AstroPlane up and Testing

[Starduster]

There can be issues with feel lines sometimes that affect the meters results, but the factors above are good goals. It is always best to take these readings for tuning purposes at the feed point and at the working height---albeit inconvenient.

I kind of figured that, the meter instructions say to use a 1/2 wave coax length, the longer the coax the more inaccuracy that you may encounter. Probably splitting hairs here anyway. I am pretty happy with my results the way they are but will get the frequency back up when it stops raining some day.


This is a bit low in frequency. When you made the mod to the antenna to make it tunable, did you change any of the original dimensions or are they the same and you just added the larger tubing?

I tried to make it the same length exactly. It don't take much of a length change maybe 1/4 to 1/2" to make a change in the frequency though and bring it into the mid band. I don't think the tube diameter hurt anything, some day I may have to assemble the original AP and get a real good comparison. Work don't allow much productive free time. Most days I am just burnt out when i get home, old age perhaps.

I think we talked about this the other day and you said when you tuned the antenna low to the Earth it was good at 27.205, but when you raised it up higher the resonance or the good readings showed the antenna drop down in frequency. That could have been another thread I was on where this topic came up.

It was this thread. Wasn't really complaining and know what has to be done. sometimes I just add information on what happened just for any discussion. Not really to get advice or cause work for anyone. You sound like a very knowledgeable person and the last thing I want to do is waste your time.

How many digits read out on the LCD display for SWR? My meter reads two places past the decimal. Yours could read only one digit past the decimal and that compared to the analog meter may differ.[/QUOTE]

It indeed reads to the 1/10ths only. I figured that it was just a cheap meter, MFJ 269B

Thanks for the reply

 

[Marconi]

This would all make sense as skip tends to scramble polarity.
Also you had mentioned that you took the top hat off of your AP and put a straight radial on top. The patent states that the upper radial should equal the length of the lower flaired radial. I wonder why you were around the 70 to 80 inch length when the lowers are 96. Unless I am missing something.
Please forget about all of the other stupid questions that I had asked.

Good point to consider about the polarity thing.

The top section is larger diameter (5/8" I think) for one thing and the tubing I used to replace the SS wires in the top hat was maybe 5x thicker. I had to try a few iterations to get to the middle of CB, but that is where I ended up. If I posted the length was 70"- 80" then I messed up. Here is Antenna Work Sheet and it shows the length of the top element ended up at 87.5" to tune to 27.205

The skew in this scan was unusual and it may have been due to some errant signal or I punched a wrong button on the meter. Notice the SWR scan that followed this work was nice and smooth.

 

[Marconi]

There can be issues with feel lines sometimes that affect the meters results, but the factors above are good goals. It is always best to take these readings for tuning purposes at the feed point and at the working height---albeit inconvenient.

I kind of figured that, the meter instructions say to use a 1/2 wave coax length, the longer the coax the more inaccuracy that you may encounter. Probably splitting hairs here anyway. I am pretty happy with my results the way they are but will get the frequency back up when it stops raining some day.

Good idea to use a tuned line if we are not too far away from a good tune already. But remember, if your antenna shows some standing wave then adding length to the cable will likely change the meter results. The longer line will add the affects of ohmic resistance to the readings and I think that might make things look a bit better than they are.

This is a bit low in frequency. When you made the mod to the antenna to make it tunable, did you change any of the original dimensions or are they the same and you just added the larger tubing?

I tried to make it the same length exactly. It don't take much of a length change maybe 1/4 to 1/2" to make a change in the frequency though and bring it into the mid band. I don't think the tube diameter hurt anything, some day I may have to assemble the original AP and get a real good comparison. Work don't allow much productive free time. Most days I am just burnt out when i get home, old age perhaps.

This antenna is so sensitive and if you did get the measurements equal to the original, the added taper with larger tubing may well have caused part or all of the change. The resonance went down and that is the response we would expect by adding diameter. It didn't hurt anything, you just have to compensate for it. However, if you did tune with a 1/2 wave line and then add the regular feed line, that too can cause a similar response. I don't know what you did, but I bet you used the 1/2 wave line to tune.

I think we talked about this the other day and you said when you tuned the antenna low to the Earth it was good at 27.205, but when you raised it up higher the resonance or the good readings showed the antenna drop down in frequency. That could have been another thread I was on where this topic came up.

It was this thread. Wasn't really complaining and know what has to be done. sometimes I just add information on what happened just for any discussion. Not really to get advice or cause work for anyone. You sound like a very knowledgeable person and the last thing I want to do is waste your time.

SD'r, everything I do with antennas is a waste of my time, but the desire for understanding is my joy and I can't get that without discussion with guys like you that are willing to exchange ideas.

How many digits read out on the LCD display for SWR? My meter reads two places past the decimal. Yours could read only one digit past the decimal and that compared to the analog meter may differ.

