[jpasadena]

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.

 

[300ohm]

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.

Yep.
Pattern at channel 10:

NEC file : http://www.wuala.com/300ohm/Documents/jpasedena_VhfLoop.nec

 

[holl_ands]

Good to see that someone is interested enough to give the Hi-VHF Loop a go....
That copper tubing looks bigger than 1/4-in O.D. Home Depot "3/8-in I.D." is actually 0.514-in O.D.
Not that it makes much difference.....

I have in mind wrapping copper tubes AROUND struts in the attic....no room for an angled screen.

When trying to run AGT, above 4nec2 file generated a bunch of Errors wrt GW61, the SOURCE Wire.
I fixed it by changing from 5 segments to just one segment (like all the other circle segments),
which required changing SOURCE Radius to 0.12 for AGT=1.0....and also segment # in EX Card.

A slight change in Loop Diameter will "center" SWR across the Hi-VHF Band, so that SWR
at Highest Freq is same as at Lowest Freq.....hence Net Gain will be flattest across the band.
As I recall, Loops are counterintuitive wrt whether they should rescale up or down....

I was going to block REPLACE the Screen Separation distance with a SYmbol to search for the
optimum distance, but "-13" occurred in way too many other columns....
I'll have to use GeomEdit to build a Screen Grid separately (I have a bunch I've already made) and
then copy/paste my original Hi-VHF Loop file (with SYmbols) into THAT and SAVEAS/RENAME.

TRICK: When building a Screen (or whatever), define an odd ball number for numbers you know
will be changed to SYmbols, e.g. "-13.003". Then it's a simple process to REPLACE it with "Sep"
....hopefully, without stepping on other columns....and if you do, change "Sep" back to the number.

PS: The C5 might be driving BOTH Loops....I'm not sure....

 

[300ohm]

That copper tubing looks bigger than 1/4-in O.D.

Yeah, it does. Good catch.

When trying to run AGT, above 4nec2 file generated a bunch of Errors wrt GW61, the SOURCE Wire.
I fixed it by changing from 5 segments to just one segment (like all the other circle segments),
which required changing SOURCE Radius to 0.12 for AGT=1.0....and also segment # in EX Card.

Yeah, but I dont know which is a worse sin, the errors or violating (Cebriks ??) the no 1 segment EX card rule in NEC2, heh. For a lot of the vhf antennas Ive modeled (uhf, never a problem), it seems the only way around the error message is to use 1 segment on the EX card. Either way, the Gain and SWR numbers look reasonable for those antennas.

 

[jpasadena]

Thanks, guys, for poking at this with the software.

That copper tubing looks bigger than 1/4-in O.D.

I'll have to put a ruler to it and verify.

A slight change in Loop Diameter will "center" SWR across the Hi-VHF Band

Probably could optimize for the particular situation. As you know, in L.A., 11 and 13 are usually weaker than 7 and 9, so one might be able to tilt things to the higher channels by tweaking the size and reflector distance.

The C5 might be driving BOTH Loops....I'm not sure....

Looking at the assembly manual and the comments of AD's jer3:
"The curved supports on the reflector are for aesthetics only." I think the element on the C5 is just the square-ish loop in front. The gain chart in that manual shows 7-7.5 dBi across the band (not sure where the 8.4 came from), which is almost exactly what 300ohm seems to get here. I am estimating for this build with SWR=3 the net gain is about 1 dB down from the raw (at channel 13 would be about 7 dBi net), and with SWR=2, about 7.5 dBi net at channel 7. So this is spot on the performance of the C5.

I am also guessing that some warping of the reflector might narrow the pattern and increase the gain. For the build (now finished) all I could do is sweep in the extra 1.5" of screen on the sides, which may not make much difference. I'll leave it to the modeling wizards to find an optimized reflector shape. In some ways, the broad pattern may be better for me, as I am seeking signal from odd paths due to 2-edge.

 

[300ohm]

I'll have to put a ruler to it and verify.

You really need to use calipers. Cheap plastic, but effective ones are a $1. Or make a crude one yourself from 2 pieces of wood and a dowel.

I am estimating for this build with SWR=3 the net gain is about 1 dB down from the raw (at channel 13 would be about 7 dBi net), and with SWR=2, about 7.5 dBi net at channel 7. So this is spot on the performance of the C5.

Close, SWR mismatch loss is .51 dbi at SWR = 2.0, and 1.25 dbi at SWR = 3.0.

. I'll leave it to the modeling wizards to find an optimized reflector shape.

You may want to keep an eye on X498 here for a possible future small mod for vhf-hi/uhf channel coverage. http://www.digitalhome.ca/forum/showthread.php?t=105660&page=34

Probably could optimize for the particular situation. As you know, in L.A., 11 and 13 are usually weaker than 7 and 9, so one might be able to tilt things to the higher channels by tweaking the size and reflector distance.

It doesnt look like you glued the fittings, so thats good. Moving reflector distance from 13 inches to either 11 or 15 inches results in slightly less net gain on channels 11 to 13. (but probably not enough to notice on the tv signal meter)

 

[liteguy]

I've used a square indoor loop suspended from a plastic coat hanger to receive real channel 9 (ABC) since the DTV change over in February. I used the classic quad formula (1005/freq in mHz)*12(inches/ft) to determine the perimeter of the loop: 63 1/2" or approximately 16" on a side.The signal strength on my converter box (APEX DT502) is usually around 65%. Quality is typically between 90 and 100%, except during a thunder storm. According to 4nec2, the raw gain is 3.24dbi with an SWR (at 300 ohms) of 2.46 which means the net gain is about 1 -- 1 db below a dipole. We have 3 digital channels: ABC on 9 (190 mHz), CBS on 3 (63 mHz) and PBS on 17 (491 mHz). The CBS outlet is also available on an analogue translator on ch 24.

