considering an M400 starduster. whats the deal?

[loosecannon]

yes marconi, LOL i meant 1/2 wave vertical dipole.

you said that the starduster is fed a bit off center?
are you saying this because the top radiator is a bit shorter than 108"?

looking at the owners manual, i get the finished length of the top radiator at about 101" give or take.

the radials, however, are 107" long, which says to me that this antenna is not fed off-center, but that the small radials on the top element form a capacity hat, thereby reducing the main radiators length.

lou franklin says that this also lowers the take-off angle in his book "understanding and repairing CB radios".

im interested to hear your thoughts on this.

as for the owners manual, here is a link:
http://www.cbtricks.com/ant_manuals/ant_specialists/m_400/graphics/m400_starduster_om.pdf

the part about the mast needing to be metal and at least 10 feet long is in the first paragraph, in the upper right portion of the page, right under where it says, "omni-directional center fed vertical antenna".


about the choke balun; we know that when erecting a vertical dipole, you shouldnt run the coax down the mast parallel with the "ground" element of the dipole. when this is done, currents are induced onto the shield of the coax.
it should be run out perpendicular (90*) from the antenna elements.
meaning that the coax will be horizontal to the ground for at least a quarter wavelength.

maybe this is why A/S wants the coax to be inside the tube.

hmmmm...

i am currently reading up on vertical dipoles and sleeve baluns.

hope to know more by tomorrow.

thanks for your interest!
LC

 

[loosecannon]

dontya just love it when you learn new things and they just create more questions? LOL

after reading numerous threads about this stuff, as well as other articles online;
im not sure if this antenna is a vertical dipole, or a 1/4 wave groundplane antenna, or what!

for example, to make a vertical "bazooka" antenna; you peel back about a 1/4 wavelength of the coax shield.

if the starduster didnt have the drooping radials; i would say thats what the antenna is. a vertical bazooka type antenna.

but then, why the need for the drooping radials?
if its a 1/4 wave groundplane antenna, then why the "sleeve", or "bazooka" formed by the metal mast?

i sure hope we get somewhere with this!
LC

 

[W5LZ]

From looking at the assembly instructions, this thing is a vertical, center fed, half wave length antenna. Or, just another 1/4 w groundplane type with really droopy radials. That depends on if that 'large hub' provides any insulation between the radiator above it and the radials and mounting pipe. I would assume that it does, or the whole thing would be just one 'electrical' piece and a terrible antenna.
I don't think I'd confuse things by describing any part of this thing as a 'bazooka'. That indicates a completely different antenna and feeding system. The coax may go through that pipe, but since the radials appear to be insulated from that pipe, the pipe is just to support the hub, not really a part of the antenna as such.
Without getting my hands on one that's about as much as I can 'guess' from the assembly instructions. It appears to be ust another 1/4 w groundplane type antenna of a slightly different shape. And all 1/4 w groundplane type antennas are the same as a vertical dipole, just in a slightly different shape.
- 'Doc

 

[Robb]

Doc, I thought this antenna was a droopy 1/4 wave vertical too. If it is 1/2 wave dipole by effect - I couldn't say. I don't know much about antennas as I should for a Ham. I'm glad that being a Ham is really a learning process...

I know that I got the IMAX from so many great reports on eHam - as well as other web pages. I almost bought the StarDuster because of the price and its great reputation. The fact that Palco Elec. has them for a really fair price makes it an attractive and effective choice. The IMAX I bought eight months ago from Palco for $85 is now selling for $95. Probably the effect of inflation in that time period - or they are more popular now. Maybe both. But if I had to budget myself and wanted an antenna with a great reputation, I would have gotten the StarDuster. It was close.

 

[W5LZ]

Just for grins, take one 1/4 w groundplane antenna. Fold the radials so that they are parallel to, and the other direction of that vertical part. Looks a whole lot like a 1/2 w dipole if you turn it sideways. Same dimensions most of the time.
- 'Doc

 

[loosecannon]

W5LZ, (Led Zepplin?)

the radials are not insulated from the lower portion of the hub.

the only thing insulated from the "ground" portion of this antenna is the vertical radiator.

both the radials, and the mounting pole are connected to the shield of the coax.


hope that helps clear things up.
LC

 

[W5LZ]

loosecannon,
Going by the assembly instruction posted, the radials are insulated from that pole, there are insulators on the radials between the support pieces holding the radials away from the pole (both ends). That makes me tend to think that the 'large hub' is also insulated from the pole, otherwise, why bother with the other insulators??
- 'Doc

(W5LZ = Lost Zipper)

 

[freecell]

"....the 'large hub' is also insulated from the pole"

it isn't. the hub, the radials and the mast are all at the same current potential.

 

[loosecannon]

doc,

the only reason those radials are attached to the mast at all is for wind support.

im not sure how else to say it.
they are radials. they are connected to the shield of the coax, and they extend down from there.

i am not guessing about this. thats the way it is.


LC

 

[15minigrass]

Hey bro. hereʻs part of a starduster just the radiator(main ant) and 1 ground radial laying on the roof and I still got below 1.5:1 SWR.

 

[Marconi]

yes marconi, LOL i meant 1/2 wave vertical dipole. you said that the starduster is fed a bit off center? are you saying this because the top radiator is a bit shorter than 108"?

