> Thank you, Bob, for clarifying this. The original letter made mention > of several types of antennas, including 1/4wave dualband and jpole, > both of which are, in your terms below, gain omnis at 440.
Actually, although a true 1/4 wave over an infinite ground plane does have 3 dB gain over a dipole, very few people use an infinite ground plane and so I consider the 1/4 "ground plane" (with 1/4 radials) and 1/2 wave vertical dipoles and J-poles all to be non gain omnis. They basically have 0 dBd gain, and my comments are pretty much still valid I think. So I think a J-pole also makes a pretty good omni antenna for satellites. (Better than all other terrestrial "gain" antennnas. When I refer to terrestrial omni "Gain" antennas, I am talking about the 5/8ths wave (which has a complete null at only 15 degrees above the horizon) and other larger terrestrial gain antennas up to 6 and 9 dBd gain. Hope that clarifies it. Bob, WB4APR (Jpole > being a 1/2wave antenna, e.g.) I first wrote the letter with these > distinctions kept intact, but then, in an attempt to keep things > simple, used 1/4 throughout, which meant that the claims about 1/4 > elevation patterns were not accurate. However they would be accurate > for the "1/4 wave vertical" that John originally mentioned, at least > on 70cm. > > So for the purpose of practical advice, I hope we can agree that: > a) a *true* 1/4 wave gp vertical makes a fine and inexpensive vertical > omni for satellite work. But this means that the 70cm's vertical > element should be around 18cm long. If it's longer, you're likely > getting gain, and that ain't good in this circumstance. > b) using a gain omni such as a jpole (or, likely, a multiband vertical > on 70cm) will produce the effects described in my letter below > c) with such an vertical you will occasionally suffer drop-outs at > very high elevation > d) I could have been clearer :-) > > 73, Bruce > VE9QRP > > On Fri, Nov 19, 2010 at 2:10 PM, Robert Bruninga > <[email protected]> wrote: > >> ... a typical 1/4 wave antenna... is super for > >> terrestrial work, where we want to have as much > >> power as possible going out to the horizon... > >> but... from a station up 20 degrees or more, say, > >> you'll find that you're working with much less ... > >> And, say, 70 degrees... with an ideal 1/4 wave, > >> you're putting out no power (and receiving none) > >> (In reality, its not that bad, but its pretty darn bad.) > > > > I think the essence of what is being said is relatively correct > > individually, but on closer inspection I think this is mixing > > apples and oranges. What is said is true for *gain* omni's, but > > not really true for the 1/4 wave vertical. In fact, the 1/4 > > wave is about the best and simplest omni antenna for satellites. > > Please see the detail explanation > > > > http://aprs.org/rotator1.html > > > > The argument being presented above *does* apply to a *gain* > > verticla omni. Yes, that is NOT good for satellite work because > > it does as stated, concentrates gain on the horizon and > > drastically falls off at higher elevation. So that is why we > > say "omnis" are not good for satellites. Because almost > > everyone uses a *gain* omni. > > > > But the 1/4 ground plane antenna does not concentrate all of its > > energy on the horizon and is why most people will not use it for > > terrestrial work because too much of it goes out at higher > > elevations. And even though it does drop off by more than 10 dB > > at high angles above 60 degrees, one has to remember that the > > satellite is 10 dB closer at that high angle! So it still works > > great. AND the amount of time that a LEO satelite is above even > > 50 degrees is only 2% of all the access time. Nothing at all to > > worry about. > > > > See the plot of gain on the above web page. It shows that a 1/4 > > vertical has nearly constant gain for a satellite from about 10 > > degrees up to over 70 degrees because of this range-gain. Of > > course below 10 degrees the satellite is as much as 3 db further > > away and hence weaker and most satellite link budgets were not > > designed to operate with such 0 dB gain omnis AT the horizon. > > > > So, the 1/4 vertical is very hard to beat for a simple omni > > satellite antenna. And by the same rationale, the terrestrial > > gain omni is NOT. SO watch out for apples and oranges > > comparisons... > > > > Bob, WB4APR > > > > > >> What would be an ideal shape for our 'omnidirectional' > > satellite > >> antenna? Let's have a muffin instead of a bagel, lop its top > > off and > >> place that on the counter instead of the bagel. Now we have > > increased > >> power at the low elevations, but still some power at the > > higher ones. > >> Note, we don't want a situation where the power is all going > > straight > >> up, because satellites spend a good deal of time in the low > > angles > >> (depending on where you are). We just want to 'fill in' the > > bits that > >> we lose from our bagel pattern. > >> > >> There are many designs that aim to provide something like this > > muffin > >> pattern. You can make your vertical longer than 5/8 wavelength > > (the > >> 19" at 440, e.g.); you can tilt the 1/4 wave vertical at > > around 20 > >> deg. from perpendicular to its ground plane; you can use > > lindenblad > >> arrangements to circularize and redistribute. I have a 2m > > qadrifilar > >> helix antenna that does its job nicely. > >> > >> One terribly important point on 70cm is that you have a low > > noise > >> preamp connected to the antenna on the mast, not in your > > shack. I have > >> a $5 70cm 1/4 wave groundplane that I made out of a female > >> N-connector. I would much, much rather use it with my ARR > > preamp than > >> a 8 element yagi without the preamp. Way more fun. The reason > > for this > >> with omni antennas is that we are having to distribute all the > >> 'receiving power' over all the elevation angles, as well as > > the 360 > >> degrees of azimuth. In other words, your muffin has to be > > smaller in > >> diameter than your bagel half, because both have to have the > > same > >> total volume. > >> > >> Finally, your question suggests that simpler antennas, like > > 1/4 wave > >> groundplanes, are not necessarily easy to use on SSB/CW birds. > > I find > >> the contrary, especially on CW. FM satellites are easier > > because they > >> don't require as frequent tuning due to doppler shift, and > > because > >> many hams already have the equipment needed to operate them. > > But they > >> aren't 'easier' in the sense that their signals are easier to > > hear > >> with simple equipment. Heck, with any sort of antenna and an > > low-noise > >> preamp you'll hear the CW beacon of HO-68 from horizon to > > horizon. > >> > >> I hope you will forgive me if this reply was aimed at the > > wrong level, > >> and I wish you all the best in your satellite station > > building. > >> > >> 73, Bruce > >> VE9QRP > >> -- > >> http://ve9qrp.blogspot.com > >> > >> _______________________________________________ > >> Sent via [email protected]. Opinions expressed are those of > >> the author. > >> Not an AMSAT-NA member? Join now to support the amateur > >> satellite program! > >> Subscription settings: > > http://amsat.org/mailman/listinfo/amsat-bb > >> > > > > > > > > -- > http://ve9qrp.blogspot.com > _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
