Low VHF antenna designs

[eggman531916]

Also, yes, it's WKBS.

[rabbit73]

Originally Posted by eggman531916

http://www.tvfool.com/?option=com_wr...90386ebd6201d7
My TVFool report. It's not very accurate as these are the old listings and the channel I'm trying to get isn't even on there BEFORE the switch. Actually, my NBC station used to come in on RF 34 which isn't on the report either. They switched to RF 35 around the same time and signal now cuts in and out. Maybe an LTE filter is needed? I don't know. I haven't had a chance to try anything yet. Hopefully over the weekend. I apologize as I just now have only had a chance to check the thread and am getting ready to run out the door to work. I'll look over everything everyone has posted ASAP. Thank you all.

Thank you for the report. Yes, there are many errors in recent TVFool reports because they are using a defective database to generate reports. Their staff is too small to make corrections and the frequent channel changes because of Repack by the FCC make it worse. Their reports still have some value. Your report allowed me to estimate your location to make a more accurate report from rabbitears.info. This is an image of your TVFool report:



This is a report from rabbitears.info which should have a more accurate list of channels:
https://www.rabbitears.info/searchma...study_id=25786

You can do your own report here; I used coordinates for the one I did:
https://www.rabbitears.info/searchmap.php

This is an extract from your rabbitears.info report:



I guess I don't need to tell you that your location is not idea for OTA reception.

Actually, my NBC station used to come in on RF 34 which isn't on the report either. They switched to RF 35 around the same time and signal now cuts in and out.

WJAC NBC has more than one transmitter.
https://www.rabbitears.info/tvq.php?...ms&facid=73120
The strongest one listed on your report is on channel 22, but it is only running 7.5 kW ERP. WJAC on RF35 is further down on the list and over 50 miles away. It is running 1000 kW. Both signals are blocked by terrain. Click on the transmitter distance in the report for each channel to see the terrain profile.





A lot of hills in the signal path that block the direct signal.

[eggman531916]

I had wondered about an FM antenna. Which sucks, because I gave one away about a year and a half ago. And I really like the idea of using the extensions on a smaller antenna, but, most likely I'm going to try the dipole outside over the weekend first. I have a pair of rabbit ears that I measured out to 2.75 feet on each side and took it to the attic late last night. When I scanned channels 2-13, it stops for about 20 seconds about halfway through. So, I'm guessing it's trying to lock onto something but there just isn't quite enough there. I know, it's in the attic. There is a spot outside on my property that I used to be able to get it before but it's at ground level. Also, I remembered I had collected and kept some elements off of an old full range antenna someone was throwing out so I will probably try and build a yagi (my first one) using those. Also, all the pics REALLY help! Thank you again.

[Tim]

Originally Posted by eggman531916

http://www.tvfool.com/?option=com_wr...90386ebd6201d7
My TVFool report. It's not very accurate as these are the old listings and the channel I'm trying to get isn't even on there BEFORE the switch.

A RabbitEars report would be helpful and that information is much more up to date.
https://www.rabbitears.info/searchmap.php

[Tower Guy]

I did this 4 element design for channel 6 using EZ-NEC:

3/8" diameter elements.

Reflector length 71.6"
Driven length 72.4"
Director #1 61.2"
Director #2 57"

spacing

Ref-Driven 22.8"
Driven-D1 3.1"
D1-D2 20.1"

The feed is direct using 75 ohm coax; no balun.

If you want to play download this: https://www.eznec.com/demoinfo.htm

[rabbit73]

Originally Posted by Tower Guy

If you want to play download this: https://www.eznec.com/demoinfo.htm

Excellent!

Thank you for the link. I didn't know he had a demo.

[eggman531916]

My RabbitFool report, err, Rabbitears report:
https://www.rabbitears.info/searchma...study_id=26219

[eggman531916]

Thank you Tower Guy for the dimensions on the Yagi. One question on it is though, what is the spacing on the feedpoint for the driven element? Though I've tried to teach myself how to use them in the past, I've never been good with NEC programs and rely heavily on people posting dimensions (or pictures with dimensions).

[Tower Guy]

The driven element is fed in the center.

[rabbit73]

Originally Posted by eggman531916

My RabbitFool report, err, Rabbitears report:
https://www.rabbitears.info/searchma...study_id=26219

Thank you for the rabbitears.info report; it's pretty close to my estimated report for your location that I used above:
https://www.rabbitears.info/searchma...study_id=25786

[eggman531916]

Originally Posted by Tower Guy

The driven element is fed in the center.

I guess what I meant was like if I was using a balun to attach the cable to the antenna. Like how many have a 1 1/4" space on the driven element of a 4 bay or a 44mm or 88mm feedpoint gap on a Gray Hoverman. I didn't know if there had to be a specific measurement on the gap or not for impedance. I know you said there's no balun, just cable. If this doesn't make sense, I apologize. I'm not fully caffeinated yet this morning.

[Tower Guy]

You may use any reasonable gap that is convenient.

[GroundUrMast]

Originally Posted by Tower Guy

You may use any reasonable gap that is convenient.

Agreed; If this was a transmitting antenna you would want to pay close attention to precise impedance matching so as to minimize power reflected back into the final amplifier stage of the transmitter. In a receiving application where you are simply trusting the design measurements and have no means of accurately testing the impedance of the actual as built antenna, the feed point gap is not going to be the one factor that makes or breaks the success of your project.

