Astrobeam

[Marconi]

I definitely do not recommend putting the rotor that way, I have learned a couple things since then...lol

BTW 2020, I just noticed that the AstroBeam thread on MD's was posted last year, and Timespy took his beam less than 2 months after installing it. He replaced it with an Imax and it probably worked better than his beam.

He probably doesn't have a clue even now...why that A/B didn't work well for him. He just claimed it started falling apart. So, we'll never know for sure if what we saw in the pictures is correct, but I don't doubt those errors are possible.

You did good to notice his possible errors.

 

[RT2020]

Avanti AV-150 "Astro-Beam"

I have the manual and have read it so many times the pages are almost wore out...lol

 

[RT2020]

OK, I messed with the beam for about 3 hours Sun. I started out with stock lengths and no pigtail to get a baseline, it was terrible 4.3 on Channel 1 and 3.4 on 40 so I took it down and installed the pigtail just for giggles. I still had a 4.3 on 1 and then ended up with a 4.7 on 40.
The next time I took marconis advice and lengthened out the driven stinger to 86" for a start and got 2.8 on 1 and 2.9 on 40 and was still confused. I lowered the driven stinger to 82" and this is when I discovered my first mistake after watching the 259B, the readings were jumping all over the place and I couldn't figure out why. In my effort to stay as far away from the antenna as possible I was pulling the coax into the hoop section, I corrected that and let it hang down straight and the readings stabilized and were 1.1 on 1 with R=59 X=0 and on 40 R=78 X=32 and the SWR was 1.9 I ended up shortening until I got to 74 5/8" and got R=40 X=4 and SWR of 1.2 on 1 and R=64 X=0 and SWR of 1.2 on 40.

I was happy until I realized my second mistake, I used a 18' piece of coax to test with,(the antenna is 15' off the ground). When I climbed the tower and retrieved my coax and went into the shack to test it I was right back where I started.

I ended up with 89" on the driven stinger in the end and have R=54 X=15 and SWR 1.3 on 1 and R=37 X=6 and SWR1.3 on 40.

Once again I am happy until I turn the power on and with the SWR meter inline the SWR jumps to a 3. I still see the same output as before, around 500 watts. Once again I am confused. I have a Palstar WM150 meter and it does not show any SWR with the amp off. Maybe my meter is not functioning properly but with the Avanti Sigma IV I have up now it shows 1.3 on my MFJ and with the power on with that antenna it doesnt show SWR with the amp on.

I wonder why the SWR changed when I switched the coax, does this show another problem somewhere?

Any suggestions Marconi?









=

 

[Lil'Yeshua]

http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf
For your reading enjoyment.

 

[Marconi]

I thought you were good to go.

How did you reach the feed point and the top element to tune that bugger with it on a tower that high? I may have misunderstood, so tell me how you are making adjustments, at what height and what is around the antenna?

Recently I asked someone to try different frequencies with an analyzer and let me know if they were able to find if and where the antenna tune looked good, and then give me some results. Was that you 2020? Too much going on here for me to remember, plus I'm closing in on 75 years old and I don't try to depend on my memory with other folks issues.

One thing I know about vertical antennas using an analyzer, the farther away you are from true resonance (x), the more changing the feed line length will mess with your head, and you could feel like you're chasing your tail trying to tune. Regardless of what the X reading shows on your 259B meter or any meter at the end of such a reactive feed line...the reading is not accurate. Does that make sense? An analyzer is a tool, not a cure all device.

I know folks will tell you as long as you use a 1/2 wave coax it will show you what is going on at the feed point, right? Well, first off an 18' piece of coax is not a tuned 1/2 wave length. (492 x VF) / frequency = 1/2 wavelength good enough for government work.

However, if your antenna is off just enough, then this tuned coax length will also be off, and the results will be wrong. You are seeing transformation by the feed line due to reactance. If you don't understand this, then just do your tuning with any length coax that is convenient. As you get closer to a good match with either a tuned 1/2 wave line or a random length line your results will end up about the same. The key here is getting the antenna dimensions close to resonance, because that is what resonance is all about.

Try this, hook up your meter to the antenna using your 259B. Read and record what results you think are important. Then tell me what that tells you, and if it tells you what is going on at the feed point and see if that tells you what to do next.

Do you remember old radios that required you to turn a dial to the station you wanted? If so, then you realize you either know the frequency number and get close before you start to fine tune, or else you start turning the knob all the way across the dial until you hear what you think is the right station and depend on luck. Antenna tuning is about the same, so knowledge about what you are doing comes first. If you are building an antenna from scratch...then things are a bit different.

