[jamesm113]

Hi folks,
Long story short, I am trying receive a RF10 transmitter that is 3 miles away but only 100 watts. Getting RF8 is a secondary objective, but that station is operating at 3kW from the same antenna farm and is easy to receive reliably. The RF10 signal is only one very small spot, in the corner of the attic, touching the roof, and the best success I've had is the Antenna's Direct VHF kit simply because it's small enough to fit where the signal is.

Attached pictures show the space I'm working with. The new antenna needs to be at least be as high as the current VHF antenna to get signal. The 2x4's make a rectangular area around roughly 23"x 25". Ideally would like to avoid going behind the 2x4's, as it that does make aiming more complicated and I suppose can lead to signal loss.

The bigger antennas I've tried (ClearStream 5, YA7000) get worse signal quality because they do not fit. With the AD VHF kit, I get a steady 98% signal quality most of the time, but still gets some drop outs at randomly. I suspect being in the attic, there is multipath and other household interference that might be mitigated somewhat by installing an antenna that has more directionality and gain than a simple dipole.

To summarize, my goal is for the new antenna to have more gain and directionality vs the AD VHF dipole kit, but small enough to fit in a small footprint.

I have a few ideas, but would love some input:

• Off the shelf VHF antenna modified to fit
  • Stellar Labs 30-2475 Fringe VHF - If I cut remove the top reflector and cut it in half, I could make it fit
  • Two piece RCA ANT751x with the UHF portion of the boom removed
  • Televes 106501 cut in half
  • Televes 106601 cut behind the dipole
• DIY dipole specifically for channel 10. The AD VHF kit is 35 1/16" wide. A dipole calculator says a 195 MHz dipole would be 28.8" wide. I don't think this would buy any more gain than the current AD VHF dipole, but due to the angle of the roof, I could move it up slightly - maybe another inch or so
• DIY 2 or 3 element Yagi designed for 195MHz (inspired by First Post, DIY VHF Hi Yagi)
  • The wireantennas.co.uk calculator spits out a 30"x8" 2 element yagi and a 31"x16" 3 element yagi
  • The Chang Puak antenna calculator spits out a 33.6" by 19.4" 3 element yagi

 

[ExDilbert]

It's probably not necessary to "build" an antenna. Simply placing a reflector, mesh or element(s), behind the dipole should improve gain. Similarly, directors could be simply placed ahead of the dipole but a good reflector should provide the best improvement. Without a boom, modelling might provide slightly different sizes and distances but that should not be critical due to the already strong signal. I would just try a reflector of the length specified by an existing model (slightly longer than the dipole) and use a signal meter to optimize the distance.

 

[jamesm113]

It's probably not necessary to "build" an antenna. Simply placing a reflector, mesh or element(s), behind the dipole should improve gain. Similarly, directors could be simply placed ahead of the dipole but a good reflector should provide the best improvement. Without a boom, modelling might provide slightly different sizes and distances but that should not be critical due to the already strong signal. I would just try a reflector of the length specified by an existing model (slightly longer than the dipole) and use a signal meter to optimize the distance.

Sounds good, I'll focus on the reflector first. Would a small strip aluminum foil glued to cardboard work as a decent prototype?

Should the reflector length be greater than the current dipole or sized according to the model? Does it make sense to shorten the dipole to match an existing model? It's currently 35.0625" wide, and it seems a 195 MHz antenna should be 28.8" wide. Seems like 35 1/6" is designed for ~160 MHz.

 

[ExDilbert]

Reflectors are always longer. The exact length and distance are not critical but they may need to be adjusted to achieve the desired results. It's best to model it with software. Looking at an existing yagi antenna and using the same ratio of dipole to reflector length and distance would be a good start. Tuning the dipole and other elements for a single channel is a good idea in this case. General purpose antennas are usually tuned for good performance over a wide bandwidth but yagis can be optimized for better performance on a single channel. I believe there are some designs for single channel VHF yagis on this site which could help.

 

[majortom]

for $23 US + shipping u can get urself a real Yagi.

https://www.newark.com/stellar-labs/30-2476/deep-fringe-directional-antenna/dp/71Y5462

Not sayin. Just sayin.