 

[300ohm]

he raw gain is 3.24dbi with an SWR (at 300 ohms) of 2.46 which means the net gain is about 1 -- 1 db below a dipole.

Nah, youre still above a 2.15 dbi dipole. SWR mismatch loss at SWR = 2.5 is .88 db.

 

[holl_ands]

The above classic quad design formula is for very thin wires...which
we need to avoid, esp in Hi-VHF antennas due to degraded SWR.
Element thickness significantly affects optimum Loop size wrt SWR & Net Gain:
http://imageevent.com/holl_ands/loops

Also note I uploaded 4nec2 runs for the above Hi-VHF Loop with Screen Reflector.

 

[jpasadena]

That copper tubing looks bigger than 1/4-in O.D.

Really is 1/4" outer diameter. Item number 23050 at Lowe's. $6.81 for 10 ft.

And thanks, holl_ands, for posting the model run results.

 

[300ohm]

Fill in some details on how you got the loop so smooth and round.

 

[wildwillie]

Fill in some details on how you got the loop so smooth and round.

Ya what did use an old ringer washer surer could use your secret.

were are you guys finding them four piont connections ? i have yet to find them in ontario canada down windsor way

 

[jpasadena]

OK, here are some details on the loop itself.

Using a pencil and string as a compass, I drew a 23" diameter circle on a large piece of paper. Since the copper tubing comes in a roll, it was just a matter of enlarging the diameter of the already-curved tubing to match the drawn template. Just a lot of eyeballing and hand-working of the copper to get it to match.

A few blows of the hammer on the tips make them flat and drill-able. holl_ands suggested drilling 4 holes, 2 for mounting and 2 for the leads, but I got a little lazy with the screws and drilling and just attached the leads at the clamp screw holes, not quite at the tips. A 1/2" #4 tapping screw is used to secure to the clamp to the PVC.

Not easy to see in the pic, but at the top of the loop, I drilled a hole (and a slot because the tips were already flattened) in the PVC and threaded the loop through.

 

[300ohm]

Since the copper tubing comes in a roll, it was just a matter of enlarging the diameter of the already-curved tubing to match the drawn template. Just a lot of eyeballing and hand-working of the copper to get it to match.

Ahh, just leaving it more or less as is off the roll, and spreading it. Good tip.

 

[jpasadena]

VHF-hi in a box

Here is a portable indoor version of the same build at the top of the thread.

I really scrimped on materials for this one, using #6 copper wire for the element, a discarded computer monitor box from work as a structure, and aluminum foil taped to the inside of the box as a reflector. Dimensions are roughly the same as before: 23 inch diameter loop, reflector distance about 13 inches, reflector in the box is about 27 by 30 inches.

The performance is pretty close to the one with 1/4 copper tubing and a larger mesh reflector, though I didn't do any rigorous tests. A build like this would be suitable for an attic or closet, but of course VHF-hi is notoriously bad indoors. But if you think there is signal in your attic, this antenna would probably catch it. Also, it's sort of portable (even has a handle!) if you are making OTA house calls.

 

[roger1818]

Dimensions are roughly the same as before: 23 inch diameter loop, reflector distance about 13 inches, reflector in the box is about 27 by 30 inches.

Is that box really that big? It is hard to determine scale in a photo, but it doesn't look more than 6" deep to me.

 

[jpasadena]

Is that box really that big? It is hard to determine scale in a photo, but it doesn't look more than 6" deep to me.

Thanks for the heads-up. I was a bit off on the box depth. It is more like 10.5 inches, which is a bit less than the 13" in the original design by holl_ands.

 

[stampeder]

A beer can/soda can would be great in photos like that.

 

[Marbles_00]

In the early going, this antenna is the BOMB!!! After the transition I was having issues in receiving RF13 CFTO. On good days I was measuring 22dB if lucky, but signal would fluctuate too frequently to make this station watchable. I was using a version of the Scatter Forager in order to try and get it. So I knew I was right on the cusp, so I decided to give this antenna a try.

After building it an testing in the early going, I've been getting a very reliable an constant 24-25dB...rock solid.

Thanks for this antenna...at this point I may build a second one of these to replace my channel-cut yagi that I'm using to try and get RF9.

The only negative thing I found out, was that the Scatter Forager was inadvertently used to pick up RF11 in the opposite direction of RF13. Now with this antenna, RF11 is blocked somewhat. I'm still getting it, but it cuts out at times.

Other than that minor issue, I'm so happy with this antenna. Great job.

 

[300ohm]

The only negative thing I found out, was that the Scatter Forager was inadvertently used to pick up RF11 in the opposite direction of RF13. Now with this antenna, RF11 is blocked somewhat. I'm still getting it, but it cuts out at times.

With less reflector, you could increase the backward gain for ch 11 at the expense of forward gain of ch 13.

If you want forward and backward gain at about the same levels you have now, you could stack two loops with no reflectors together with a Winegard SD-3700 3000hm to 75 ohm coupler.