How to add VHF with UHF M4?

[shipfaced]

Made an M4 using 1/8 inch copper thru-out, not using a reflector. It is up 30+ feet on a mast, the house has a metal roof approx 23 feet high. Presently receiving ch11, 67 miles away but only at about 50% strength, it's a network so it would be nice to have a signal better than hit an miss.

The mast is placed on the south side of the house and the wanted VHF signal is to the north so it must look over the metal roof. Most of the remaining UHF signals are 67-70 miles away to the N and 25-45 miles to the south. Getting all the other available signals in this area and very happy with the signal strength. Have a CM7777 with the UHF/VHF option - on combined.

My questions are have I left out any important info?
How much spacing is required between the UHF and VHF?
How small can I make the antenna to get ch11 VHF only?
Should the VHF antenna go above or below the M4?
What style or shape antenna can be used (what does it look like)?

If there is a source with these answers please point me there. I have rattled around this site for a good long time and have not found it. SF

[mlord]

Ideally, the two antennas would be placed such that their receive aperatures have exactly zero overlap.

Failing that, common practice says that the absolute minimum spacing is half the receive aperature of the UHF antenna. More than that is better.

So the minimum, in practice, is normally about 2', center to center (or boom to boom). More than that is even better.

Cheers

[shipfaced]

mlord, thanks for giving me the "receive aperatures" snippet. Using this term has been useful in tracking down the info needed inside and outside this site. What a can of worms, as you know and I found out there are too many variables for a quick-draw response.

From what I gather, a ch11 only folded dipole would be roughly 15 or so inches overall length but the resulting signal disruption to the M4 might not make us happy. The one wave spacing (if I got it right) would be close to 5 ft, and that is only for the VHF side. Not practical in this case.

Would an M8 be a better/simpler way to go?
Or a custom tuned M4, knowing that I need RF from 11 to 48?

BTW I tried the Narods per OTA Canuck with dismal results.... it was worse.

Remember I'm a layman at this so dumbing it down is appreciated.

[holl_ands]

You should enter your location into www.tvfool.com and then post the results URL (webaddress at top of browser)
so we can see your signal strengths. Also describe any nearby buildings, trees, etc. affecting reception.

You could simply mount a Hi-VHF antenna lower down on the mast, presuming you don't need to rotate it.

[mclapp]

The M4 should have near the same gain as a dipole especially with some sort of reflector but even without it should be around 2 dbi on VHF-hi. If the M4 isn't getting the channel you need a dipole probably won't either.

[shipfaced]

mclapp, is there anything that can be done to antenna to improve it's reception .. is 8ga copper OK?
This M4 is 10 inch elements, 9.75 spacing and 1.25 between feeders. It was very carefully put together this time with emphasis on accuracy, straight whiskers, new 6G and good quality balun. Distance is about 75ft from antenna to TV. We did get ch 11 better at first with the odd interruption but it's degrading slowly.

holl_ands, If a VHF antenna goes much lower the line to the tower will be behind the roof line. Not much in the way as far as buildings or trees. I am in coastal FL right at sea level, the towers are inland a bunch so it's gotta be "way hi" at maybe 300-400 ft.


fool scan here, getting 16 RF's with good signal ... 70% or better http://www.tvfool.com/?option=com_wr...41055374660650

[holl_ands]

Ch11 is weak at your location. I would recommend Antennacraft Y5-7-13 which has
reasonable Gain, and should still allow Ch10 to come in on the back of the antenna.

Adjacent Ch10 might be causing a problem to weaker, adjacent Ch11 using the M4,
although I haven't worked out the respective signal levels after including M4's VHF gain,
not forgetting to include a fading margin of perhaps 10-20 dB when Ch11 signals fade.
A Hi-VHF antenna with reasonable Beamwidth and F/B Ratio should be adequate to
suppress Ch10 when aimed at Ch11...even when Ch11 suffers a multipath fade....

Y10-7-13 (and W-G YA-1713 equivalent) are higher Gain but higher F/B also reduces Ch10.
This may or may not cause a problem receiving Ch10, so I would try Y5-7-13 first....

