[dhahne]

I have a Radio Shack VU-90XR that does a great job most of the time for DTV stations here in Hampton Roads, VA. My biggest problem is dealing with multipath signals that occur on some of the stations depending on weather conditions. What I would like to do is narrow the beam width on this antenna somewhat to both increase the gain and reduce the sensitivity to ghost signals. All the transmitters are located in less than a 10 degree arc from my location.


Question: Can I decrease the beam width by adding more directors to the from of the antenna? Based on what information I have been able glean from several sites this seems like the right approach but I have seen nothing definitive. Any advice would be greatly appreciated. Thanks.

 

[eric.krieg]

I am not an electrical engineer, I'm a Mech E. But I have a copy of the antenna engineering handbook. If you are going to mess around with your antenna, so should you. I got my copy at the library.


The table of interest to you will be "Optimized Lengths of Parasitic Dipoles for Yagi-Uda Array Antennas of 6 different lengths".


It appears to me that, as the number of directors is increased (and the length of the antenna increases), the length of the reflector may increase, but the lengths of the different directors may decrease. It says in the text that the length of the driven diplole has little impact on the gain.


Now, this was in the text for a single channel Yagi, god knows how a broadband one would operate. Is there any link on the Internet so I can take a look at this antenna?

 

[eric.krieg]

Does that thing have a corner reflector for the UHF?


Oh yeah, what channel is giving you trouble?

 

[pc_load_leter]

Using the same RS antenna. All other channels are fine except CBS 2 in Chicago.


I live up north near GURNEE, IL. I initially pointed to CBS chicago with no luck and tried pointing North (5 deg) to Milwaukee. No luck.

 

[Mac The Knife]

Yes, in general adding directors increases the gain and reduces the beamwidth whereas adding more reflectors tends to have very little effect.


But, adding directors to an existing antenna may or may not give satisfactory results since the number of directors and their optimum spacing and relative lengths tend to interact.


Another possibility you might consider is replacing the driven dipole element with a loop.


Personally, I'd look for other solutions (unless you just enjoy doing antenna design as a hobby). How about planting a nice thick (high water content) tree to block the multipath (I'm only half joking about this. The best way to deal with interference is to eliminate the source rather than change the antenna).

 

[dhahne]

Quote:

Originally posted by eric.krieg Does that thing have a corner reflector for the UHF? Oh yeah, what channel is giving you trouble?

Yes, the VU-90XR does have a corner reflector. I can have problems with just about any channel given the right conditions. The PBS station I have trouble with transmits on UHF16, the WB station on UHF38 (both at reduced power). Can't remember what UHF the NBC station transmits on (I think it is around 29) but I have always gotten major ghosting on even the VHF analog broadcast so that one is primarily due to some reflections in the path somewhere.

 

[dhahne]

Quote:

Originally posted by eric.krieg I am not an electrical engineer, I'm a Mech E. But I have a copy of the antenna engineering handbook. If you are going to mess around with your antenna, so should you. I got my copy at the library. The table of interest to you will be "Optimized Lengths of Parasitic Dipoles for Yagi-Uda Array Antennas of 6 different lengths". It appears to me that, as the number of directors is increased (and the length of the antenna increases), the length of the reflector may increase, but the lengths of the different directors may decrease. It says in the text that the length of the driven diplole has little impact on the gain. Now, this was in the text for a single channel Yagi, god knows how a broadband one would operate. Is there any link on the Internet so I can take a look at this antenna?

Here is the link to the Radio Shack site ( http://www.radioshack.com/product.as...5Fid=15%2D2152 ). Not a great deal of detail. The technical library where I work has a copy of Thomas Milligan's anatena design book that I plan to look at once it is returned.

 

[George Molnar]

Although maybe not practical for you, you could get a 2nd identical antenna and stack with the first and combine both.


This would sharpen the directivity.


