AVS Forum banner

1 - 13 of 13 Posts

·
Registered
Joined
·
236 Posts
Discussion Starter · #1 ·
To turn it into a VU-90XR

A VU-90XR has eight forward directors, and VU-75XR has 4 in front of the driven element.


I can buy a piece of square aluminum tubing and easily form directors. Leave the corner reflector alone.


A 3 foot piece is about 10$, some aluminum sheet metal screws and flashing folded and attached, and the two booms joined with 2 pieces of the square tubing cut length wise and screwed together.


I could make quite a lot longer boom of 2.5 feet added onto the nose of the antenna.


Lots of pictures of long booms with lots of directors, question is can the corner reflector arms remain the same length, or does it also need extending?
 

·
Registered
Joined
·
6,199 Posts
No one has performed such a mod that I'm aware of....

Although Ken Nist (HDTVPrimer Website) prepared models of the VHF sections for several R-S Combo Antennas, he DID NOT model the UHF Sections....and I have never seen DETAILED Measurements and Photos (with Ruler for Scale) for any of these Antennas from which a 4nec2 Model could be constructed....
 

·
Registered
Joined
·
6,473 Posts
To turn it into a VU-90XR
A VU-90XR has eight forward directors, and VU-75XR has 4 in front of the driven element.

Is there something you're hoping to achieve by doing this or just a fun experiment? At best you'll get a couple dB more gain. If you're having reception problems it's unlikely a couple of dB is going to solve them.
 

·
Registered
Joined
·
22 Posts
Is there something you're hoping to achieve by doing this or just a fun experiment? At best you'll get a couple dB more gain. If you're having reception problems it's unlikely a couple of dB is going to solve them.
A little of both. I have a channel that is weak but watchable when I can get it. I could try additional directors very easily. I also thought about optimizing the UHF dipole length for the weakest channels, RF 39 and 41. It is 13.75 inches tip-to-tip but I assume the center insulating block figures in there somewhere. Regardless, that seems to me like that length would tune the lower channels better. At 620MHz, a 1/2 wave dipole is 9 inches and a full wave is 18 inches.
 

·
Registered
Joined
·
6,199 Posts
Active Dipole is ALREADY close to OPTIMUM for those channels....and even if it wasn't, we would be talking about TENTHS of a dB difference anyway....

I ran following PARAMETRIC SWEEP using 4nec2 on a Optimized Ch14-51 7-El Folded-Dipole Yagi. The Length of the Dipole was varied on either side of the OPTIMIZED FULL-UHF BAND Length of 10.6-in. Note that on Ch40, there can be a significant impact on SWR, but only one or two tenths of a dB impact on Gain...essentially NO DIFFERENCE. And, of course, performance on OTHER Channels is ALSO being affected, perhaps degrading performance to unacceptable levels:

Code:
Run     SWR     Gain    F/B     F/R     R-a     X-a     Eff.    L2      

2-0     1.4131  10.95   16.88   16.88   260.26  88.602  99.87   9       
2-1     1.4106  10.94   16.93   16.93   291.71  101.94  99.87   9.2     
2-2     1.4378  10.93   16.97   16.97   327.63  111.08  99.87   9.4     
2-3     1.4891  10.91   17.01   17.01   368.03  114.49  99.87   9.6     
2-4     1.5589  10.9    17.05   17.05   412.4   110.26  99.87   9.8     
2-5     1.6429  10.89   17.09   17.09   459.43  96.22   99.87   10      
2-6     1.7383  10.88   17.13   17.13   506.71  70.294  99.87   10.2    
2-7     1.8434  10.87   17.16   17.16   550.54  31.087  99.87   10.4    
2-8     1.9573  10.86   17.19   17.19   586.14  -21.27  99.87   10.6   OPTIMIZED LENGTH
2-9     2.0795  10.85   17.22   17.22   608.52  -84.35  99.87   10.8    
2-10    2.21    10.84   17.25   17.25   613.81  -153.3  99.86   11      
2-11    2.3489  10.83   17.27   17.27   600.7   -221.8  99.86   11.2    
2-12    2.4969  10.82   17.29   17.29   570.94  -283.4  99.86   11.4    
2-13    2.6545  10.82   17.31   17.31   528.71  -333.5  99.86   11.6    
2-14    2.8225  10.81   17.32   17.32   479.2   -370.2  99.86   11.8    
2-15    3.0019  10.8    17.32   17.32   427.24  -393.6  99.85   12      
2-16    3.1942  10.79   17.32   17.32   376.44  -405.4  99.85   12.2    
2-17    3.4008  10.78   17.32   17.32   329.07  -408    99.85   12.4    
2-18    3.6235  10.77   17.31   17.31   286.28  -403.7  99.84   12.6    
2-19    3.8643  10.76   17.3    17.3    248.4   -394.4  99.84   12.8    
2-20    4.126   10.75   17.29   17.29   215.29  -381.8  99.83   13
 