[eggman531916]

Well, I tried a pair of rabbit ears measured out for RF 6 with no luck. The small TV I carried up onto my shed roof again tries to lock on to something, but can't quite get it. Without the rabbit ears, my channel scan zooms right through with no stalls. For kicks I hooked up one of those RCA preamps just to try it and still nothing. I wimped out and ordered a FM yagi because it will get here before I get a chance to build anything. My wife is gonna kill me when she finds I bought yet another antenna as I ran into what I thought was a sweet deal at Ollie's (good stuff cheap!) on some Antennas Direct DB8 antennas for 50$ a piece. So I bought 3 of them to play with and use for future installs. Guess I should get used to sleeping on the couch.

[rabbit73]

Originally Posted by eggman531916

Well, I tried a pair of rabbit ears measured out for RF 6 with no luck. The small TV I carried up onto my shed roof again tries to lock on to something, but can't quite get it. Without the rabbit ears, my channel scan zooms right through with no stalls.

Sounds like the TV tuner tried to lock on to the signal, but the signal wasn't quite strong enough above the noise for a good SNR.

For kicks I hooked up one of those RCA preamps just to try it and still nothing.

Yeah, I tried that too. It made the signal AND the noise stronger for no net improvement.

So I bought 3 of them to play with and use for future installs. Guess I should get used to sleeping on the couch.

I don't usually watch the Roseanne show, but she said something interesting:
"A wife should let her husband have his hobbies so that he doesn't notice she is running his life."

[eggman531916]

Success! Well, sort of. My FM antenna was delivered today so even though it was pretty cold here, I had to go out and play with it. Something is up with my rooftop antenna setup, so I installed a temporary setup behind my shed where I can just snag my local stations around the neighbor's house. It's not ideal, but it'll get me through the winter. I had to get it up about 15' to 20' up, but the FM antenna will lock onto RF 6, though HEAVILY pixelated and pretty much unwatchable. BUT it's a step forward. I ran it through the RCA PRAMP1R on the VHF input. I want to try just using a straight cord to the TV in the next couple of days and see how that works. Would've done it today, but I lost daylight too quickly. I should also mention that this is being sent to 4 different TV's. Anyways, I bought this antenna:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I don't know of any Low-VHF only antennas, although I understand that the larger ones (usually made by Channel Master, R.C.A., and Winegard) have long dipoles near their ends that are designed to receive low-VHF channels. Rabbit ears still seem to be readily available, although I assume indoor reception would be a problem regarding the effects of impulse noise on the low-VHF signals since they would already be weakened by the exterior walls.

I'm also not sure if there are any ways to improve gain. I don't imagine that the corner reflectors seen on U.H.F. antennas would be practical as they would be rather large and cumbersome, so I would assume the only way to improve gain for the low-VHF channels would be to add more parasitic elements/directors.

I hope this helps.

[eggman531916]

So, I've decided to start gathering materials needed for a 5 element Yagi found on the Yagi calculator page that GroundUrMast posted earlier. I decided to go with the folded dipole calculator as well. All to the tune of 86 MHz. Now this causes me to ask a question or two. The main question is, how important is it that I stick to the rod diameter for the folded dipole? It's calling for a diameter just a hair under 1/2" and I was wondering if 3/8 would work just as well? I ask because benders can get a little pricey. Also, I've seen folded dipoles now with both a curve on the end and squared ends. So my next question is, could I just keep the same distances (with the exception of the curves) and simply use 90 degree elbows and square it off? Or, in my ignorance, am I missing something here? Thank you in advance for any wisdom you all can bestow upon me.

[Tim]

Originally Posted by eggman531916

So, I've decided to start gathering materials needed for a 5 element Yagi found on the Yagi calculator page that GroundUrMast posted earlier. I decided to go with the folded dipole calculator as well. All to the tune of 86 MHz. Now this causes me to ask a question or two. The main question is, how important is it that I stick to the rod diameter for the folded dipole? It's calling for a diameter just a hair under 1/2" and I was wondering if 3/8 would work just as well? I ask because benders can get a little pricey. Also, I've seen folded dipoles now with both a curve on the end and squared ends. So my next question is, could I just keep the same distances (with the exception of the curves) and simply use 90 degree elbows and square it off? Or, in my ignorance, am I missing something here? Thank you in advance for any wisdom you all can bestow upon me.

I can't answer your question about diameter with a formula or antenna modeling facts, but from practical experience building ham radio antennas, I would suspect that making your tubing diameter smaller would change your resonant frequency by 2 or 3 MHz at a design frequency of 86 MHz and that technically you would have to change the element length a small amount to compensate. It would also technically affect what your spacing should be between elements. From a practical viewpoint, I suspect that it would have very little effect on the performance of your antenna for receiving purposes.

As far as the folded dipole driven element, from a practical perspective I suspect that either curved ends or squared ends will work fine as long as you keep the relative dimensions the same.

Another thought to keep in mind is that an antenna is usually more broad banded when it is constructed of larger diameter elements.

There are many, many variables in yagi antenna design. Someone with antenna modeling software could probably give you some more specific answers than my general ramblings and observations.

[rabbit73]

I agree with Tim on the general answers and that precise answers would require antenna modeling.

Channel 6 is 82 to 88 MHz, so center freq would be 85 MHz.