Trying to fine tune at a point where the antenna is wildly out of tune or there are a construction issues...will drive you crazy, because every time you make a change somewhere the 259B results will likely be wildly different too, and NO special length tuner line will help you...........that is where the luck comes in.

This is why I have been trying to find someone to confirm what the radiator length should be at 27.205 for the AstroBeam.

On my AstroPlane I find 87.5" gives me resonance at 27.205. Did I send you my Antenna Work Sheet results. Once you get physically close to the correct setup and have good construction, you should easily be able to fine tune that bugger, even if you only have to move something 1/16" inch or less may be all you need.

When you get this close it is like tuning a guitar string and antennas respond just that way. It is not always easy, but that is the way it is. That is also why Avanti made the A/B kit like they did, with the instructions worded like plug and play.

I have never built or used an A/B antenna, so I'm shooting in the dark with what you have. So I have to take it for granted that everything is correct in your construction and is fully tight, but not striped. IMO, most of the problems with this antenna are physical construction problems. I know that from just reading the manual. The manual does not talk about tuning, and thus this antenna was not meant to tune. It can be tuned, but the kit was not designed that way.

When I have trouble with tuning a beam, I take all the elements off except the driven element, and see if it tunes right just by itself sitting on the boom. It's a lot of work, but you may have to do it.

I am also concerned about the fact you told somebody that the #2 top element for the radiator was not and exact fit. Bad fitting aluminum tubing can present issues and they may present as intermittent.

We also discussed briefly an issue with your antenna frequency and the short radiator you first used. If it was short or shorter than it should be then it should be high in frequency...and you said yours showed about 26.000 mhz.
On the other hand we've already talked about it, and we disagree on the results you reported. I just say that is an issue that needs to be resolved, but I'll leave that up to you to figure out.

How about some closeup pictures of the important construction areas on the antenna?

 

[Shockwave]

Keep in mind the Astro Beam will not tune the same as the Astro Plane. Each parasitic element you add will reduce the impedance of the driven element. This also means that if you tune the AB without the reflector or director, it will not be the same as when you add them.

 

[HomerBB]

If the parasitics reduce the impedance, and the antenna is now lower than 50 Ohms at the feedpoint resulting in a high SWR, would that not lead us to assume we need a longer driven stinger (I haven't tuned an Astrobeam, either)? <--- serious question

Read the MFJ-259b and determine if your impedance is too hgh, or too low with a e1/2^ jumper to decide which way to go with your length assuming the previous question is answered yes.

And, I would be sure the spreader rod that sets the distance of the two decending elements (skirt) is set properly as tuning the Astroplane can be a combination of the spreader positioning (to a limited point) and the length of the upper radiator length and width of the Cap Hat. You have no cap hat on the beam so you will depend on subtle adjustments of the spreader (not a big player) and the upper radiator.

I tend to agree with Marconi that your feedline may be screwing up your reading based on how it is cut. AND I agree with him that your antenna must be dead on or very close for the feedline to matter one way or the other. If your antenna is good with the e1/2^ jumper, then look at the condition of your feedline.

 

[Marconi]

Keep in mind the Astro Beam will not tune the same as the Astro Plane. Each parasitic element you add will reduce the impedance of the driven element. This also means that if you tune the AB without the reflector or director, it will not be the same as when you add them.

SW, I agree and I've made the same distinction before. If I didn't state it here, I should have.

For some reason I was surprised to hear Tuner's beam was not working right. Is this just another of a long line of scenarios that explains the antenna is an enigma, and probably junk...that has been way over sold.

The only thing my Eznec model shows for this one is a very broad power lobe low to the horizon that ranges in near max power from 6* degrees up to 32* degrees and it possibly should work DX well. I don't see the big rejection we often hear talked about, but right off the backend is maybe a narrow beamwidth that is 20-30 db down from maximum, but it looks to go to heck in a handbasket fast on moving a little.

What more can I say?

 

[Shockwave]

The only real advantage I can see with the AB is that is removes some of the problems with the mast being so close to the driven element on a 3 element beam. Unfortunately it does this by turning the mast into part of the antenna. I suspect it may be more susceptible to CMC issues as a result.

