my 88-92mhz antenna half done??

kyrie59

Member
0306181946.jpg 0326181714.jpg Been working on my 88-92mhz specialty antenna for months, in spare time... and seems to be going OK. Building this from a 20 yr old VHF/UHF/FM antenna.
Was hoping for some thoughts opinions etc... Here's a couple pics... Thanks!!
 
What are the element lengths and spacing?

If it is going to be used outside, are the zip ties UV resistant?

What type of feed will you use?

This is the link to the antenna... found it here on AK.
http://ham-radio.com/k6sti/88-92.htm
Great idea about the UV ties, I didn't think of it. That woulda been a blast, start finding the elements laying in the yard.
I'm still trying to figure out the info on the feed from the dipole... not much info on the site for laymen like myself.

Thanks for the help, a big help!!
 
Here is what i would do Cut driven element to length and remove 1" from center .Use arrow shaft to fill center in into each element 4" . Add bolts to attach 75 ohm lead and you are done. Arrow shaft or what ever works. A solid 8db across the lowerend of the band. band is good. A few years back I built a log periodic 8 element that gave me a solid 8 db across the entire band. Had to take it to the dump last month as I am moving. No one wanted it. It was free.
EddyR
 
A solid 8db across the lowerend of the band.


dBi or dBd? Antenna gain numbers are meaningless unless this is specified.

If a balanced antenna is directly fed with coaxial cable, the common mode current of the outside of the coaxial cable can have a negative impact on the performance of the antenna. One should at least us some ferrite common mode chokes at the feed point.

As you are investigating antenna design, remember antenna gain is more about boom length than the number of elements.

Although the impedance of the dipole in free space is about 75 Ohms, this is altered by the proximity of the parasitic elements. Simple matching is typically addressed by adjusting the length of the fed element.
 
Here is what i would do Cut driven element to length and remove 1" from center .Use arrow shaft to fill center in into each element 4" . Add bolts to attach 75 ohm lead and you are done. Arrow shaft or what ever works. A solid 8db across the lowerend of the band. band is good. A few years back I built a log periodic 8 element that gave me a solid 8 db across the entire band. Had to take it to the dump last month as I am moving. No one wanted it. It was free.
EddyR
Thanks for the help... fiberglass arrow, I was trying to think of what to use, but nothing so far.
Sorry about your antenna, too bad you couldn't take it with you.
 
" I like Music " All that you said is true but the article he is using tell how to match the lead to the driven element. It mentions 1/4" spacing. But as you know the impedance goes up as you move the feed off center of the driven element. The article use the term db and dBd. I always avoid the close spacing of reflector and first element to the driven element as it drives the impedance down. I would think this antenna is down around 20-30 ohms.
" I Love Music" Why not show what he can do or direct him to a article rather than just point all the the problems.
 
Here is the impedance and mismatch loss for this antenna if the op build it per design and uses it in more less free space.

upload_2018-3-28_14-20-49.png

It appears to somewhat on the high side.

I Love Music" Why not show what he can do or direct him to a article rather than just point all the the problems.

I believe that this is what I did.

If a balanced antenna is directly fed with coaxial cable, the common mode current of the outside of the coaxial cable can have a negative impact on the performance of the antenna. One should at least us some ferrite common mode chokes at the feed point.

As Brian as points out, a coaxial common mode choke provides a reasonable amount of common mode current reduction, but using several ferrite common mode chokes may be easier to implement for some.

solid 8 db across the entire band.

I do not understand your issue with using the correct terminology. Is the quoted gain above dBi or dBd, it makes a difference, that is all.

I have shared accurate information for OP and others that might be interested in building an FM broadcast band antenna.

For those that are looking for plans, it is not a bad idea to understand the role that boom length plays in the gain of the antenna.

Here is what i would do Cut driven element to length and remove 1" from center

I do not know the actual length of the OP's driven element, but if it is per Brian's design, there should not be a need to make it shorter. Brain gives the dimensions of the driven element and how he matched it, impedance wise. Why would it need to be shorter? Why would one need to remove an inch?