It indeed reads to the 1/10ths only. I figured that it was just a cheap meter, MFJ 269B

I think it should suit your needs, you see mine messes up too and I know I show a couple of ohms low. That said, even my calibration procedure is not perfect either. It is off and I can't adjust it.

Thanks for the reply

!!!!!!

 

[Marconi]

bob, i always wanted to try that with mine, but i dont want to cut my push up pole. they are getting hard to acquire.

i sure hope you end up trying it.

now im off to find that CB radio magazine article!
any guesses as to the year?
LC

Hey LC, don't cut your push-up pole, just add the insulator to the end and put the AP and a 18' mast on top of the insulator like bob is talking about--- if you can reach and handle the setup. It's always risky business doing that, but Jay's I-10K gets by doing something similar and all you need is a couple of inches. That 4# AP up there on the 18' mast will make you say your prayers though.

BTW, I think I have located the CB Radio magazine. I just hope we get a real interview or report and not one of those carton charades like is all too common.

Good luck,

 

[bob85]

eddie,
you could be correct about the astroplane, i hope its an informative article and not just advertising jargon too, if you can find it i would be very much interested in what it says,
to me it looks like a 5/8wave loop folded around the mast to form a three wire tapered transmissionline,
the guys at the blackboard look to be talking about transmissionlines, i hope thats lou or herb,


starduster,
nice workshop, nice work

 

[loosecannon]

marconi ive already been thinking the exact same thing since i posted last.

not sure i can do it where i live currently, but my future plans include putting this antenna up to where the tip is 60' in the air, and the insulating and "choking" will be part of that install.

i am also very curious to see what that CB magazine article has to say.
the existence of that article has got me thinking that Avanti's engineers might have done more interviews or articles in other trade publications.
not necessarily cb/ham publications, but other technical realms.
hmmm...
LC

 

[Starduster]

Good point to consider about the polarity thing.

The top section is larger diameter (5/8" I think) for one thing and the tubing I used to replace the SS wires in the top hat was maybe 5x thicker. I had to try a few iterations to get to the middle of CB, but that is where I ended up. If I posted the length was 70"- 80" then I messed up. Here is Antenna Work Sheet and it shows the length of the top element ended up at 87.5" to tune to 27.205View attachment 2629

The skew in this scan was unusual and it may have been due to some errant signal or I punched a wrong button on the meter. Notice the SWR scan that followed this work was nice and smooth.

Getting ready to see if your results will be dupicated when I ditch the top hat. Will be trying it tomorrow.

 

[bob85]

LC,
its worth a try, i would not cut a pushup pole unless i had to, the patent says what happens if the mast is shorter than 9ft below the hoop, it does not say what happens if its longer, people get variable results from astro's which makes me wonder if thats down to how they are installed.

 

[Marconi]

Getting ready to see if your results will be dupicated when I ditch the top hat. Will be trying it tomorrow.

That's great SD'r.

 

[Starduster]

That's great SD'r.

Ran into some complications with my coax. Made a new 1/2 wave length of LM 400 then set the element lengths to what the original Astroplane were. See the doc attachement. I hope to try the modification tomorrow and post results of the straight element w/o top hat.

 

[Marconi]

Ran into some complications with my coax. Made a new 1/2 wave length of LM 400 then set the element lengths to what the original Astroplane were. See the doc attachement. I hope to try the modification tomorrow and post results of the straight element w/o top hat.

I don’t get it, what happened? Results today are different than what you said earlier. It tunes low again near 27.105 and the other day it tuned at 27.205. Of course it could be the new 1/2 wave jumper, but if that’s so, then it don’t speak well for tuned lines. Does this surprise you or did I misunderstand what happened?

Just got my copy of an old Astroplane completed and up for testing. The SWR is lowest at channels 1 -6 (1:1). I had it tuned for channel 20 when it was lower to the ground but now it had changed on me....

You need a good base point to start, so I wouldn't make the mode until I had a good handle on the default AstroPlane tune to the middle of the CB band. Then I would keep the height for all tests the same in order to eliminate variables and for sure since going a few feet higher made such a difference.

Good luck,

 

[bob85]

starduster,

in your pics you have the coaxial fastened down the side of the mast,
the radials beside the mast form a tapered transmissionline whos impedance is the dominant factor in determining the antennas input impedance,

transmissionline impedance is determined by mast and radial tube diameters / spacing and the electrical length of said transmissionline,
i don't know how much offsetting the coax to one side effects impedance when the mast is present, it may have very little effect,
ideally the coaxial should be taped dead central between the radials as shown in the assembly instructions,

tuning the astroplane is not so simple as tweaking element lengths although that may give you a low vswr

 

[jazzsinger]

I don’t get it, what happened? Results today are different than what you said earlier. It tunes low again near 27.105 and the other day it tuned at 27.205. Of course it could be the new 1/2 wave jumper, but if that’s so, then it don’t speak well for tuned lines. Does this surprise you or did I misunderstand what happened?