·
Registered
Joined
·
6,473 Posts
A little of both. I have a channel that is weak but watchable when I can get it.

Of course that's how digital TV is. If you can receive it you can watch it. :)

Let me give you an idea of what a couple of dB improvement might do. If you have a station that's out for a few minutes a couple times a week then 2 dB more might fix that. If you had a station that's in only 50% of the time then 2 dB more might approach 60% of the time. In order to get a 50% station to approach 100% you'd need 10 dB or more improvement which would be a MAJOR antenna upgrade over what you have.

A 2 dB improvement is subtle and you might have a hard time telling that there is any improvement at all.

If this is something you wish to explore further than you should follow the sticky above on antenna threads adding your location to the title and a link to your TV Fool report.
 

·
Registered
Joined
·
22 Posts
Active Dipole is ALREADY close to OPTIMUM for those channels....and even if it wasn't, we would be talking about TENTHS of a dB difference anyway....

I ran following PARAMETRIC SWEEP using 4nec2 on a Optimized Ch14-51 7-El Folded-Dipole Yagi. The Length of the Dipole was varied on either side of the OPTIMIZED FULL-UHF BAND Length of 10.6-in. Note that on Ch40, there can be a significant impact on SWR, but only one or two tenths of a dB impact on Gain...essentially NO DIFFERENCE. And, of course, performance on OTHER Channels is ALSO being affected, perhaps degrading performance to unacceptable levels:
I somehow missed this until today. If the VU-75XR was a folded dipole, It's performance would probably be much better. Instead, it uses a half-wave straight dipole, which should be 75 ohms. It is then connected to a 300:75 ohm balun. That can't be good. So would a folded dipole short the VHF reception? I'm not sure how the commercial models do that. I also thought about using a bowtie as the UHF element. I have seen X or bowtie shaped directors on yagis but not as the driven element.

As far as my location and TV Fool report, I will try to post it later. I already have a thread in another group but I'm not sure if I'm allowed to link to it from here. I know I need a better antenna but have to wait for some extra cash. I was just thinking in the meantime.
 

·
Registered
Joined
·
6,199 Posts
Ken Nist never posted an EZNEC Model for the UHF portion of the R-S VU Series Antennas, so I picked a Folded Dipole 4nec2 model that I DID have that CLEARLY illustrated the futility of "tweaking" the Dipole's Length. It's interesting that VU-75 and VU-90 used Stick Dipole Driver with 300:75-ohm Balun, whereas VU-120, VU-190, et. al. used Folded Dipole Driver....

Note that (see image #14) when a [20% Longer] Stick Dipole is terminated into a 300-ohm Load, the SWR Curve becomes much flatter across a wider range of Frequencies, versus when terminated into the "normal" 75-ohm Load, which results in much lower SWR, but only over a very narrow range of frequencies:
http://imageevent.com/holl_ands/dipoles/uhfstickdipole

The Stick Dipole doesn't "short" anything, although it DOES affect the Impedance of the resultant Antenna SYSTEM....which presumably Radio-Shack design engineers included when they originally modeled it.....and NO I would NOT recommend changing ANYTHING without re-modeling the proposed change.