Combining identical antennas is tricky and requires the right vertical separation and the right coupler and the same length of wire from each antenna to coupler.


If not done properly, actually would worsen antenna's response.


Search for "antenna stacking."

 

[dhahne]

Quote:

Originally posted by Mac The Knife Yes, in general adding directors increases the gain and reduces the beamwidth whereas adding more reflectors tends to have very little effect. But, adding directors to an existing antenna may or may not give satisfactory results since the number of directors and their optimum spacing and relative lengths tend to interact. Another possibility you might consider is replacing the driven dipole element with a loop. Personally, I'd look for other solutions (unless you just enjoy doing antenna design as a hobby). How about planting a nice thick (high water content) tree to block the multipath (I'm only half joking about this. The best way to deal with interference is to eliminate the source rather than change the antenna).

No, antenna design is not a hobby but I do enjoy learning new things and tackling interesting problems, not to mention just plain tinkering. I am actually looking at the possibility of both adding directors and replacing the driven element with a folded dipole (or more likely bridging the ends of the existing dipole to convert it to a folded dipole). I have played around with some antenna design software (the ham radio guys develop lots of stuff) but it seems geared more towards transmission characteristics as opposed to reception. The antenna is in the attic and I have "designed" a little bracket that will allow me to "hang" additional directors off the front end. Right now my plan is to scale some Yagi designs to the existing antenna and just try a few configurations. The mods I plan on making will all be easily reversable. The original antenna performs good most of the time, I'm just looking to decrease the impact of atmospheric variables.

 

[dhahne]

Quote:

Originally posted by George Molnar Although maybe not practical for you, you could get a 2nd identical antenna and stack with the first and combine both. This would sharpen the directivity. Combining identical antennas is tricky and requires the right vertical separation and the right coupler and the same length of wire from each antenna to coupler. If not done properly, actually would worsen antenna's response. Search for "antenna stacking."

Thanks George. I have looked at this and figured I did not have the room to stack antennas properly.

 

[eric.krieg]

I have a lot of the same problems that you have. I get ghosting on various channels depending on the weather conditions.


I have a very similar antenna from Phillips Magnavox. I only got the antenna a few weeks ago, and the weather here in Chicago has been too extreme to get on the roof and tweak it.


I also have some emf interference on Channel 2. It's really an odd thing. I did a half assed grounding job when I installed it, I'll have to take care of it when spring comes around.


Anyway, what might be interesting, if you really don't mind hacking up your antenna, is abandon the UHF portion. Then you could at least a) get a better UHF antenna and b) use multiple UHF antennas in an array.

 

[eric.krieg]

pc, I live in Mount Prospect. Antennaweb says that I am less than 21 miles from the Hancock. I have an absolutely HUGE antenna (my 80 year old neighbor says that it is the biggest he has ever seen!).


My Ch 2 reception sucks. I have ghosting and EMF interference.


I believe that Ch 2 is the toughest channel to get. When I lived on Long Island I had all kinds of problems with Ch 2 out of NYC.


I am going to bet that in Gurnee, you need a huge antenna to get Ch 2. What does antennaweb say?


You probably need a good installation, too, and a good preamp. There are SO MANY things that can go wrong with an antenna. I understand why most people get cable.


With that said, I have to say that, when conditions are right, especially on certain channels (Ch 7 and 9), the reception from my antenna beats the snot out of the crappy picture I used to get from AT&T/ Comcast analog cable.


I was watching the Bulls game last night (yeah, I know, why bother) on Ch. 9 and it almost looked like high def.


Oh, and my new Zenith 32" HDTV with built in ASTC tuner gets delivered this afternoon! Joy!

 

[eric.krieg]

Dave,


I thought of a good idea for you to try as an experiement.


The handbook has a table and some formulas for how the number and size of the rods on the corner reflector effects the gain, and especially the front to back ratio.


