted *way* too much money for them. For their price I was able to get a few to experiment with and have money to spare even after shipping. Anyway, I assembled everything tonight, and just for grins left out the amp. Absolutely fantastic. VHF signal went up about 20% and quality hit 100% on both channels. As for UHF, even with the simple loop, by aiming it a bit off axis I was able to pick up the towers that are separated by quite a few degrees. I think if I take your advice and put in something with more gain than the loop, everything will be rock solid.

I appreciate the help and all the work you all are doing.
 
#48 ·
Modifications needed to optimize for VHF Ch9?

I'd like to build one of these loop antennas for my parents to pick up their single VHF station (channel 9, 189 MHz center frequency). The antenna will be used in an indoors location (void space in a fireplace chimney) above the roof line, stone and vertical rebar structure. The space is ~10' wide, 20' high and 3' deep, so I can't go with a horizontal YAGI. This design seems like the perfect choice for this space, and at a much better price than the Antennas Direct C5.

I installed a Solid Signal HDB2X over the holiday to pick up the four UHF antennas of interest, planning to combine that with a VHF/UHF YAGI to tune in the VHF channel, but was surprised when CH9 was picked up by that antenna as well. The signal on CH9 is marginal, particularly during bad weather, so I'm considering adding a VHF antenna and USVJ to improve reception.

There are no other VHF transmitters within 85 miles so I'm not expecting interference, but I'd like optimize the design to get the highest gain on the channel of interest. Are there any modifications I can make to this design (loop diameter, reflector size, gap between loop and reflector) to get the best reception on CH9?

Thanks,
-phil
 
#49 ·
Frequency Response of Hi-VHF Loop + Screen Grid Reflector is VERY FLAT, so no NEED to re-Optimize....just follow one of the fol. designs:
Hi-VHF Circular Loop + Reflector

Details re Hi-VHF Loop construction....note that Loop Diameter is different whether using 1/4-in O.D. [i.e. Refrig Ice Maker] or 3/8-in O.D. [which is actually 1/2-in O.D.]:
VHF Square & Circular Loops

When you say "void space in a fireplace chimney" I'm hoping it's not actually IN the Brickwork...which severely reduces signal strength and the corrosive flue would rapidly degrade the metal Antenna. And if it's BESIDE the Brickwork, then bear in mind that it will severely reduce signal strength from that direction. Perhaps an Attic location would be better????
 
#50 ·
holl_ands,

Thank you for the feedback. The antenna is in the brick work (in this case it's natural stone, though I suspect that's not much different than brick). The metal flue is a 12" diameter pipe about 8' away from the current HDB2X location, and the stone facade surrounding the fire box and flue is close to 12' x 4'. The flue is at one end, the antenna at the other. With the HDB2X antenna, CH9 is picked up so I'm confident that an antenna better tuned for HI-VHF should as well. The house doesn't have an attic, so that is not an option, unfortunately. If this doesn't work, then I'm just out ~$50 worth of supplies and my time, I'm willing to take a shot to improve the reception and to experiment with building my own antenna.

Do you have a feeling for how the gain would change if I used a 1/2" grid for the reflector rather than the 1"x2" grid in your simulations? What if I changed the reflector size to 28"x28"?

-phil
 
#51 ·
holl_ands,

Do you have a feeling for how the gain would change if I used a 1/2" grid for the reflector rather than the 1"x2" grid in your simulations? What if I changed the reflector size to 28"x28"?

-phil
I think I've answered my own questions. I downloaded 4nec2 and modified holl_ands' HIVHF loop design, replacing the reflector with a 28"x28" square reflector of 1/2", 19-guage wire mesh, which can be found at Home Depot. I then ran a Frequency sweep of the HIVHF band. After 4+ hours of simulation it returned a slightly better gain (peaked at 8 dBi between 189 and 199 MHz) and similar SWR results than the original design, so I think going to a finer grid will be slightly helpful.

I've also found reference on the web to a minimum reflector size for HIVHF of 34", which is the half wavelength of the lowest frequency of interest (174 MHz), so that makes sense.

I think I'll use a 34" wide reflector of the 1/2" grid. Now I'm going to try to simulate shorter reflector sizes to see if a 34"x24" design will work well as I can easily find 24" wire mesh.

-phil
 
#52 · (Edited)
Although it's a DIFFERENT Antenna, I did a Parametric Study of mclapp's M4 on UHF and Hi-VHF Performance, analyzing the Effect of Different Bowtie-to-Reflector Separations as well as Screen Grid Reflector WIDTHS....it might be of interest in how you conduct similar Parametric Study with Loop+Screen Grid Reflector Antenna:
M4 (9.5x9) + Various FLAT Refl. Widths

I did a similar Parametric Study of mclapp's M4 on the Effect of Different Reflector Rod WIDTHS [= 2*Yrod] on UHF and Hi-VHF Performance.....which yielded DIFFERENT Results:
M4 (9.5x9) + Various Number of RRs - OPT

I also OPTIMIZED a Hi-VHF 2-Bay with two different Number (and hence Spacings) of Reflector Rods...they were HUGE:
http://imageevent.com/holl_ands/multibay/hivhf2bayreflrods

===========================================
I've tried Screen Grid Reflectors with same Width and different HEIGHTS....Gain increases as Height is increased...however, even higher Gain is possible if you BEND the outer edges Forward to narrow the Elevation Gain Pattern [emulating a Parabolic Reflector]. Or, as done in the M4/FF4's Double Angled Reflector, Gain improves by narrowing the Azimuthal Gain PatternJ:
http://imageevent.com/holl_ands/multibay/4bayrefl/uhffreeform4baydblanglreflopt

And I compared different Horizontal Spacings for different Screen Grid Reflectors [with readily available Grid Sizes], using CM4228 UHF Antenna....so if Rescaled to Hi-VHF Frequency Band, the equivalent Horizontal Spacings would range from 1.5 to 12-inches. Note that there was no benefit in going beyond 1-in Horizontal Spacings [equivalent to 3-in when Rescaled for HI-VHF Band].....improving F/B and F/R Ratios more than minimal Gain improvement. Of course, you may find different Results with simple Loop + Screen Grid Reflector:



 
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