UHF Loops - simple bi-directional antennas (See Post #109)

[JOHN IN YORK,UK]

300 OHM.
Thank you for your reply.The one in question is the one made by MrVanwinkles,there is a photo of it on Post 48.
After a few calculations with my trusted calculator,the loop is more or less a full wave loop for the lowest frequency.The whiskers have me stumped,as I would have thought that they would also have a length for the lowest frequency,but no.They have a length of 10 inches,which gives a frequency of 280 mhz.The nearest that I can get the whiskers to,is that they are 3/8 wave long.

Regards,John.

[300ohm]

Thank you for your reply.The one in question is the one made by MrVanwinkles,there is a photo of it on Post 48.

OK, so that's this one :
http://imageevent.com/holl_ands/loop...720z51.goose_s
Keep in mind, simple rescaling also implies rescaling the element diameter. But with 4nec2, you can go in and change the diameter quickly and then evaluate the performance.

1).For operation on other frequencies,is it a matter of rescaling the original figures.Or is there a formula that you must adhere to.For example,using it on (88-108) mhz and (174-230) mhz.Or maybe (118-137) mhz and (230-400) mhz.

So whats your intended frequency range and what material will you be using ?

If going down to 88 MHz, its gonna be bigger and RG-6 wont cut it for outdoor use. For outdoor use down to 88mhz, I would use 3/8" (10mm) copper tubing for ease of soldering.

[JOHN IN YORK,UK]

Thank you for the reply.
As yet,I am not sure of the fx's that I will be using.It is the first time that I have seen this antenna,so I thought that I would have a play around with one.
The materials could range from 10mm*2mm aluminium strip,to 2.5mm sqr. wire.
With these materials,I do appreciate that the bandwidth will be degraded.The antenna would be for inside use,so I could use anything.
Without using the NEC plotter,I am not really bothered regarding all the different lobes of the antenna.It was just a matter of does it work better than a antenna nest.I might even try and make a 2TFD antenna for VHF,that gives a bandwidth of roughly 4:1.

Regards,John.

[Xauto]

John UK: I did a crude model of CJU ate 50 ohms looks like 440Mhs is 6.4dbi & SWR 1.56, for loop size 232 by 107.

[JOHN IN YORK,UK]

Xauto.
Thank you for your reply.I have looked at various web sites regarding the CJU,I was interested in what gain the CJU gave.The nearest that I got,was a gain equivalent to a 4 element beam.So 6.4 dbi is not bad for a 2 element beam,I would say it beats a Moxon.Did you just rescale the dimensions of the original CJU,which was built for 435.3 Mhz and 145.920 Mhz.Did you make the loop slightly smaller on purpose,as my calculations come out for 440 Mhz as 235.45*107.Are your dimensions for outer,inner or centre to centre of the conductor.
We operate a radio system over here that is similar to the Stateside FRS,but ours operate on 446 Mhz.I have been meaning to give the CJU a go,but as yet,have not got round to it.
Have you tried the loop just on its own,with a 4:1 balun.I might try it on the DAB radio frequencies,in my location,it is 223 Mhz.
Maybe experimenting with a director could give 7.5-8 dbi.

Regards,John.

[300ohm]

Are you dead set on loops ?

With the CJU loop it looks like some kind of "NAROD" effect is in action.

[Xauto]

John UK: I do not know the math for this funny yag. I think that the loop is one wave length. If you are looking for a 300 ohm match, you can move the reflector rod to 0.121875 wave length. I'm look at it for VHF-Hi (177 to 213 Mhz).

[JOHN IN YORK,UK]

Thank you for the replies.My apologies for the delay in replying,as I have been working 12 hour night shifts.
The shorted bow tie loop seems an interesting antenna,but the one that has caught my eye now,is the Stealth Hawk.This seems to be more easier to construct.But I am having problems in working out the dimensions for other frequencies.So far,I think that the diamond is a 1/2 wave in height for mid band,roughly 580 mhz.Each leg (whisker),is a 1/4 wave for mid band VHF Hi.I am using 2808/fx as a reference,which is for wire,so I could be wrong in my calculations.
Maybe if OTA Canuck reads this,he might be able to answer a few questions for me.
1).Is the diamond tuned for mid UHF band,or tuned for 470 mhz,utilising the wide band characteristics of the antenna.
2).Would it be possible to use a 6:1 balun,then use 50 ohm coax.This would allow the antenna to be used with a scanner.
3).Is the antenna only horizontally polarised,or is there a vertical component also.

Regards,John.

[holl_ands]

To rescale to a different operating frequency, simply multipy each dimension, incl. the
wire size, by the Rescale Factor, F = (old mid-freq) / (new mid-freq).

