Bow Tie TV Antenna Designs (mclapp's M4 featured)

[300ohm]

The naked copper wire, preferably 14 gauge or thicker. The phone line wire is four (or 6) strands of 20 gauge or less.

[ScaryBob]

I would recommend #10 AWG for an M4 if it is going outside. For an attic install, #14 will probably be Ok. Two conductor house wiring with the insulation taken off is probably the cheapest solution.

I agree with the previous advice regarding 8 bay vs 4 bay. An 8 bay M4 will be very bulky and not provide a lot of benefit for the extra cost and complexity. One trick used by cable companies was to mount 2 antennas at different heights to handle tropo, multipath or terrain problems. For example, mount one M4 at 40' and another at 30'. Optimizing the height for problem channels is even better. The two RG6 feeds can then go into a combiner, switch or separate tuners to provide the best signal.

[mirtec001]

I was looking at mclapp's m4 design and was impressed, but I thought it could use some minor tweaking. Here's a concept drawing. The issue that I'm working through is what kind of material to make the arms.

I was going to use:
1/2" Sched 40 for the outter frame: 34" X 40"
1" Sched 40 for the central frame
1" Tees to use as stand-offs
8 AWG wire for the element
?? for the arms (replaced the wiskers)
#12 pan heads for the screws to keep it all together
UV resistant zip ties to hold the 1" X 2" screen in.

Thoughts?

Thanks,

-Pete


http://www.digitalhome.ca/forum/album.php?albumid=673&pictureid=3439


http://www.digitalhome.ca/forum/album.php?albumid=673&pictureid=3440

[Dave Loudin]

The v-shape of the whiskers is important. Switching to a solid body would add weight and wind resistance without adding any performance benefit.

[300ohm]

Those are like the PR4400 whiskers. They work alright, but no real advantages and less bandwidth. Mclapp built a similar model.

[stampeder]

mirtec001, keep in mind that while solid cones are theoretically the ideal form for a bowtie antenna element, it was found long ago that the effect could be replicated with two wires, each representing the upper and lower edge of a similarly sized cone. Testing has proven beyond a doubt that the performance difference between the "real" and "imaginary" cones is negligible. In your case using rectangular strips of metal with parallel edges that do not follow the pattern of a cone does not replicate the theoretical cone, so limitations result.

See "How does a bowtie reflector antenna work?" in Post #5 of the OTA FAQ. Please keep challenging designs though!

[mclapp]

mirtec001,
As was mentioned I have built antennas almost exactly like that, there may be some pictures of them earlier in this thread. I used .060 aluminum and made them about 1" wide with a bead roll down the middle for stiffness. They work well and I found they seem to have a little better VHF-hi bandwidth but I did see a slight loss in bandwidth on UHF compared to the whisker style .
The measured gains were similar between the 2 as well with the whisker style maybe having a very slight advantage on UHF on certain channels.

It appears that the solid bar elements need to be shorter in relation to the bay spacing unlike the whiskers which show max gain over a wider bandwidth when they are slightly longer than the bay spacing.

I've never made up any drawings or dimensions for them since most people find the whiskers easier to make and the materials easier to come by but some may find the solid element (winegard style) elements to be just the thing if they happen to have the materials.

[mirtec001]

Thanks for all of the great responses. I can see how the wind resistance would play a big factor and I didn't think about the cone design. I'll do both styles and post my real world results.

Thanks again.

-Pete

[logray]

Finnaly accumulating some of the parts I need for this build but still have a few questions.

I would think a C to C spacing of about 22 to 24" between the 4 bays would be good. -mclapp

On the horizontal 8 bay, do you see any reason why I couldn't try a C to C spacing of 1.5 or even 2 wavelengths, for example 36" or even 48"? That way I can keep the wider mesh (perhaps to aid lower UHF channels). Of course the beamwidth will narrow and be more difficult to aim. I've had good luck with wider spacing with my 91XG's.

The line I used is 450 ohm ladder line and then the feedpoint would be the center of that. A open wire feeder would be fine too the spacing of the open wire feeder isn't real critical some where in the 1 1/2 to 2" range would be fine. -mclapp

I'm having trouble visualizing the impedance here... to maintain 75 ohms, are you using a 2.5:1 balun with that 450 ohm ladder line?

With an open wire feeder connecting the two 4 bays, would I need to use a 2:1 balun, or just a 4:1?

Thank you!

[300ohm]

I'm having trouble visualizing the impedance here... to maintain 75 ohms, are you using a 2.5:1 balun with that 450 ohm ladder line?

The logic behind that is, 300 ohms parallel gives you 150 ohms. 450 ohms plus the 150 ohms divided by 2 gives the 300 ohm match.

[mclapp]

You could go wider with the spacings as you suggested it will change the beamwidth but doesn't do much for gain because on some channels the side lobes will get larger.

Don't worry about the 450 ohm ladder line and balun impedance too much. Those antennas are all over the place impedance wise so the 450 ladder doesn't hurt much there and it will still be 300 ohms in about the same amount of places on the 450 ohm ladder as 300 ohm or some sort of other open wire feeder.

Ladder and open wire line are not like coax the mismatches don't hurt you as much.

[300ohm]

If you keep the CM4228HD spacing, you could try Ken Nists harness, tested by tballister here:
http://www.antennahacks.com/Hacks/NistHarness.htm

Looks like a decent match.

[logray]

Hmmm.... antenna pron. Looks to be above my wire bending skills, but he did a fantastic job. :-)

And I'm guessing that two "straight" bare wire phase lines would not achieve good results. sooo.....

Perhaps I'll just go with two 4:1 baluns and a 2:1 coax combiner initially.

Or maybe source some .75" twin lead as mclapp has done.

Edit I found some 300ohm twin lead laying around perhaps I will try that as well.

Also in the 4228hd hack, is that a 1:1 choke balun he soldered straight to the phase line?

[300ohm]

I asked him the same question, heh.
His response:

RE: Balun
No balun. That was the point of Ken's design. That the transmission line itself serves as a transformer. The design does include the ferrite bead I indicated, and in fact he'd recommended two of them; a second one 5" down from the 1st. But as I moved the second one back and forth, close to the 1st one and then up to 3' away, there was neglible change to the response.

http://www.digitalhome.ca/forum/show...103749&page=16

[300ohm]

Edit I found some 300ohm twin lead laying around perhaps I will try that as well.

300 ohm twinlead probably isnt going to work well, unless maybe you have one of these antique Channel Master 2 set 300ohm indoor couplers (which I happen to have, heh) used in reverse and of course weatherproofed. One of these days, if I get a get a fair testing setup, Ill test them out.

Picture:


If you twist my arm, I may post a schematic with wire lengths and values.

Looks to be above my wire bending skills, but he did a fantastic job. :-)

Nah, take your time and youll do fine. But instead of non UV resistant PEX tubing separators, use pieces of 15 cent grey pvc gutter ferrules available at ACE Hardware.