Without knowing ALL of the Dimensions of the VU-75 from which to prepare a 4nec2 model, I can't answer ANY of your "WHAT-IF" questions....
 

·
Registered
Joined
·
22 Posts
Ken Nist never posted an EZNEC Model for the UHF portion of the R-S VU Series Antennas, so I picked a Folded Dipole 4nec2 model that I DID have that CLEARLY illustrated the futility of "tweaking" the Dipole's Length. It's interesting that VU-75 and VU-90 used Stick Dipole Driver with 300:75-ohm Balun, whereas VU-120, VU-190, et. al. used Folded Dipole Driver....

Note that (see image #14) when a [20% Longer] Stick Dipole is terminated into a 300-ohm Load, the SWR Curve becomes much flatter across a wider range of Frequencies, versus when terminated into the "normal" 75-ohm Load, which results in much lower SWR, but only over a very narrow range of frequencies:
http://imageevent.com/holl_ands/dipoles/uhfstickdipole
Thank you for this valuable information. There is some method to their madness after all.
The Stick Dipole doesn't "short" anything, although it DOES affect the Impedance of the resultant Antenna SYSTEM....which presumably Radio-Shack design engineers included when they originally modeled it.....and NO I would NOT recommend changing ANYTHING without re-modeling the proposed change.

Without knowing ALL of the Dimensions of the VU-75 from which to prepare a 4nec2 model, I can't answer ANY of your "WHAT-IF" questions....
When I said "short" I was referring to a folded dipole that is basically a loop from one terminal to the other and forms a DC short. I was wondering if it shorts any other frequencies and how manufacturers avoid that.

As for adding extra directors, I think we can lay that one to rest. I tried it in a very temporary fashion with a stick and heavy aluminum wire. If there was an improvement, I couldn't tell. The VU-90, also has extra reflectors and an additional VHF dipole. The slight gain increase is probably from a combination of all of these changes. It would be interesting to see what a bowtie would do in place of the stick dipole but I'm guessing it wouldn't be good. I haven't seen anything with straight directors/reflectors and a bowtie driven element.
 

·
Registered
Joined
·
6,199 Posts
Do NOT confuse a "DC Short" with what happens at RF Frequencies. In a Folded Dipole...or a simple Hoop Antenna (e.g. UHF part of Rabbit-Ears, which is a "stretched" version of a Folded Dipole) there is NO "RF Short". The Total LENGTH of the Antenna Element is ROUGHLY one wavelength in the Frequency Band of Interest....so it's a RESONANT structure.

Adding just ONE Director isn't going to do add very much Gain...to add 3 dB of Gain, MULTIPLE Directors need to be added at the RIGHT locations until the LENGTH doubles. AND they have to located at the RIGHT Separations from the rest of the Antenna, or it can actually DEGRADE performance....hence WHY we use Antenna Modeling Software to FIND the RIGHT Dimensions....
 

·
Registered
Joined
·
22 Posts
Do NOT confuse a "DC Short" with what happens at RF Frequencies. In a Folded Dipole...or a simple Hoop Antenna (e.g. UHF part of Rabbit-Ears, which is a "stretched" version of a Folded Dipole) there is NO "RF Short". The Total LENGTH of the Antenna Element is ROUGHLY one wavelength in the Frequency Band of Interest....so it's a RESONANT structure.
That much I know but how does it affect other frequencies such as a combination antenna? Could an improperly implemented UHF dipole affect VHF performance?

Adding just ONE Director isn't going to do add very much Gain...to add 3 dB of Gain, MULTIPLE Directors need to be added at the RIGHT locations until the LENGTH doubles. AND they have to located at the RIGHT Separations from the rest of the Antenna, or it can actually DEGRADE performance....hence WHY we use Antenna Modeling Software to FIND the RIGHT Dimensions....
I added 4 directors based on the OP's description. If you would like to model the VU-75 and toy around with it, let me know what to measure. It is sitting in the garage for now.
 
1 - 13 of 13 Posts
Top