Tuning Yagi Antennas

thanks.

If the antennas are spaced or stacked 1/4 wavelength apart you get a 3 dB gain.

However, your feed line impedance changes, I think.

If I had a receiving antenna on one side of a hill connected to an antenna on the other side of the hill (pointing in the opposite direction), would the second antenna act as a transmitting antenna?

Sure would. It will also generate a multi-path signal path with the diffraction pattern generated at the top of the hill from the original transmitter.

It shoul be noted that we have to talk about the TV standard too.

If it is analog NTSC / PAL then the Yagi should be horizontally oriented.

If Digital TV (especially DVB-T) is used the polarisation is vertical and the Yagi should be Verically oriented to tap the COFDM modulation better. For DVB-T there are exceptions possibel when the provider uses horizontal polarization.) I have no clear idea about performance ATSC antennas.

For the over-the-hill application even with digital TV the following configuration is possible: Active Receiving antenna or preamp supplied via a small photvoltaic panel & battery (total cost ~60 US$) - Yagi connected to the output of the AMP. The active antennas for DVB-T require around 50mA at 5 or 12V which a small photovoltaic panel should be able to deliver. The yagi is used in this application due to directivity od the antenna. And to maximize the output also on the receiving end a yagi is recommended. Only issue: You have to hope that the tree where you have hidden the stuff (battery & Photpanel) is not cut down by the owner...

Tuning? One does not tune yagis. They are refined well and needed to be left alone. You might want to stack them, one an average half wavelength or more above the other. You amplify them individually, and combine their signals downstairs with a standard splitter cabled in reverse, or use one here, the other there. Splitters have losses, and may eat up much of the combined power.

Raising them high is necessary in any case, singly or doubled.

If you don't mind spending a few minute, and a couple of dollars, here is a very good HDTV fractal antenna.You can build it yourself.Check out the website below: website.The one by BADDABING is very good.It fits inside of a picture frame.

These have higher gain than higher-priced pre made units.

Good luck

http://www.instructables.com/id/How-to-make-a-fractal-antenna-for-HDTV-DTV-plus-/

Re #5. Yes, but for the inevitable cable losses. A simpler way is to stretch a long wire on the top, visible from the transmitter, and the receiver in the valley, both. The long wire (about 10 times the lowest wavelength, nothing precise), will do the dual task of retransmitting fine.

I may be opening myself to criticism here as antenna theory was never my strong point, I have a good friend that does it far better!!!

Why bark when you have a dog? (hope he doesn't read this!!)

Further on with Redfred's great post:-

Depending upon how far away the transmitters are and assuming they are all terrestrial, eg all of a similar vertical angle to the Yagi's, you may find that placing them in adjustable mountings (one over the other for starters as there will be little signal phase difference when transmitters are horizontally wide apart). Then carefully move one up and down relative to the other and watch the signal quality/strength, tighten at the best level.

If it shows nothing positive, try them side by side, forward and back, away and together.....

That is the way another friend of mine did it with two Yagi's 25 years ago, he's been dead 10 years and they are still on his house......his mother is in a home and his father has been dead even longer. The house will be sold soon.....he placed them one above the other......with success.....

We live in an area that was served badly with signal levels in the days of conventional TV terrestrial transmitters......today we get DVBS and DVBT of good quality.....

I am told/informed that twin Yagi's always work best when they are setup to point directly at the transmitter, then "tuned" while either looking at the picture (assuming TV, or look at the receiver quality and signal level displays in many receivers nowadays.......I have never done it myself though, sorry!!....

Yes, this approach is exactly the approach I was originally thinking of recommending, but then I noticed a critical word by the OP. This is for a digital television signal. The RF reception quality will always be an analog signal but with digital modulation techniques, the relative quality of the RF signal no longer appears as picture noise quality improvements. The picture can be decoded or it cannot be decoded. So slight adjustments in orientation will not provide any usable feedback. If the OP has a spectrum analyzer, this can be a quick and easy adjustment to maximize signal quality, but I'm certain that the OP does not own this expensive test instrument.

