I horizontally stacked 2 Televes DAT75 antennas hoping to get more reliable reception from Philadelphia digital signal 72 miles away(my elevation is 500 feet).I purchased equipment for the stack from ATECH.
Previously I had one DAT75 in the same location.After changing to the double stack I noticed one of the signals(digital67) was worse.
Today I conducted an experience disconnecting one antenna from the jointenna and then the other to see how they performed independentally.I also compared the results to how they performed together.
The results:
Digital 27,32 and 34-Better results when stacked.
Digital 54- solo left antenna had highest signals,solo right antenna had second best signals.Combined stacked worse results.
Digital 64-Solo left and stacked had similar results(strong steady signal).Solo right antenna was awful-no signal.
Digital 67-Solo right-strong reliable signal. Stacked second best but weak signal with breakups. Solo right-awful-no signal.

Shouldn't the stacked results always have the best results?Is there a problem in my setup? The wires to the jointenna are the same or measured the same.

BTW,I used the signal measures from a Sony HDD500 besides observing the picture with my eyes.

Shouldn't the stacked results always have the best results?Is there a problem in my setup? The wires to the jointenna are the same or measured the same.

BTW,I used the signal measures from a Sony HDD500 besides observing the picture with my eyes.

Pulling one antenna off the joiner is not the way to do it. You need to bypass it completely. Results are unpredictable when one side in unloaded.

Does the HDD500 have a real signal strength meter or only a signal quality meter (i.e., 0-100)? On my Sony SXRD, the signal strength is the AGC number on the diagnostic screen. The lower the number the stronger the signal. The signal quality number is a combination of signal strength and multipath and may not be giving you true results.

It would be a strange situation indeed if one antenna produced a higher signal strength than 2 antennas.

How far apart are the two antennas?

It would be a strange situation indeed if one antenna produced a higher signal strength than 2 antennas.


If one takes a look at http://www.hdtvprimer.com/ANTENNAS/16bay.html the author had the same experience. He states that both antennas have to be in an equal strength zone (not one lesser and one higher).

I had the same experience in stacking antennas. If one antenna was in a lesser strength zone, it in fact contributed to diminishing the overall strength of both antennas. The only antenna stack that works for me is one that I have to get ABC - I measured the position of each antenna prior to joining them together to determine if both were in equal strength zones (4' apart).

Each antenna measured by itself in its space and location was 25 db. Stacked together with a low-loss joiner (0.2 db insertion loss) gave me 28 db overall.

However when I attempted to stack two antennas for another channel (CBS), the stack underperformed. In that scenario, one antenna measured at 25 db. The second one came in at 19 db. The overall count with both antennas joined together was 23 db (2 less than the one 25 db antenna).

The HDTVPrimer author went through similar experiments and has some observations regarding equal strength zones for stacking (hotspots):

"If one 4228 is in a stronger field, then part of its signal will be retransmitted out the weaker antenna. This loss may equal the little bit of gain you had hoped for from the weaker antenna."

Other people have observed this same phenomenon working in human relationships and marriages...

Shouldn't the stacked results always have the best results?Is there a problem in my setup? The wires to the jointenna are the same or measured the same.


The symptoms that you describe could be caused by a combination of out of phase connections and/or slight misaiming.

The antennas are 3 feet apart. My first thought was the wires are slightly different causing the signals to be out of phase,but wouldn't all channels suffer?
The HDTV primer article was interesting.Thanks-I never heard heard of the hot and cold spot problem with stacking.
I might have to search the roof for hot spots-something I'm not looking forward to.

I would tend to agree with Tower Guy's comments. I've been using two Triax Unix 100 wideband antennas horizontally stacked for 4 years with very good results. Occasionally, a few distant stations are stronger with one of my other single antennas (which simply could be the exact location of the single antenna) than with the stack. Pic of the stack attached. IIRC, the spacing on these two antennas is around 38-39." The height is 30' AGL.

Are you using a rotor to aim the stack? The two antennas narrow the beamwidth, making the aim much more critical than with a single antenna. Sometimes just a degree or two can make a huge difference.

Steve

I, too, am interested in having a 2 yagi UHF stack. I have been looking at the Antennas Direct model 91XG, which is available for $95 shipped. Is there any real difference between the 91XG and the European models? I would have mine about 40' AGL. Thanks.

Mike Glass
Indianapolis

The antennas are 3 feet apart. My first thought was the wires are slightly different causing the signals to be out of phase,but wouldn't all channels suffer?


I believe the difference can be up to 15% in phase for stacked antennas to work - I had one stack working (3b gain overall) where one wire was 1 foot longer than another. My experience has been that inches don't matter in wiring stacks.

