Hi George, Thanks for mentioning that antenna system thermal noise eventually establishes the limiting factor where additional preamp gain will not provide any additional improvement in performance. Earlier today I was looking for a previous discussion by Tom (W8JI) that explained this very well and I believe he provided some examples but I have not yet located it (but still looking).
73, Don (wd8dsb) On Tue, Feb 23, 2021 at 8:20 PM GEORGE WALLNER <[email protected]> wrote: > The smaller the flag the lower its gain and its signal output. The limit > of > usefulness is reached when a weak signal is below the thermal (Johnson) > noise of the system (~ loading resistor). Below this point a pre-amp will > no > longer help. That limits the usefulness of small flags for weak signals, > regardless of RDF. > > 73, > George > AA7JV/C6AGU > > > On Tue, 23 Feb 2021 18:37:03 -0500 > John Kaufmann via Topband <[email protected]> wrote: > > As a follow-up to my original post, here are a few additional comments. > > > > Don, you mention that you designed the mini-flag for a deep null off the > > back at low elevation angles, which is entirely understandable. As I > said > > in my earlier post, the null is very pronounced in the AM BCB on local > > groundwave signals. However, I also see pretty significant nulls on > > higher-angle signals, too. Just a short time ago, I was listening to > W1AW > > on the low end of 160. They are located only ~100 miles from me. Their > > signal has to be arriving at a pretty high angle, but the null is still > > quite pronounced. > > > > My homebrew preamp, that I mentioned in my post, uses a cascade of UTO > 511 > > and UTO 533 mini-amplifier modules. I used this preamp, not necessarily > > because it's optimal, but because I already happened to have it on hand. > > The gain of the 511 is given as 16 dB typical while the 533 is 17 dB, > which > > should yield a net gain of ~33 dB for the cascade of the two. The noise > > figure on the 511 that serves as the input amplifier is specified as 2.3 > dB, > > but its spec sheet gives an operational frequency range of 5-500 MHz, so > I > > can't be sure the noise figure (or the gain) holds up at lower > frequencies. > > Nonetheless I can hear the ambient noise in my receiver increase on 160m > > when I connect the mini-flag to the preamp, which suggests the noise > figure > > for this preamp is at least adequate at my location. I use a Yaesu > FT-817ND > > "backpack" radio as a portable radio with this antenna. > > > > In EZNEC I calculate the RDF of this mini-flag as 7.4 dB on 160m at a 20 > > degree elevation angle. That's essentially the same as the K9AY loop or > > other similar pennant/flag antennas. For use as a receiving antenna, the > > important thing is the noise figure of the preamp. The DX Engineering > Web > > site does not give the noise figure of their preamp. > > Don, perhaps you know? > > > > The other thing that might degrade the antenna is common-mode signal > pickup, > > which can be a problem for very low gain antennas where you are working > with > > very small signals. However, based on what I observe in terms of antenna > > pattern for this mini-flag, I can't say that I see any pattern effects > that > > might be attributable to common mode degradation. Don, maybe you can > > comment here as well on this aspect of the antenna. > > > > As I also mentioned in my earlier post, the dimensions of the DXE > > implementation are somewhat smaller than what's given in the QST article. > >For me, that works out well because the width of the DXE mini-flag just > > manages to fit inside the trunk of my mid-size sedan. A wider flag would > > not fit. > > > > 73, John W1FV > > > > -----Original Message----- > >From: Topband > > [mailto:[email protected]] On > Behalf > > Of Don Kirk > > Sent: Tuesday, February 23, 2021 5:22 PM > > To: [email protected] > > Cc: Top Band List List > > Subject: Re: Topband: The WD8DSB mini-flag antenna > > > > HI Steve, > > > > Thanks for the nice implementation comment. > > > > The portable flag front to back ratio is highly related to the elevation > > angle and frequency of operation (just like any terminated loop), and > > therefore I did not want to overstate the front to back ratio in my > > portable flag article. I designed the portable flag for direction > finding > > local RFI (ground wave based signals) and therefore made sure I selected > an > > appropriate termination resistor to provide a very deep null at low > > elevation angles on 160, 80 and 40 meters where I often deal with RFI > (the > > portable flag has a very high front to back ratio at low elevation > angles), > > and because of this it also has exceptional front to back ratio at low > > elevation angles down in the AM Broadcast Band. Very small flags have > just > > as good front to back ratio and RDF as a full size flag as long as the > > appropriate termination resistor is used. The problem is when the flag > > becomes too large for the frequency of operation which causes the > > directional properties to degrade. You can see some front to back ratio > > vs. elevation plots for my portable flag on my simple portable flag > website > > and here is the URL to that site: > > https://sites.google.com/site/portableflagantenna/home > > > > Problem with very small flags is that the noise figure of the preamp > > becomes a critical parameter, and because of this I don't recommend > > attenuators be placed before the preamp as this causes degradation in the > > signal to noise ratio. I stumbled upon this issue when doing field tests > > on one of the DX Engineering prototype preamps, and had them change the > > design so the attenuators now come after the actual amplifier stage which > > solved the problem. > > > > Everything I said above about the performance of very small terminated > > loops assumes no interaction with surrounding objects, and ignores issues > > related with feedlines since the feedline is very short on the portable > > flag. > > > > P.S. I make no money from DX Engineering as I agreed to not be paid in > > order to keep the price of the portable flag as low as possible. > > > > 73, > > Don (wd8dsb) > > _________________ > > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > > > > _________________ > > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