Now that I've written the above comment, I think I have a method that one can "tune" a single or multiple Yagi antennas to point in the correct direction using a digital TV reception response. This will be a pain in the ass but you do this only once. This technique will work if there is no multi-path reflection that the receiver will be picking up. (A big assumption, but you have to start somewhere.) Start with one and only one antenna. To state the obvious, set your television equipment to receive the appropriate transmitted signal. Rotate the antenna orientation until you can capture a TV picture (This will be a slow tedious process because of buffering delays.) Once you have a picture, choose a method to mark the antenna position but do not mark this position but try to remember this location. Very slowly and with as consistent an angular velocity as possible rotate the antenna in one and only one axis and direction while watching the TV image. When the image starts to freeze video frames stop. Mark your position. Return the antenna back to about where you captured video again and wait for a recapture. (Remember that the buffer will complicate and delay a captured signal from being displayed.) Once you get a signal again, rotate the antenna away from your previous mark at the same angular velocity as you did previously and mark where the signal dropped out again. Your optimal setting will be very close if not exactly on the midpoint between your two marks. The uncertainties in this approach is how complicated (how many parts of the screen were changing at the same time) the image was at the instant of losing communication. This will change the amount of usable buffered signal was displayed before freezing.

Once that you've done this for one antenna, you could try this with adding a second antenna added to make a phased array. But you will likely find that adding the second antenna and tuning this up will have a more narrow acceptable angle of the second antenna.

Now a higher element count Yagi antenna will increase the RF signal gain with only a modest amount of noise increase. The drawback of a higher element antenna is that there is a narrower angle for this much gain improvement.

The advantage of using a 30 db amplifier is that considerably more signal gain will happen with the amplifier than is practical of having more elements in the Yagi antenna. The drawback of a 30 db amplifier is that the incoming noise will also be amplified along with the signal.

That answer HAD to get a GA from me.....Brilliant.

By the way, that is approximately how I used to set up my Sat dish years ago before I bought a meter.....

The major difference was that I had a really strong signal, so to make the signal weaker, I piled old blankets on the dish till even the strongest signal area was weakened, then it was easy to put a sticky label on both axis, move them to a certain level of noise/artifacts, mark it, move in the other direction for about the same level of noise, place in the middle. Do this for the other axis as well......

What also helped was having 90cm dishes, it had to be a really bad rainstorm before the picture was lost on some transponders.....generally only 60cm are used here.....but they lose signal earlier with the same sensitivity of LNB of course......

Adding to the good advice you've already gotten:You didn't say how the two antennas were spaced - horizontally or vertically, or by how much distance or how many quarter waves.

I didn't use yagis, but cubical quads; quite a few years ago a new station I didn't care for was installed nearby, at nearly the same frequency as a distant one I wanted to receive. By simple geometry (knowing where the two transmitters were located), I was able to calculate a horizontal spacing that provided constructive interference for the desired station while providing destructive interference for the unwanted one. It was hardly perfect, but it worked!

If you do have horizontal spacing between the two antennas, then the directional properties are enhanced, and only a small deviation from the optimum direction will reduce the signal drastically. Note that if there are tall objects between you and the signal source, diffraction and/or reflection may cause the optimum direction to be something different from the line-of-sight direction to the transmitter.

One issue we have forgotten to tell the OP.

The number of elements is also a criteria to increase the Yagi's directivity and the signal output. So the longer the Yagi antenna is (and the more elements it has) , the better is the directivity, the antenna gain and and related to it - the signal output.

This might help more than to stack two antennas.

Another issue is if the antennas are stacked, it is recommended to have them mounted in a way where the distance between antennas can be optimized for best combined fieldstrength. (See the previous messages regarding the distance between the antennas.) In some cases it might even be required that the "line of sight" of the antennas is lightly different due to reflections of buildings, roofs etc. effective at the location of the antenna.

Furthermore if the Yagi has a reflector behind the receiving dipole - the mast carrying the antennas should best be located behind the reflector. Whether this is mechancal feasible (wind load) might depend on the used frequency range and the effective size of the antenna.