Hence an antenna designed for the 470-698 MHz UHF Band (mid-freq = 584 MHz) can
be redesigned for (say) the 2400 MHz Wi-Fi Band by making all dimension SMALLER by
F = 584/2400 = 0.243. [And conversely BIGGER for lower freqs.]

Modeling should still be done, using the actual assumed wire size, esp. to see if there
needs to be a final tweak up or down in size and to determine the rescaled Bandwidth,
which typically shrinks at higher frequencies due to higher ohmic "skin" losses.

You know how to build a 6:1 Balun? Although simple voltage transformer winding ratio
type transformers are feasible for lower frequencies (e.g. 30 MHz and below), internal
losses at higher freqs are only avoided when using transmission line type designs:
http://www.naturgrise.dk/oz9wi/artik...204_sevick.pdf
Note that two cascaded 2.5:1 Baluns will result in 6.25:1 ratio...but the additional
loss might not be worth the effort compared to the usual 4:1 balun.

Not sure which StealthHawk you're looking at. Here are nikiml's OPTIMIZED versions:
http://clients.teksavvy.com/~nickm/sh.html
And my analysis of some earlier versions by ota_canuck, XAuto and holl_ands (myself):
http://imageevent.com/holl_ands/chir...vhfstealthhawk

And then there is the Super StealthHawk....sorta like TWO StealthHawk antennas:
http://digitalhome.ca/forum/showthread.php?t=144918
Which look like a chopped, unequal sided variation of an equal-sided 4-Square Chireix,
just as the G-H is a chopped variation of the 2-Square Chireix:
http://imageevent.com/holl_ands/chireix/chireix

[JOHN IN YORK,UK]

Thank you for the informative reply.I think that the antenna went as far as a Mark 10 version.But you have given me the information that I required,the diamonds dimensions are for mid FX.
Will the bandwidth be altered pro rata as you lower the design FX.For example,would it give a bandwidth of (116-174)mhz if the design FX was 145 mhz.
Another thing that I have notice,is that the legs (whiskers) are roughly 3/4 wavelength of the mid UHF FX.This will give a slight gain,but what would be the results if these legs were 1/4 wavelength for mid UHF.I know this would mean sacrificing VHF Hi.
You comments regarding the balun are noted,I think that I will stick with the 4:1.
Have we got a sort of bidirectional biconical antenna here.

Regards,John.

[holl_ands]

It is difficult to design an antenna that will cover EITHER Hi-VHF or UHF Bands with
good Gain and SWR.....and much more difficult to cover BOTH BANDS. I'm not going
to speculate on any design variations...feel free to download the FREE 4nec2 program...

PS: Many BROADBAND antennas (esp. Bowties) do NOT follow quarter-wave & half-wave
"rules" well known to narrow-band ham radio antenna designers....you have to MODEL it.
For example the G-H antenna dimensions are much smaller than the Chireix from which
it was derived...after opening up the top and bottom elements. And sometimes it makes
sense to treat what you think is a 75-ohm antenna with simple stick elements (e.g. VU-90)
as a 300-ohm antenna instead, degrading SWR some, but providing a much wider bandwidth.

Bandwidth percentage is roughly the same after rescaling, except for some shrinkage
at higher frequencies as I mentioned above. You can apply the Rescale Factor "F" to
mid-band frequency as well as upper and lower freqs to be in the ballpark of the
rescaled frequency response.

[JOHN IN YORK,UK]

Thank you for you informative reply.Sorry about the delay in replying,12 hour night shifts again.
I will certainly have to read up about Bow Tie antennas,I was always under the impression that the diagonal side was for FX A and the horizontal was for FX B.The end result being an antenna covering from FX A to FX B.Of course there were variations such as the biconical.
I am now going to set you 2 posers.

1).In the stealth hawk antenna,would it be possible to extend VHF coverage if the whiskers were converted into a OCFD.

2).What would the results be if you were to rescale 2 bow tie elements and use them for OCFD elements.Would this increase the bandwidth,or would the elements become useless lumps of wire.

John.

[holl_ands]

OCFD??? Do you mean Off-Center Fed Dipole:
http://n9wn.com/data%20files/OCF%20Dipole.pdf

If so, you're on your own...and as always, feel free to model it yourself using the FREE 4nec2 program.....

============================================================ =======
I analyzed effect of varying the SWEEP of the Bowties (away from the Reflector) as well as investigating
the effect of varying the TINE SEPARATION for the Bowties (slides 7-8) in an 8-Bay Bowtie antenna:
http://imageevent.com/holl_ands/mult...yrefl/cm4228hd
Since a narrower Tine Separation of about 2-in improved the overall bandwidth, your above hypothesis
wrt FX A and FX B is clearly invalid.