You can mount it 3-5 feet below your UHF rotator.
BTW: VHF bends over obstacles much easier than UHF.

[mclapp]

Shipfaced,
Since your VHF-hi channels are in the upper portion I would make a 9 1/2 x 9 version it will only slightly hurt the gain on the lower UHF channels by a db or so but could make 2 or 3 db net gain on ch 11.

You don't mention if you have a reflector, if you don't adding one 36" wide would help a lot. Looking at your TVFool plot I think you would really want the bidirectional beam that the non reflector version gives you.

If you're into experimenting you could try a minimal reflector which will give you slightly more gain in the weaker 325 degree direction and take a little away in the 170 degree direction. Using only 4 reflector rods 36" long, (1 rod centered behind each bay) and spaced either 4" or 15" (try 4" first) could be enough. For reflector rods you can use most any metal, for testing you could even use 1/2" wide tin/alum foil strips on cardboard or some wrapped around a wooden dowel or stick.
That should work as a fairly decent VHF-hi reflector without degrading the UHF in the other direction too much, it should help reduce any ch10 interference as well.

You could also go with more reflector rods to further inhance the 325 direction if needed it just depends on how much signal you can give away in the 170 direction.

A vertical stacked 8 bay would also be an option it will give you a few more db's on both VHF-hi and UHF.

A small ch 11 yagi would also be an option a 3 or 5 element version would probably get the job done. There are many Yagi antenna calculators on the web that will get you in the ballpark. I built a larger one for ch 11 using a folded dipole driven element and it worked very good using the lengths and spacings provided by the calculator.

[shipfaced]

Thanks guys,

Going to make the 9.5 X 9 M4 tomorrow. Hopefully it can make do w/o reflector.

Holl_ands, good to know VHF bends, keeping that one in the bank for later use.

mclapp, in case I do need a reflector, does the "finished" reflector material need to be rods (round)? I have rolls of thick copper flat stock metal 5/8" wide all over the place here.

BTW CH 10 is more of a nuisance than anything (I really don't need to be saved) - so if it's lost ....

[mclapp]

Shipfaced,
No the reflector doesn't have to be round, that flat stock will be good. You may want to try that first before building a new sized antenna, the reflector will most likely do more for you on ch11 than the resize (reduce ch10 and enhance ch11). My only worry about the reflector is how much it will hurt the UHF's in the 170 degree direction.

[shipfaced]

After getting thru half of the M4-M8 bowtie thread (whew) it's up to 36 pgs now. We found a new item that I had overlooked ... the phase lines were flat against the plastic frame of this antenna for the entire length. So after correcting this, another better M4 went up the mast.

Results not good, still no CH11 but CH10 was back with a vengeance - like it or not I might get saved after all!

Then 4, 36" reflectors were added, CH10 stayed and CH11 came back at the original 50% but unwatchable.

Additionally, the last 4-5 ft of the mast was temporary 1 1/2" plastic plumbing pipe, it was replaced at this time with a longer more solid fiberglass mast from a windsurf rig, this also added height. Total HT now 36-38 ft.

It was incredibly windy during this experiment, guessing at 20 knots at times. Does wind have anything to do with reception aside from the sway of an antenna?

[mclapp]

If you're still getting good reception through the minimal 4 rod reflector maybe you could add more reflector rods to help cut ch10 some and boost ch11, just keep track of the signals on UHF.

[shipfaced]

Update. This morning the 5/8 in reflectors were replaced by 2.5 inch wide reflectors, used this size because I had it on hand. Finally, troublesome CH10 was eliminated and the elusive NBC network on CH11 is back with some authority. Getting 70 to 100 percent strength on 18 RF's, yielding 51 channels. CBS has never had such a clean signal, this is especially appreciated on Superbowl Sunday!

Want to thank everyone for their help.
special thanx to mclapp

[mclapp]

That's great news, that is the beauty of these antennas a little custom tailoring of the elements and reflector and you can dial it in for your particular situation.