Of course, if one has access to a VNA or other form of antenna analyzer, one can teak the antenna how they want.

The article use the term db and dBd.

Yes it does, but for different things. There is a difference that needs to be understood. dB by its self is just a ratio. Because an antenna is a passive device, gain must be relative to some reference. When it comes to antennas it is usually an isotopic (non-existent) ideal radiator or a dipole antenna.

dB by its self is used when he talks about antenna loss and F/R ratio because a simple ratio is correct and dBd is used when he talks about antenna gain, because it is relative or referenced to the performance of a dipole antenna.

He could have used dBi as the reference and that would have increase his numbers by about 2.15 so it can make a difference when specifying the performance of an antenna or when comparing two different antennas.

Again, I mean no disrespect, I am just curious about the gain of the antenna that you built.

Is the gain in dBi or dBd?

upload_2018-3-28_14-50-37.png
 
kyrie59, if you are careful with your build you build, the performance of your antenna should come quite close to Brian's numbers.

I was building antennas before the transistor was invented (wow that makes me feel old) because of my interest in amateur radio as a hobby and that lead to a stint as a professional antenna engineer among other things. It is cool when the signal that you are listening to is being picked up by an antenna that you built.

Good luck with your project and keep us updated.
 
I'm 60 yrs old, but know very little about antennas, maybe shouldn't be attempting this build, but what the heck, ain't going get younger no time soon.
I've been living by the motto "Reduce Reuse and Recycle" for decades, and always encourage others to do the same. I've gotten the UHF end of the old antenna, cleaned and ready to go...am using 7 of 15 elements and boom for this build, and have 8 elements left for another project. Now that's Recycling!!

About the time I'm finished, they'll kill OTA broadcasts... but lets hope not.

Thanks again to you both for even more help... You guys have a nice tomorrow!!
And I'll update, and hopefully share something others can use.
 
It looks like "kyrie59" has elements cut to length. The driven element needs to be cut at center that is why I mentioned the one inch cut out at center. The article calls for 1/4" Moving the center spacing to one inch will lead to less chance of failure at the center and will bring the impedance up a small amount. I said" this is what I would do " I agreed with you.What more do you want than for someone to agree with you.
Here is a link to the antenna I built.http://www.rescueelectronics.com/FM_Antennas.html
I built this antenna in the single and dual boom version.
I started in the day of TV Rhombic Antennas long before the CK722 but who cares.
EddyR
 
Okay, to the gain of your antenna is in dBi.

upload_2018-3-29_13-16-22.png

It is no big deal, I was just curious, that is why I asked. This information is needed to make a fair comparison of antennas,

For comparison to the antenna the OP is building, this antenna would have 6.1 dBd of gain if both were compared to a dipole antenna. I only mention this because there a lot of antennas on the market the give their gain in dB only and that can mean anything.

And that difference of a couple of additional dB can help overcome, for example, some feed line loss.

Antenna theory tells us that in general log periodic antennas trade a little bit of gain for more even gain over a wider range of frequencies.

Each type of antenna has it application depending on the needs of the user.

The OP has the plans to an antenna from a very knowledgeable source with the data to confirm the dimensions. This data includes feed point impedance if built as designed. I just mentioned that there is no reason to deviate from the published dimensions unless one has a way to verify the results of the changes.

The driven element needs to be cut at center that is why I mentioned the one inch cut out at center.

There are others that are interested in building FM antennas. A one inch spacing is not the same as cutting out 1 inch of material from the one piece driven element when the dimension of the element is already given. That is why I mentioned it.

I said" this is what I would do " I agreed with you.What more do you want than for someone to agree with you.

QUOTE="EddyR, post: 11463852, member: 7950"]Here is what i would do Cut driven element to length and remove 1" from center[/QUOTE]

Again, I meant no disrespect. I asked you a basic question about the gain of your antenna, and why you would recommend deviating from the antenna dimensions given by Brian by removing an inch of material.
 
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