============================================================ =======
When modeling a simple UHF Stick Dipole, a 75-ohms load SWR approached 1.0 near 570 MHz, but SWR was
excessive SWR at both low and high frequencies....even when using fat QICT (Quarter-Inch Copper Tubing):
http://imageevent.com/holl_ands/dipoles/uhfstickdipole

A 20-percent wider Dipole provided a much better broadband SWR match to 300-ohms:
Each Whisker=5.27-in with Gap=0.75in for Total WIDTH of 11.29-in, which is a HALF-Wavelength at 523 MHz
(just below middle of the band).

Simple UHF Bowties are inherently broadband, permitting the use of much thinner AWG10 wire sizes.
In the 4-Whisker example, optimum performance across the (old) UHF Band was provided when
Total WIDTH is 21.5-in, which is a ONE-Wavelength at 549 MHz (middle of the band). Contrast this
to a Total WIDTH of 11.29-in for the Stick Dipole, which is a HALF-Wavelength at 523 MHz.
http://imageevent.com/holl_ands/dipoles/uhfbowties

[mrvanwinkles]

23" bowtie in loop - roof mount, tv fool, reception results - OTTAWA

OK, so I finally mounted this antenna in a good spot for tests, and hooked it up to a receiver with a signal meter ... so here are the conditions and the results.

... Finally, some real % RESULTS for this antenna to share





TV Fool,

Near by, approximate location taken was the gas station at the nearby major corner. Petro Canada gas station at the corner of Baseline Rd & Greenbank Rd in west end Ottawa (Nepean). Address: Petro Canada Gas Station: 2577 Baseline Rd (at Greenbank Rd), Ottawa, Ontario, CANADA postal code: K2H 7B3

TV Fool :

http://www.tvfool.com/?option=com_wr...2df91b53613eb5

Keep in mind, the information / stations / channels / rf channel given - in this TV Fool seems to be a little bit different than some of the info in the reception results I will give below.

Things regarding TV broadcasting here in Ottawa Canada are still in a state of change - and TV Fool may not be totally up to date with the current conditions. But it shows the general reception conditions for most channels where this antenna is set up.

Reception Conditions and Results:

Antenna: 23" Bowtie in Loop of 3/8 soft copper tube - as shown in picture. (Design and computer modelling of the antenna is Holl_Ands / Wildwillie. Build and mount of the antenna is my improvisation )

Downlead: Approx 60-70 feet of RG-6 coax to reach the receiver. No pre-amp. Good quality compression connectors.

Location: West end Ottawa (Nepean), near Baseline & Greenbank Rd

Height of Antenna: Approx 40 ft AGL

Aiming of Antenna: Antenna is bidirectional but aimed straight at the HC Herberts Corner / Manotick Ont. transmitter by eye, using land reference points from a TV Fool "lines to transmitter" image. So it is at a bit of an angle to the CF Camp Fortune Transmitter, since the CF transmitter is closer and signals are generally more powerful. Also, previous tests and aiming show this is the best aiming for our location (best compromise). So per the TV Fool, aimed approximately at the 128 degrees true from true north. Aiming is pretty accurate I figure - even if "by eye". Anyway, this antenna has a broad pattern - but aiming does make a difference - shown by previous practical tests - esp. trying to receive the weaker/further HC transmitter channels.

Conditions: Clear and Calm, Night, cool, maybe 10 deg C.

Receiver: Convertor Box. Electronic Master DTV 5000 HD. Digital ATSC HD/SD TV Tuner

Date Time of Reception Results: Wed Oct 10, 2012 approx 1 AM.

Distance to two main Ottawa Transmitters: from TV Fool:
11.6 miles to CF Camp Fortune Transmitter, north up in Quebec, 345 deg true
13.9 miles to HC Herberts Cornert transmitter, south easterly in Manotick Ont at 128 degrees true.

Reception Results (from the "signal" screen of the Convertor Box):

Virt rf? callsign netwk %signal
4-1 25 CBOT-DT CBC 57%
6-1 6 CIII-HD Global 49%
9-1 uhf? CBOFT-D CBCfr 57%
9-2 9? CBOFT-D CBCfr 38%
11-1 ? CHCHDT1 ind? 49%
13-1 13 CJOH CTV 58%
14-1 ? OMNI2 OMNI 41%
24-1 24 TVO TVOntario 57%
30-1 ? CIVO-DT TeleQuebecfr 49%
34-3 34? CH1-HD CFGS-DTfr? 44%
40-3 ? CH1-HD TVAfr 53%
41-2 ? CIII-SD Global 49%
42-1 ? CTS-HD religious 29%
43-1 ? CHRO "CTV TWO"? 42%
60-1 ? OMNI-1 OMNI 35%
65-1 ? Citytv CityTV 43%

Signal Quality is showing 100% on all channels. So far everything is looking stable with no pixellation so far - but time viewing with this antenna is still minimal. More time viewing or recording will give longer term results.