Since you are in an attic, and don't have wind load as a consideration, why not take some aluminum foil and make the reflector into a solid plate. This might make your f/b ratio as good as it is going to get. It might minimize any ghosts that you are getting from behind.


I think I read some posts from people who used foil around their attic to minimize reflections. That might also be an area for you to experiment with.

 

[Mac The Knife]

"My Ch 2 reception sucks. I have ghosting and EMF interference."


Eric:


Are you certain it is EMF? The low VHF channels are very prone to FM radio interference. Have you tried an FM trap?

 

[eric.krieg]

I have no idea the source of the interference. I speculate that it is EMF because it is on channel 2 (analog), and like I said, it kind of looks like random, white pixels on the screen. It is not herringbone, which I have read in the FCC interference pamphlet is what FM interference looks like.


The good news is that my Zenith 32V37 HDTV came in, and digital is great. WBBM-DT (Ch. 3) was coming in great last night, just a few dropouts.


I think once the thaw comes, I will be:


1) replacing my 4' boom with a 6' or larger boom


2) doing a thorough and professional grounding job. (I only put the antenna up the last week of December, and the weather didn't allow me to do a perfect job).


3) Aim the antenna using the 32V37 signal meter.


If that doesn't get WBBM-DT in perfectly, I will consider a pre-amp with a FM trap. I want to try the cheap stuff first.

 

[dhahne]

Quote:

Originally posted by eric.krieg Dave, I thought of a good idea for you to try as an experiement. The handbook has a table and some formulas for how the number and size of the rods on the corner reflector effects the gain, and especially the front to back ratio. Since you are in an attic, and don't have wind load as a consideration, why not take some aluminum foil and make the reflector into a solid plate. This might make your f/b ratio as good as it is going to get. It might minimize any ghosts that you are getting from behind. I think I read some posts from people who used foil around their attic to minimize reflections. That might also be an area for you to experiment with.

Excellent idea Eric. I will give it a try along with my experiment with additional directors.

 

[wward]

Quote:

Originally posted by eric.krieg I am not an electrical engineer, I'm a Mech E. But I have a copy of the antenna engineering handbook. If you are going to mess around with your antenna, so should you. I got my copy at the library. The table of interest to you will be "Optimized Lengths of Parasitic Dipoles for Yagi-Uda Array Antennas of 6 different lengths". It appears to me that, as the number of directors is increased (and the length of the antenna increases), the length of the reflector may increase, but the lengths of the different directors may decrease. It says in the text that the length of the driven diplole has little impact on the gain. Now, this was in the text for a single channel Yagi, god knows how a broadband one would operate. Is there any link on the Internet so I can take a look at this antenna?

Intersting book; went through the table cf contents I didnt see any thing on Log Periodic antennas.

 

[eric.krieg]

Look at the chapter on frequency independent antennas. It is Chapter 14 in the second edition.

 

[wward]

Quote:

Originally posted by eric.krieg Look at the chapter on frequency independent antennas. It is Chapter 14 in the second edition.

Cool thanks.

 

[Mac The Knife]

Quote:

Originally posted by eric.krieg ...), and like I said, it kind of looks like random, white pixels on the screen. It is not herringbone... If that doesn't get WBBM-DT in perfectly, I will consider a pre-amp with a FM trap. I want to try the cheap stuff first.

I'm also a big proponent of trying all avaliable options before buying new equipment.


It also sounds like that isn't FM interference, although an FM trap did reduce some noise I had on 3 and 5 that also looked like random white pixels. I think in some cases that strong FM signals just desensitize the receiver and it looks like a weak snowy signal rather than the typical herringbone pattern. BTW, my FM Trap is just a $7 RatShack trap. So it is cheap and easy to return if it doesn't work.




dhahne:


My local library has a copy of "Modern Antenna Design" (I don't recall the authors name, but he was an antenna designer for Martin Marietta). It does a good job of covering the theory for Log-Periodics. You could probably get it via interlibrary loan at your local library.