I would tend to agree with Tower Guy's comments. I've been using two Triax Unix 100 wideband antennas horizontally stacked for 4 years with very good results. Occasionally, a few distant stations are stronger with one of my other single antennas (which simply could be the exact location of the single antenna) than with the stack. Pic of the stack attached. IIRC, the spacing on these two antennas is around 38-39." The height is 30' AGL.

Are you using a rotor to aim the stack? The two antennas narrow the beamwidth, making the aim much more critical than with a single antenna. Sometimes just a degree or two can make a huge difference.

Steve
No rotor.My 4248 Channel master has the rotor.The distance between booms is about 39'. They are close since I think multipath is a problem.Strong signals from Spring to Fall even on non trop days but as soon as the leaves fall from the trees my digital reception decreases.Example-digital Ch.54 is solid day and night from Spring to Fall,but received less than half the time in Winter.

A few comments:

1) A 36" spacing is fine. Spacing has more to do with setting the location of the nulls and the lobes than it does the gain.

2) Two antennas may have a greater impact on multipath reduction than gain increase. I have severe multipath but plenty of signal on my "locals" (54 miles). Two antennas made a huge improvement in multipath. Do you know if your problem is signal strength or multipath?

3) On general location of antennas..... There is probably no location that is best for all stations. I've noticed two issues on antenna location, especially on distant stations. A particular station may have a permanent hot spot or a cold spot. I tried my antennas 90' apart and discovered one station was always 10 dB stronger with the antennas in this one location. Unfortuantely many others were weaker. The second issue is that cold and hot spots are constantly changing with tropospheric conditions so there likely is no ideal location for all stations.

4) Two antennas vs one...... At UHF cold spots and hot spots can be only a couple of feet apart. Even though you may find that one antenna out performs two at times, overall two should always be better than one because the cold/hot spots are always moving. The hot antenna may be the cold antenna an hour later or any other possible combination. Try repeating your test several times during the day.

5) Phasing harness length. A wavelength at channel 20 is about 2 feet. If one cable was electrically a foot longer than the other, then the signals would arrive at the combiner 180 degrees out of phase and cancel each other out assuming equal amplitudes. I'd say the two cables should be within an inch or less of each other in length to work properly.

6) My experience here at my very difficult location (most stations show as purple on the Google overlays) is that any non line-of-sight station is constantly changing in signal strength and will at times have drop-outs. Most stations over 60 miles are "part timers." I've seen short term 30 dB drops in signal strength even in my local stations. No antenna can overcome that. :)

A few comments:

1) A 36" spacing is fine. Spacing has more to do with setting the location of the nulls and the lobes than it does the gain.

2) Two antennas may have a greater impact on multipath reduction than gain increase. I have severe multipath but plenty of signal on my "locals" (54 miles). Two antennas made a huge improvement in multipath. Do you know if your problem is signal strength or multipath?




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The vast difference in signal stability from Summer to Winter leads me to think that multipath is a problem for me.

I believe the difference can be up to 15% in phase for stacked antennas to work - I had one stack working (3b gain overall) where one wire was 1 foot longer than another. My experience has been that inches don't matter in wiring stacks.

Chalk off your experience to dumb luck. The combination of about a foot of coax and a backwards connected balun will offset each other on the lowest channels. If the balun was connected in-phase, the extra coax becomes a full wave near the top of the band.

If the antennas were connected properly, the difference in length would skew the main beam left and right in the middle of the band. If you have a rotator, you might have compensated for the mistake and never knew it.

The vast difference in signal stability from Summer to Winter leads me to think that multipath is a problem for me.

One way to find out is to look at some analog stations that transmit from the same towers to see how clean those stations are. Also at 72 miles from the transmitters, even with 500' of elevation on your end, it's likely you do not have line of sight to the transmitters. Have you gone to tvfool.com and entered your location? It will tell you if you have LOS, 1 edge, or 2 edges between you and the transmitters. 1 edge might not be too bad if it's a low angle but 2 edges make for difficult reception.

At your distance, it's almost a guarantee that tropospheric ducting will cause periods of enhanced and degraded signals. It could be that the types of ducts you get in the winter vs the summer are responsible for the variability.

My bet is that at 72 miles there is no antenna you can put up that will give you 100% reception of all your desired stations under all conditions.

Went to tvfool.com. It shows analog Philadelphia as edge2 or not listed.When I had a VHF antenna on my roof I received 3,6,10 strongly and 12 with some snow.The major UHF channels 17,29 and 57 were just as strong until digital(29) and LP analog(17) popped up locally.
Tvfool lists only two digitals as edge 2.Based upon the analog results I don't have confidence in the digital results.