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Converting UHF Yagi to a VHF Antenna

15K views 103 replies 11 participants last post by  rabbit73 
#1 ·
I have a 5 foot long UHF Uda Yagi style antenna that I have no use for but could use another VHF antenna for a VHF channel broadcast in a different direction from the main VHF stations. I tried giving it away and there were no takers so I was wondering if I could possibly re-purpose it with the right modifications?

I haven't posted in awhile and have to now try to figure out how to post a picture of the antenna which I will figure out and post later.
 
#5 ·
#6 ·
Majortom,

Thanks for the reply. I know there are a few VHF Hi antennas available for purchase.

I just did not want to put this antenna in metal recycling without seeing if it were possible to put it to use as a VHF antenna after modifying it a bit. Seems like a big waste to just toss it.
 
#8 ·
I just did not want to put this antenna in metal recycling without seeing if it were possible to put it to use as a VHF antenna after modifying it a bit. Seems like a big waste to just toss it.
It is possible to modify it, but it would be a major modification. You would be able to use the boom and have to add longer elements for the driven element, reflector, and directors. You would also need a design for the added VHF channel you want. What channel is it?
My situation may be unique. I am in a difficult reception location. I am only about 30 miles from the NY CITY broadcast towers and at a good elevation (close to 700 feet above sea level). However you can see from the previously posted pictures that my line of sight to the broadcast towers is blocked not only by trees and other obstacles but I am on the back side of a hill facing the broadcast towers.
Where are you located? Can you show us your TVFool report and give us the callsign of the VHF channel you want to add with the VHF antenna?
 
#7 ·
You have to bear in mind that things like element length and spacing are important factors in antenna design. Both are significantly different between UHF and VHF, so trying to convert that antenna would be more trouble than it's worth.
 
#9 ·
Another reason I posted this is because there are a lot of experts on this site who just love to tinker and modify antennas. So I gave it a shot thinking someone would have some ideas.

I live in Randolph NJ. The VHF station is NJN VHF 8.1 which previously to broadcast from Montclair as a UHF station 50.1 and was in my LOS to the NY broadcast of VHF stations 7.1(WABC),11.1 (WPIX)and 13.1 (WNET).

It now transmits from Basking Ridge NJ in a completely different direction from the NYC transmitters. So my current VHF antenna does not pick it up.

What I would like to do is remove the first 20 inches of the front of the boom which includes the fist 4 elements to make it a smaller antenna to work with. It is easy to separate as it is held on by a screw. I can show a picture of that in another post as soon as I take it.

Next I would need advice on how many of the remaining elements I would need to keep, their spacing from one another and increase in lengths?

I am open to the possible transformation. It's either that or to recycling center.
 
#14 · (Edited)
I live in Randolph NJ. The VHF station is NJN VHF 8.1 which previously to broadcast from Montclair as a UHF station 50.1 and was in my LOS to the NY broadcast of VHF stations 7.1(WABC),11.1 (WPIX)and 13.1 (WNET).
What VHF antenna are you using now for the NY channels? Will it pick up WNJB if you aim it at WNJB?
What I would like to do is remove the first 20 inches of the front of the boom which includes the fist 4 elements to make it a smaller antenna to work with. It is easy to separate as it is held on by a screw. I can show a picture of that in another post as soon as I take it.
You might need the full length of the boom because the VHF elements are about 3x as long as UHF elements and the spacing of the elements on the boom is also about 3x as much for VHF.

I don't know your exact location, so here is a generic TVFool report for Randolph, NJ:
http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d9038509fbae8d3

This is an image of the generic TVFool report.



This generic report is from rabbitears.info. It should have a more accurate list of channels. WNJN is no longer transmitting; they sold their license in the FCC Repack reverse auction. They are now a Guest on the WNJB transmitter.



https://www.rabbitears.info/market.php?request=station_search&callsign=WNJB

This is the repack plan for NY:
https://www.rabbitears.info/repackchannels.php?country=US&city=&state=&mktid=1&owner=&sort=&ph=&lss=&status=
 
#13 ·
The main boom should be reusable for building a VHF-hi yagi. Start by stripping off all the existing elements and use the boom for building a VHF-hi yagi from and existing design. Check out available designs first as there may be better choices for VHF-hi than a 5' yagi. It might be worthwhile to collapse and store this antenna as-is for future use.
 
#15 ·
Picture of the VHF Antenna I am currently using and aimed at the NYC Broadcast Towers (Empire State Building and ! WTC) almost due east of me).

Below it is the UHF UDA YAGI Antenna no longer mounted and now looking for a new home or to be modified for use as another VHF antenna.




 
#18 ·
Have you tried receiving VHF with the UHF antenna. Not ideal, but depending on signal levels it should work. My CM 4221HD receives RF 8 (about 12 miles away) easily.
Sometimes replacing the balun can improve VHF reception as well.
 
#20 ·
Jorgek,

Actually today I am using a spare CM4228 to pull in that VHF station. It works but the signal strength is in the 80s and quality level is in the 20s. I want this to be a temporary solution.

No I have not tried the UHF UDA YAGI as I did not think it would do a better job than the CM4228 does.
 
#21 ·
Hollands has a 4 element UDA YAGI Antenna that looks like it might not be too hard to build using the parts from this UHF antenna. Might even be able to make it 5 elements?
I was just going to suggest the same thing. I've built two of holl_ands 4 element yagi and they work quite well. This one I installed yesterday in cottage country for my friend, and now able to receive CJOH 13, which is a 2-edge station where he lives. I can provide the dimensions, if you aren't familiar with NEC2 cartesian coordinates...

 
#22 ·
Old Sparks,

Yes send me whatever dimensions etc. that you have on Hollands Small VHF Yagi. I think I have the basics but maybe I missed something.