You need a good base point to start, so I wouldn't make the mode until I had a good handle on the default AstroPlane tune to the middle of the CB band. Then I would keep the height for all tests the same in order to eliminate variables and for sure since going a few feet higher made such a difference.

Good luck,

What length did you cut your 1/2 wave of lmr400 too? it has a velocity factor of .85 or 85%.

so for 27.205 which i assume is what you intend centring the antenna on the 1/2 wavelength in that coax should be 4.69m or 15 feet 41/2 inches or thereabouts.If its not then it may explain your slight drop in resonant frequency as could a poor solder joint in one of the plugs,either before or after you changed transmission line.

chances are if you are only using a halfwave of coax to measure the vswr/impedance then your body is too close to the antenna and could interact/detune it,you might be better using 1 wavelength or 1.5 wavelengths of coax.

 

[Starduster]

I don’t get it, what happened? Results today are different than what you said earlier. It tunes low again near 27.105 and the other day it tuned at 27.205. Of course it could be the new 1/2 wave jumper, but if that’s so, then it don’t speak well for tuned lines. Does this surprise you or did I misunderstand what happened?

You need a good base point to start, so I wouldn't make the mode until I had a good handle on the default AstroPlane tune to the middle of the CB band. Then I would keep the height for all tests the same in order to eliminate variables and for sure since going a few feet higher made such a difference.

Good luck,

I agree that I need a good base point to start with, that is why I am starting with different and better coax. I would disregard the first set of test numbers as the coax was not 1/2 wave length. Now that I have a base point I will do the mod without changing anything other than the top radial. I am not so concerned about being in the center of the band so much as to see of the straight radial is more broad banded. I can always go back and retune as it only takes about 5min. to lower the antenna down and make a change.
Sorry for the miss information earlier but I am learning as I go. I cut the new jumper to 18'-15/16" acording to the antenna book this should be 1/2 wave but now I am hering that it should be 15' 4-1/2" ???

Bob85 suggest that the coax be dead center down the mast. That it is. I was considering running it down the inside of the mast also. unlike the original my mast pole is axactly centered between the mast pole the original design has the mast pole slightly ofset. I achieve this alignment by putting a bends in the top bracket as you can see in the photos. The angle of the bends and all of the dimensions are such that the original dimensions between the mast and lower legas as well as between the two lower legs are the same.I am taking the measurements on my roof with a mast pole about 19" long stuck into a 3" tripod. The 1/2 wave jumper is just about at the level of the top of the tripod and that is where I hook the analyzer. There is about 9" from the top of the tripod to the bottom of the loop. This keeps my body away from the antenna and directly below it.

 

[jazzsinger]

I cut the new jumper to 18'-15/16" acording to the antenna book this should be 1/2 wave but now I am hering that it should be 15' 4-1/2" ???

I am taking the measurements on my roof with a mast pole about 19" long stuck into a 3" tripod. The 1/2 wave jumper is just about at the level of the top of the tripod and that is where I hook the analyzer. There is about 9" from the top of the tripod to the bottom of the loop. This keeps my body away from the antenna and directly below it.

As soon as you wrote you had made up a halfwave jumper I figured you'd got the length wrong,it's a very common mistake due to all the misinformation being perpetuated on the internet and in books by those who think they are specialists,with the emphasis on the word "think".

The formula for measuring a halfwave in coax is as follows:

300 million metres per second (speed of light/radio waves) / 27.205 Mhz (your chosen centre frequency for US cb band) (300 and 27.205 are brought about by cancellation of zero's from 300,000,000 m/s and 27.205.000 Mhz) = 1 fullwave in freespace /2 which gives a 1/2 wave in free space, x.85 (the velocity factor of your particular coax,because wave propagation is slowed in coax compared to free space)

300/27.205= 11.027 metres /2 = 5.513m x 0.85 = 4.686 metres. to convert to feet you multiply metres by 3.28 =15.372 feet which is approximately 15' 4-1/2" give or take a fleas bawhair.

You have either read the book wrong and its the formula for 3/4 wave impedance inverter if using 75 ohm coax to match quads/co phased mobiles etc or a 3/4 wave length of 50 ohm coax for coax with a velocity factor of 0.66 (rg58c/u,rg213/u,rg214/u,rg11/u,etc),either that or the book writer hasn't taken velocity factor into account which means the guy who wrote the book is clueless in seattle.

I tend to think the latter is most probably the case,as there ain't no shortage of people writing books about subjects they know precious little about and who understand even less on the subject than the majority of readers who take for gospel anything thats put down in print.

As for being clear of the antenna,its best being at least 18 feet from the antenna,approx 1 full halfwave in free space if you want to avoid interaction.I'm assuming by " you mean feet and not inches,feet is generally signified by ' and inches by ".

From what I read on this forum,Bob 85 and Shockwave are two guys whose information you can rely on when it comes to antennas,especially avanti ones.Theres a few other well clued up guys here too.

Good luck with your testing/modifications.