 

[Marconi]

If the parasitics reduce the impedance, and the antenna is now lower than 50 Ohms at the feedpoint resulting in a high SWR, would that not lead us to assume we need a longer driven stinger (I haven't tuned an Astrobeam, either)? <--- serious question

Read the MFJ-259b and determine if your impedance is too hgh, or too low with a e1/2^ jumper to decide which way to go with your length assuming the previous question is answered yes.

And, I would be sure the spreader rod that sets the distance of the two decending elements (skirt) is set properly as tuning the Astroplane can be a combination of the spreader positioning (to a limited point) and the length of the upper radiator length and width of the Cap Hat. You have no cap hat on the beam so you will depend on subtle adjustments of the spreader (not a big player) and the upper radiator.

I tend to agree with Marconi that your feedline may be screwing up your reading based on how it is cut. AND I agree with him that your antenna must be dead on or very close for the feedline to matter one way or the other. If your antenna is good with the e1/2^ jumper, then look at the condition of your feedline.

Homer, I agree your point is serious too.

Tuner and I talked about his results earlier. I see the tune results he noted as being the opposite of what I figured. At first his top radiator was <>72" inches and his 259B showed resonance at 26.000. I claimed he might need to make his top radiator longer at about 87.5", but that would make his frequency go even lower, thus our ideas are the opposite. I'm leaving that further understanding up to him.

Regarding the spreader and the tune. I think this antenna is fixed in this regard, and the spreader may not truly be adjustable. In my Exnec model it shows to effect the tune OK, but its proper location looks to be very sensitive and very tricky as to its position relative to the tune and I think I see mostly resonance changing. If this is like I see in my model, and if its construction is like it is with my knockoff version (not adjustable), then this is just another argument supporting my claim...this antenna's construction is plug and play, and was never meant to be tunable or sold as tunable.

Excepting maybe for the 2nd element on the top radiator for the beam, which is a new item added, so the A/P driven element is at a full 1/4 wavelength for the radiator.

On your old original A/P, was the spreader adjustable? If so, did you ever check out moving it to change the bow in the down radials and thus your tune?

I also don't reject the idea that the short dual RG59/u tuner jumper can play a part in the tune of this antenna however, but it doesn't make sense that some use it and some don't, but still claim it works just fine.

This one is truly and enigma.

 

[Marconi]

The only real advantage I can see with the AB is that is removes some of the problems with the mast being so close to the driven element on a 3 element beam. Unfortunately it does this by turning the mast into part of the antenna. I suspect it may be more susceptible to CMC issues as a result.

On my Eznec model the top of the mast shows more current amperes than the radiator does with the antenna at 36', but according to the patent the mast in this area can be extended looking to replace the top radiator, so it is indicated the circuit works the same. The mast does show a lot of current flowing, but so do the two radials hanging down around the mast. There looks to also be a lot of phase relationships going on that I don't understand, so there could also be cancellation in the area as well.

 

[HomerBB]

Yes, I moved the spreader up and down, and it was a small change in resonance I saw, and limited at best. I have read someplace that changing the length of the upper stinger, and changing the length of the Cap Hat wires would shift resonance much more dramatically than the spreader does.
He must have a tune somewhere on the bands in order to have a point from which to move the resonance.

If he ever had a tune anywhere, then having a good coax with e1/2ƛ multiples should show the same tune at the other end, or very close owing to coax loss.

I'm with you. Get all the dimensions right, and all will be well. Plug and Play. Notwithstanding loose antenna parts connections, open or shorted coaxial connections and such.

 

[RT2020]

Well guys I thought I had it right, the only 2 questions I had was why it changed when I changed coax length which was answered and why does the SWR go up when I apply power. I am not into antenna theory like you guys and I wish I could answer your questions but your over my head a little, I just want the damn thing to work and it is frustrating but I am trying not to give up, not to mention my wife wants her clothesline put back up, I have been using that spot to work on the beam and attempt to tune it. I tried to set it to the upper portion of the band where I talk more and that is what i got but cannot seem to use power.
I dont know if it will change when it is at 48' or not from the 15' is is at now but I dont want to put it up to only take it right back down.

Sorry to be a pain guys.

 

[RT2020]

Just went and rechecked with the SWR meter and it is actually 2.5 on 27.205 with the power on with a dead key, but when I turned the mic gain back up and modulated it dropped to 1.2. Go figure.

 

[Staybolt]

The Astrobeam is a junk antenna. Its best to scrap it and buy a new Maco.. I should know as i have had 4 of them and NONE of them worked right....