It looks like you put the directors and reflector right through holes in the boom? Did you do the same thing with the folded dipole?

Thanks!
 
#23 · (Edited)
I mounted all of the parasitic elements through the boom. I use about 3 cm of heatshrink tubing on each element to insulate it from the boom. Once the element is centred in the boom, I use hotmelt glue to hold it in place.

In my first prototype, I mounted the driven element centred above and below the boom using pieces of plexiglass and plastic coax standoffs. In the 2nd yagi, I mounted the driven element through the boom and used a PVC cap to shelter the connections from the weather. All of the elements are 1/4" aluminum rod. I bent the driven element in a jig, using vice grips and a propane torch. I drilled and tapped the driven element for stainless m3 machine screws and connected the balun to the machine screws.

I measured the impedance with my newly constructed antenna analyzer - you could add 5 mm to the length of each element to account for boom effect, but it's really not critical with a wide-band antenna.

 
#25 ·
FYI: Performance for the Hi-VHF 4-el FD-Yagi I Optimized is found here...Dimensions are in 4nec2 *.doc/*.pdf File:
https://imageevent.com/holl_ands/yagis/hivhf4elfdyagiopt

Many more Yagis found here: https://imageevent.com/holl_ands/yagis
Home page entry to over 800 Antenna Analyses: https://imageevent.com/holl_ands

For Full-Band Antennas in Lo-VHF, Hi-VHF and UHF Bands, Boom Correction for Passive Elements is negligible if attached to Top of Boom [but may be Important for Narrow Band, esp. UHF Antennas]. Since Active Element is isolated, away from the Boom, Correction is not needed [although Narrow Band, esp. UHF Antennas may need tweaking to minimize SWR perturbed by stray Capacitance]. Correction is higher if Thru Boom, so you should do your own research and calculation. FYI: My Spread Sheet Calculator example for UHF Band found here:
https://imageevent.com/holl_ands/yagis/uhf13elfdyagi2rropt
 
#28 ·
holl_ands, just a heads up, noticed below Typo in title of each of the charts, graphs, drawings, etc in that VHF Yagi.

eg - "UHF 4-El FD-Yagi - OPT"
 
#29 ·
If your shortened Boom is at least 32-in Long, then you COULD reuse it to mount the much longer elements in the fol. K6STI's Hi-VHF 5-Element Swept Dipole Yagi design [but 75-ohms, so you would need to use 1:1 Coax Balun], to which I added my own Rev A 75-ohm version and my own 300-ohm Swept Folded Dipole version (which is what I recommend for ever so slightly better performance and higher rigidity for the Swept Element):
https://imageevent.com/holl_ands/yagis/k6sti

Hi-VHF Band Gain ~ 7.5 to 8.5 (+/- 0.25) dBi, F/B & F/R Ratio Min = 22.0 dB and SWR (300-ohms) under 1.3.
Boom Length = 30.2-in (75-ohm K6STI), 30.4-in (75-ohm RevA) or 31.4-in (300-ohm FD).

Code:
SY Zf=1.00		' Optimized Separation between Folded Dipole Elements
' Following are Element positions along X-axis:
SY X1=0.00000		' Reflector Element (At X-axis Origin)
SY Xt=4.80000		' Tip of Driven Dipole Element
SY X2=8.74093		' Center of Driven Dipole Element
SY X3=11.68676		' Director #1
SY X4=17.29980		' Director #2
SY X5=30.20938		' Director #3
' Following are Element Half Lengths:  [Double to find overall Width Tip-to-Tip]
SY L1=16.31922		' Reflector
SY L2=15.14195+TT	' Driven Dipole Element
SY L3=12.96533		' Director #1
SY L4=12.47244		' Director #2
SY L5=11.28278		' Director #3
SY Relem=0.1875		' Radius (inches) for All Elements (O.D. = 3/8 inches)

=========================================
If you decide to use the entire 60-in Length of the Boom, then you could use the fol. Optimized Hi-VHF 6-El FD-Yagi (300-ohm) design, which provides a bit more Gain on Ch12/13:
https://imageevent.com/holl_ands/yagis/hivhf6elfdyagiopt

Hi-VHF Raw Gain = 7.9 to 11.2 dBi, F/B & F/R Ratio Min = 17.7 dB and SWR (300-ohms) under 2.4.
Boom Length = 60.06-in

Dimensions extracted from 4nec2 text file: [Only need accuracy to nearest 1/4-inch]
Code:
' Cumulative Element Spacings and Delta's (min, max):
SY S1=0
SY D2=10.45
SY S2=S1+D2
SY D3=4.06
SY S3=S2+D3
SY D4=7.756
SY S4=S3+D4
SY D5=20.33
SY S5=S4+D5
SY D6=17.46
SY S6=S5+D6
'
' Element HALF Lengths:  [Double to find overall Width Tip-to-Tip]
SY L1=17.10
SY L2=15.02
SY L3=12.87
SY L4=12.44
SY L5=11.46
SY L6=11.42
'
SY FW=1.5		' Distance between Folded Dipole Elements
SY F1=FW-0.875	' Distance between Center of Boom and Folded Dipole Element
SY Relem=0.375/2	' Radius (inches) for Passive Reflector & Director Elements
SY RACT=0.375/2    	' Radius (inches) for Driven Element
 
#30 ·
Thanks for noticing and reporting my little Cut&Paste error....from 3-years ago....it took 30-min, but is now fixed.

BTW: Although imageevent is very speedy for most chores, for some reason EDIT is super-SSSLLLOOOOOWWWW.....even after I lowered priority of both PYTHON.EXE processes that are attempting to Optimize 4-El and 5-El FLF (Flat Loop Fed) Yagi's...
 
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