My reasons for my off center claim has to do with the diameter of the SD's tapered elements which likely average 2-3 times larger than that of a 102"-108" SS whip or a similar size wire. Having thicker diameter elements I would think the SD'r elements should be considerably shorter than 107" in order to maintain resonance. The rules of resonance have to apply and if we substitute SS whips we would have an overall lengthof 204"-216" and that tells me that the SD'r would either be way too long or it has to be loaded off center a bit with the radials being longer than the stinger. The physicals of a SD'r follow pretty close to the 5%> rule that applies to ground plane elements and is often mention in discussions regarding radials. Maybe my saying "off center loaded" is not a good description of what is really going on, but that is what I was thinking.

looking at the owners manual, i get the finished length of the top radiator at about 101" give or take. the radials, however, are 107" long, which says to me that this antenna is not fed off-center, but that the small radials on the top element form a capacity hat, thereby reducing the main radiators length.

The stinger of my original SD'r is 99" made up of three tapered elements, 1/2"X51.25", 3/8"X20", 1/4"X27.75". On-the-other-hand my knock-off SD'r is 98" made up of two tapered elements, 1/2"X51", 3/8"X47". Assuming both antennas are designed for the middle of CB, which is likely, this one inch difference is totally ascribable to the diameter differences and that diameter difference is probably small as a percentage. So if we consider the differences between a whip and these elements we might expect to see a sizable difference in length while the GP elements fall near the top of the 102"-108" standard of length for the mobile whip.

lou franklin says that this also lowers the take-off angle in his book "understanding and repairing CB radios".

im interested to hear your thoughts on this.

I don't know what Lou Franklin knows, but if he thinks and you believe that the these small projecting radials have an effect on TOA, then that is fine with me. I can't prove or test TOA and I don't consider such information very important in the performance of the mono-frequency vertical antennas we use in CB. I hear references to the term in CB chatter, but too often it is just BS on the subject, a subject that is probably only derived thru a very complicated understanding and application of physics. Frankly I think the real issue of TOA is controlled mostly by the conditions of the soil below the antenna and in the far field. And to a lessor degree controlled by the height of the antenna. For me TOA is just another word used by guys that try to impress and come across with categorical claims about what they know about antennas.

as for the owners manual, here is a

link: http://www.cbtricks.com/ant_manuals/ant_specialists/m_400/graphics/m400_starduster_om.pdf

the part about the mast needing to be metal and at least 10 feet long is in the first paragraph, in the upper right portion of the page, right under where it says, "omni-directional center fed vertical antenna".

Thanks for the link. That is an original SD'r spec sheet. I think it is just common sense about the 10' foot mast if the fact is the coax exiting the antenna inside is shielded from the inductive RF in the lower half of the antenna, which IMO is probably a good idea for some improved performance.

about the choke balun; we know that when erecting a vertical dipole, you shouldnt run the coax down the mast parallel with the "ground" element of the dipole. when this is done, currents are induced onto the shield of the coax. it should be run out perpendicular (90*) from the antenna elements. meaning that the coax will be horizontal to the ground for at least a quarter wavelength.

maybe this is why A/S wants the coax to be inside the tube.

hmmmm...

i am currently reading up on vertical dipoles and sleeve baluns.

hope to know more by tomorrow.

thanks for your interest!
LC

LC, you make a good point, but your last statement is arguable as well, considering that just about every dual polarity yagi multi-element antenna pretty much violates the rule you site, plus you hear very little discussion about this issue concerning beams---as though it is not even a problem.

Like I said before, "...we are just very lucky that Mother Nature does not require us to be perfect in what we do concerning our radios and antennas."

 

[regencycb]

considering an M400 starduster. whats the deal?

loosecannon,
Going by the assembly instruction posted, the radials are insulated from that pole, there are insulators on the radials between the support pieces holding the radials away from the pole (both ends). That makes me tend to think that the 'large hub' is also insulated from the pole, otherwise, why bother with the other insulators??
- 'Doc

(W5LZ = Lost Zipper)

As others have noted the radials are not insulated at the hub. They are supported lower down the radial away from the mast by insulated spacers or rods; for mechanical and rf stability. The radials are resonant and if the spacing away from the mast changes so does the tuning. The starduster when properly tuned has the same amount of current flowing in the radiator as the radials. As for the name you choose to use to describe the Starduster that's open : 2 1/4 wave elements same as the old 1/4 droopy Groundplane,the original coaxial vertical and the newer coaxial bazooka
designs. I believe that the Starduster was designed with the radials at the angle they are was an attempt to raise the impedance of the antenna closer to 50 ohms. The radials also act as a sleeve with the large diameter at the bottom of the radials producing a high impedance point. The earlier vertical coaxial dipole designs suffered from bad feedline radiation due to the sleeve being pipe or alum of 2 inches in dia. Reason being the ratio of the feedline dia to the sleeve was not great enough to produce the high impedance needed at the bottom of the sleeve for the antenna to operate correctly.
The only improvement i can think of would be to increase the number of radials on the Starduster to simulate a complete sleeve. Have any of you
seen the ISOPOLE brand of 2m/440 antennas same theory with more decoupling.

I once built a coaxial vertical for 10 meter FMand had to use 4" alu for the sleeve to reach acceptable decoupling. It is really great to see all the discussion on antennas.

73 RCB

 

[loosecannon]

marconi, you are right. i hadnt considered the way coax is run to a beam antenna.

so, the way i see it now, is that this antenna is a 1/4 wave groundplane antenna with a sleeve over the coax to help reduce coax radiation.

thanks to all who replied.
good thread.

LC

 

[loosecannon]

well, im really glad i learned everything from this thread that i did, but its not going to matter for me as i just made a deal with someone to buy their "top one" astroplane copy.

i cant wait to get it and put it up as i was very happy with my last one.

this is the antenna that i really wanted for my birthday anyway. LOL
LC

 

[15minigrass]

Happy Bday LC. Them Astro are top choice too, hope to catch you on the band!!!

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