 

[jamesm113]

for $23 US + shipping u can get urself a real Yagi.

https://www.newark.com/stellar-labs/30-2476/deep-fringe-directional-antenna/dp/71Y5462

Not sayin. Just sayin.

Thanks, didn't realize the deep fringe was cheaper than the fringe one. I'm definitely open to using an off the shelf antenna, just curious if I can hack that one up to make it fit and still have it work better than my current antenna. I'd probably need to remove everything in front of the first director and maybe just use one reflector element instead of all 4.

Also not sure if the folded dipole would still fit where the signal is.

 

[old sparks]

It would likely be cheaper to buy the Stellar Labs antenna and modify it, rather than buy the aluminum and building one (like I did).

I'm sure some kind soul could model a shortened version with 4NEC2, since the input file exists on Hol_lands webpage

HiVHF 13El FD-Yagi - Stellar Labs 302476 (imageevent.com)

 

[old sparks]

So, as a simple 4 element yagi with 1 reflector and 2 directors, the 30-2476 free space pattern is something like this

 

[jamesm113]

Thanks for posting that!

I'd likely only be able to fit a 2 or 3 element yagi - so 1 reflector and/or 1 director.

I've tried playing around with 4nec before, but did not have much luck. I'll try again!

 

[old sparks]

Here's the 4 element nec2 input file.

https://www.qsl.net/v/va3rr//hdtv/cutdown302476.NEC

 

[jamesm113]

Ok gave it a try. Basically just deleted lines from your model file. I kept getting a "Errors or warnings found, run segment check" error, but managed to get some output anyway.

Made 4 different diagrams - one for just a dipole, one for a 3 element, one for reflector only, one for director only.

 

[old sparks]

Good job! Segment warning aren't a big deal, as long as the various elements have similar length/segment ratios.

I meant to mention that having the UHF antenna in front of your VHF yagi will likely skew the pattern of the yagi somewhat...

 

[jamesm113]

I can probably move it to be more underneath the yagi. It also needs to be fairly high up, but it's not as sensitive to location as the VHF is

 

[ljhavener1]

Segment warnings will go away, if you set auto segment on. Per comments autoseg(13) should take care of the segment warnings. Change to Auto Segment under the settings tab and set segments per half wave to 13. This will give you more accurate results, but won't really change your results that much.
As you can see, the reflector makes the most difference, for your application, and it looks like you should have enough room to add a reflector.

 

[jamesm113]

I picked up an RCA 754E from Walmart. I was hoping for a 4 element version of the RCA 75xx, but when I opened the box it's the 3 element. Figure it's easy to return if it does not work out.

Rough measurements:
Boom Length 16 1/16"
Rear LPDA element 34 1/4" wide, 1 1/8" from the end of the boom.
Front LPDA element 29 1/4" wide, 10 7/16" from rear element
Front director 25" wide, 13 15/16" from rear element

Couldn't get signal in the exact same spot as the old antenna, but if I put the rear element behind a 2x4, I can get 98% signal quality again. Will run this for a few days to see if drop-outs improve.🤞

 

[jamesm113]

No improvement with the RCA. If the stars align I'll get a 98% signal, though most of the time it hovers around 90% and signal drops into the 70s are common, with drop-outs below 70% (aka impacting picture) somewhat frequent.

Switching the VHF-1 back in, but keeping the USVJ and cables setup used with the RCA, the signal quality immediately went up to 98%. So the issue is either with the antenna, it's placement or the balun. I don't think I could have done a much job placing it.

I have a stellar labs 30-2475 (regular VHF, not the fringe previously discussed) on order, arriving next week.

If no luck with that, I'll consider trying a shorter VHF-1 kit and perhaps DIY'ing a reflector and director out of 3/8" or 1/2" copper pipe and a PVC boom. Antenna's Direct support staff told me the current VHF-1 kit dipole length is 31.5" and said the ClearStream 2V's VHF dipole's length is 38". They said the different lengths were for various product design reasons and have negligible impact of reception performance. That being said, if I could shorten it, I can get the antenna higher maybe an inch higher, which seems to make a big difference in the reliability of signal quality where I am.