Looks pretty good so far though ...

... sorry, it's hard for me to actually confirm the real rf frequency of the signal received. This Convertor box does not give it, and TV fool seems off, and things regarding TV broadcasting here in Ottawa are still in a state of change, migrations to UHF and broadcaster work still going on here and there, antenna and tower and power changes etc. etc. . Anyway, most of high channels are actually UHF, and probably match the virtual channel ... since most of Ottawa is UHF now. The exceptions might be Global 6 still on VHF lo 6 (converting soon to UHF only, I think)... and maybe CBC french 9 - one of the CBOFT channels on VHF hi 9 , and probably for sure CJOH CTV is definitely on VHF hi 13.

P.S. - Oh ya - forgot to mention - the mount, the two parallel rods are fiberglass fence spreader rods, and the 3/8" clips to secure the copper tube are black UV resistant plastic. So NOT metal there ... insulative material, not conductive, and should therefore NOT affect the antenna performance much.

The Mount (important):

I wanted to mount this antenna in a certain way, as shown. It was a bit tricky to figure out how to do this well. There are actually FOUR (4) fibreglass fence spreader rods / fibreglass fence tension rods used. They are doubled up / twinned for more strength and less flex.

I wanted to design a mount to get it above and clear of the mast (metal), and I wanted to keep metal clear of the center connection / feed gap / bowtie center area. Also I wanted to be able to have the mount help strengthen the center tubes, so they would not bend or break apart at the solder joints, by combining / joining / connecting them together with something stiff and non-conductive - i.e. by using the mount rods themselves for this purpose as well.

Also, this will help prevent the thing, hopefully from twisting back and forth / oscillating in the wind.
There will be some bending and oscillating in heavy winds anyway - but better to bend or twist a little, instead of breaking.

The 3/8" black UV resistant plastic clips are important. They grab the copper tube with just the right amount of strech and grip. They were found in the wiring dept of a Home Depot I think, as 3/8" plastic cable clamps.

Those fibreglass rectangular rods are great - but a little difficult to drill (change or sharpen drill every 4 or five holes, and feed drill slowly and carefully - I used drill press, more accurate, more stable, and alot more control), and even more difficult to mount to with screws. I used what I had - #6 x 3/4" stainless steel screws - but the hole has to be drilled just a hair smaller than the screw threads ( 1/8" drill this case ) or there is real trouble putting the screws in. By the time you get thru to the second rod, you need too much force to drive in the screw (tough material those rods, and do not give much), and end up breaking off the head with the twisting force of the screwdriver. Then getting it out to try again is difficult.

Also, the sharp end of the screw tends to stick through, and you have to cut it off and grind it flat.

If I did this again, I would probably recommend using #6 x 3/4" machine screws and nuts to secure on the other side of the double rod, and drill it so the machine screw goes through easily with just a little bit of clearance - easy to assemble (or disassemble later). Semi tight, not too tight, and blue thread lock so they do not unthread easily.

I'd use a tough glue as well, to glue together the double rectangular fibreglass rods. And maybe even paint them with a grey paint (same colour) for long term UV protection.

I wanted / had to make and drill and bend the ends of a couple special AL flat bar brackets for the U-bolts (U bolts are SS stainless steel, 5/16" x 1-3/4" ... to grab the 1-1/2" mast tube). The U-bolts go through holes in those wider flat brackets and clamp the two doubled fibreglass rods between them, at the same time as they clamp the mast. I did not want to drill the fibreglass rods, for mounting, in the area where the top clamp is (or even bottom clamp). That is exactly where the FULCRUM is, where most force will be, for wind loads. So better to do it this way, and clamp the rods, rather than drill to mount somehow in this area.

It's a bit complex for a mount, but this is what I came up with for this particular antenna.

Everytime you do something ... you learn something ...

[mrvanwinkles]

The 3/8" black UV resistant plastic clips

I have the little package in front of me now.
Again - found them in the wiring dept of a Home Depot h/w store I believe.

Details off package:

GB - Gardner Bender
PPC-1538UVB
Cable Clamp
3/8" 9.5mm

[ UV ]

( Qty, 15 in a little red, black & clear plastic pack )


on back of pack:

- for outdoor use, resists cracking

GB Gardner Bender
Milwaukee, WI 53209
www.gardnerbender.com
ZK862
Made in China

bar code: 0 32076 89599 0