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Discussion Starter · #1 ·
Although my reception of the digital locals has been fine during the fall and winter, I always experience multipath this time of year. For backround, I live in a low-lying area but have a line of sight to the towers, and have a Zenith DTV1080. To try and combat the spring vegetation:


*I started out with a Radio Shack UHF-only yagi and rotor--regardless of the orienation of the antenna, I often have a couple of dropouts an hour on St. Louis locals KTVI, KMOV and KSDK. The strange thing is that sometimes I'll go days without any dropouts and then have several a hour on certain days. No obvious weather conditions seem to contribute to this mystery.


*Then, I bought the pricey Blake JBX21 yagi that's supposed to be great at multipath rejection. It really hasn't reduced the number of dropouts or multipath. This antenna may actually provide too much gain, since I'm within 15 miles of the broadcast towers. I've been searching for a sweet spot for all stations with my rotor to no avail. I've adjusted the tilt with no significant improvement.


*Next, I decided to vertically stack both the Radio Shack and Blake antennas with a splitter. Although this seems to have increased my signal strengths on the locals, the signal intermitently bounces from good to low normal, especially on KMOV. Needless to say, I still have dropouts with this setup.


*Now, I've read the informative atechfabrication.com website that makes a great case for horizontally stacking antennas. Would it be advisable to stack the Radio Shack yagi and Blake JBX21 or must the antennas be alike? I'm not keen on spending another $170 for a Blake. Also, could I build a field goal -like mount to stack these antennas with PCV or will it not support the antennas? Basically the mount would look like this:



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The scale would be a 2 ft. bottom T and two 2 ft. uprights 36 inches apart, if that makes any sense. A total of 4 ft. above the roofline and the rotor. I know you can buy a horizontal stacking mount for $115. However, I'm trying to reduce additional costs since I already have over $400 in this project. Thanks for your help in advance.


Andrew
 

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Do you have multi-path, or just drop-out? If your line-of-sight to the transmitting towers does not include a ground-level reflecting plane, it may be ray-bending from temperature or humidity changes in the direct path. If you do have a reflecting plane, perhaps a high gain yagi is worse than a dipole, since the yagi may get more energy from the reflecting plane. You might try tipping the yagi up during a fade, to see if it is getting low angle radiation. A broadband dipole may be better than a yagi. Unfortunately, I haven't played with antennas for a long time, and we have no OTA here, so I don't know what's available.


The other difficulty you will run into by stacking antennas is the different frequency of the stations causes the wavelength of the line tying the antennas together to change. At one frequency the signals at this junction will therefore add while at another they may cancel. Stacking is good only over a limited frequency range.
 

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Stacking definitely requires the same antenna to be used. Have you tried just raising the existing antenna a few more feet off the roof peak? Or maybe trying an attenuator if you feel you are getting too much signal?
 

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Discussion Starter · #4 ·
Onemoretime,


I'm not sure what you mean by "reflecting plane." Can wind cause reception problems, becuase the JBX21 yagi definitely sways in the wind? It's so big it's tough to anchor down with the one U-bolt. Do you think a double bow-tie antenna would be better? Aren't these mult-directional antennaes though?


I beleive I have multipath becasue the signal will go from great to zero when the dropout occurs. I did try tilting the antenna up, but that didn't increase the signal levels. It's hard to say if tilting would reduce multipath because I only tried this on the roof with someone giving me signal readings inside. I can't really raise the antenna any more because the mast is mounted to a rotor. I've tried an antennuator but that was when I had a DTC-100 and the Radio Shack yagi. It didn't seem to help.


Thanks,

Andrew
 

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If your antenna overlooks a broad flat expanse (reflecting plane), it will pick up the signal reflected from it, and cause drop-outs as the propagation path changes One way of curing this is to lower your antenna, so the plane appears smaller, or even better, obstructed. This will handle multipath that is coming from the same direction as the transmitter.


Multipath that is reflected from objects to the side of the path e.g. buidings, are usually more stable, and cause ghosting in analog TV's. This is cured by using directional antennas such as yagis.


Yagis are also used to increase a very weak signal.


If you don't have a weak signal, and ghosting multipath is not a problem, then a yagi may not be worthwhile, and I think it could cause more problems during some multipath conditions. As I suggested, the performance (gain and pattern) of a stacked pair may vary widely with frequency, so the result for one station may be quite different for another, and even for different propagation conditions. I would in general limit the use of a double yagi to reception a long obstructed path.


Ignore my statement about feedline interference - I haven't thought about antennas for a long time, and it doesn't seem right when I read it again.


Bow-ties are broadband semi-directional dipoles. A stacked pair would help in a marginal signal area. Is there a dealer in the area you can ask to see if bowties provide enough signal in your neighborhood? Maybe there's an easy calculator somewhere, or the TV stations can help.
 

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As has been noted, you might be getting reflections off the atmosphere or ground plane. I don't have a line of sight like you, so all my local signals must either reflect off the atmosphere or bend over the edge of the 1000' ridge that blocks my signal. I have found that sometimes I will lose reception on some channels as the sun is setting for about 30 minutes. Maybe this type of effect is causing some problems for you this time of year?


You say the antenna sways a lot, how tall is the mast? some thinner gauge masts can flex quite a bit without guy wires. Just a few degrees of movement can throw off a stacked antennas reception at that close range.


If you are getting side reflections, the vertical stacking can make it worse. Ideally the antennas should be matched, you might want to try another 4248 as it is cheaper and you probably don't need the high gain. Stacking will help cure some of the side lobe problems with the 4248. You should not have any trouble selling your Blake.


PVC is too flexible for stacking unless you use large diameters. Adding vertical sections of mast to the boom will make flexing even worse.


Since you have such a close line of sight, you might want to try a bowtie antenna, like the 4 bay Channel Master or the 8 bay if you need more gain. These antennas are already stacked, 1x4 and 2x4, but they are not as directional as a yagi, so multipath performance is worse if that is actually what your problem is. This is the type of antenna I would recommend first in that type of location.
 

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Discussion Starter · #7 ·
Glenn,


The mast below the rotor is about 6 feet. The mast above the rotor is about 5 feet--a total of 11-12 ft. These are standard Radio Shack masts that do seem pretty thin gauge. Can I use guy wires with the rotor? The mast on the rotor with the antennas mounted on does sway a little. Do you think I should try a double bow tie, even though I think multipath is my problem?


Thanks,

Andrew
 

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Andrew,


A common setup is to have 10' of mast below rotator with guy wires just below rotator and 2-3' mast above rotator. If you go higher, you want to have another set of guy wires on the mast in the middle, like at 10' and 20'.


The RS bowtie has 2 pickups, where the CM's have 4 and 8 pickups for more gain. Since it is only $20 and you can return it, it would be worth a try.


Do you really have line of sight with all the towers at 15 miles? A RS bowtie inside your house may be all you need in this situation.


If the low area you are in is blocking your line of sight, then multipath is most likely since the same signal might take different paths to get over/around the hill or buildings.
 
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