Agreed. However, the short 18" antenna is commonly used for handheld and aircraft Loran receivers. However marine Loran antennas are typically 8ft long.
That's not the only reason that Loran antennas are rather short. If the antenna were longer, the impedance would increase, causing it to pickup more percipitation static, atmospheric noise, and 60Hz harmonics. A longer antenna would also not be as narrow band and low impedance as a short (loaded) antenna. The relatively narrow bandwidth is helpful for eliminating broadcast, beacon band, and other forms of interference.
Incidentally, that's also one reason why some remote Loran systems have a pre-amp that really burns some watts. It needs to handle the out of band overload and stay linear. If the antenna were made longer, the amplifier would need to handle proportionately more power (and probably melt). Some details in the patent at:
The "background" section is worth reading. The other reason for the amplifier is to give the antenna system a 50 ohm output impedance so that cheap coax can be used.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I haven't looked at the back of a radio in a long time, but those that I have looked at usually had a trimmer capacitor that helped tune the antenna to the radio input. This is the impedance match you are looking at.
I make amplifiers with 50 Ohms input impedance and 300pV/rtHz input- referred noise. By your assertion, that should have been impossible. The trick is that the input impedance obtained by feedback: The cooled resistor trick. It works.
Yes. If anything there's been a bit of a rebirth in antenna sales as people start hearing about free over-the-air (ATSC) HDTV.
(Speaking of HDTV... Fry's is advertisiting ATSC->NTSC converters for $59, coming very close to the $49 I was predicting a while or so ago... Joerg will be pleased. :-) )
This implies only that the antenna/receiver *matching* is good enough... yes?
(I'm thinking that you would still sometimes prefer a highly directional antenna over just a dipole even though both increase the background noise. I.e., in both cases the antenna matching is good enough, but without the directionality the antenna itself might not be good enough to eliminate interference, overloading, etc. from sources other than the one you're interested in.)
Over the air HDTV is superior to cable and satellite HDTV. They may sell more outdoor antennas as soon as it becomes clear that, from the 1996 telecommunications, not only did the FCC ruling on small satellite dishes overrule the homeowner association covents restricting dsmall satellite antennas, they ALSO overruled covenants restricting outdoor TV (i.e.yagi type) antennas...this was done to promote fair competition for cable, satellite and broadcast TV. I did put up a regular radio shack yagi style outdoor TV antenna back then and the HOA did contact their lawyer and found they could do nothing about it. If I ever splurge and buy my 1080p, I will do it again because the OTA picture is absolutely the best.
A home brewed double Gray-Hoverman. Apparently, they work pretty well and if you build a really impressive one, the HOA will really get their panties in a bunch.
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Paul Hovnanian mailto:Paul@Hovnanian.com
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The only tools one needs in life:
WD-40 to make things go and duct tape to make them stop.
Nice. Too bad it only goes from about 400-800MHz, which covers the UHF channels. It doesn't work for the VHF channels. Looking at the
4NEC2 plots, it's not very good on VSWR, has reasonable gain, but only on the lower UHF channels (no problem because the higher channels are being auctioned off by the FCC).
The local OTA digital TV is on Channel 8 (KSBW) at roughly 87MHz. The reflector of a suitable Gray-Hoverman antenna should be about 1.5 meters wide. I think the aesthetics committee will be banging on the front door rather quickly. What's that giant fly swatter doing on the roof? I wonder if a TV antenna can legitimize a solar cell array if the array is mounted on the TV antenna?
"Comparing some commercially available antennas"
I really like the negative gain (i.e. loss) in some of the commercial VHF antennas. Too bad they didn't run antenna patterns, as many such TV yagis have more gain in the reverse direction, than forward.
Incidentally, I once built a vertically polarized omnidirectional OTA TV antenna intended as a disguise antenna (if you consider a 2.5 meter white fiberglass pipe to be suitable disguise). It worked well enough from about Channel 8 to about Channel 47. Not much gain (about 4dbi) but the big problem was that it was prone to receiving ghosts (reflections). I should probably resurrect the design, rename it "Ultimate Digital Disguise Magic Super-Antenna", and join the hype instead of fighting it.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Does your HOA forbid solar arrays? Some of them can be pretty anal retentive. Mine won't let me put up my thermonuclear clothes dryer. :-(
Ghosts are a big problem where I live. Or multipath and the resulting dropouts the case of DTV broadcasts. That's why I like highly directional designs (both horizontally as well as vertically). Other than that, I can get all of our local stations with rabbit ears and a UHF loop.
All of our DTV stations broadcast on UHF as the VHF band in the Seattle area is pretty much filled up with the legacy analog stuff. I'm not certain what the stations plan on doing once the digital cut over is complete. They might move their digital signal down onto their analog slot or abandon the analog slot altogether. That will be a factor in what kind of antenna I put up.
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Paul Hovnanian paul@hovnanian.com
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Procrastinators: The leaders for tomorrow.
No home owners association in my neighborhood. (However, I am the self-appointed chair person and bill collector for the private road committee). If we had an HOA, I would be the first to be lynched as I have 2 dead cars in front of the house, a huge mess ready for recycling, and an antenna farm on the roof.
However, I have friends that bought into the "planned community" philosophy and are stuck with CC&R's from hell. Basically, anything that can be seen from ground level is unacceptable. In the CC&R's I've read, solar arrays are certainly not allowed, especially on the roof.
I've found the local cities and county bureaucracy on behalf of various homeowners. Batting average is about 50%.
I live too far away from the local digital TV xmitters to get reliable reception. My rule of thumb is that if OTA analog TV reception is marginal, digital TV will be worse.
Highly directional antennas are the right way to eliminate ghosts (reflections). However, I keep running into problems with f/b (front to back) ratio problems, where the ghosts are reflected from behind the antenna. That's where the lower gain, but higher f/b ratio antennas, such as a barbeque grill backed bowtie array, makes more sense. My preference is to use single channel narrowband yagi's for maximum gain, but that gets really ugly as one per channel is required.
We have Channel's digital 10 (KSBW) and digital Channel 12 (KNTV)
I once did a paper design for a trapped yagi, designed solely for channels 8 and 11 (the analog channels) and no others. It probably would work, but nobody was interested in buying one.
All of them. The station owners would need to be insane not do do digital. It's a free extra channel with the oportunity to sell additional services (i.e. data broadcasting).
Duh... I never thought to ask the local broadcast engineers what the channel lineup is going to be after the Feb 17, 2009 fire drill. I'll ask.
Every day, I go on an extended exercise constitutional. One day, I decided to count outdoor TV antennas in my mountain neighborhood. My guess is that I passed about 100 houses and only saw three outdoor antennas (which looked ancient and inoperative). I'm wondering if anyone watches OTA TV in my area.
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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS
Check your state laws, solar is allowed, period, here in AZ... irrespective of HOA regulations.
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
The State of Schwarzenegger has had a "Solar Right Act" since 1978. See:
The Calif Civil Code Section 714 limits the power of HOA's and governments to restrict solar installations.
The problem is that term there seems to be some variations in what constitutes "reasonable restrictions" which are allowed by the law. If the purpose of the CC&R's is to maintain some semblance of aesthetic integrity, the local courts have ruled it "reasonable" for them to block the installation of just about anything with a visual impact. The interpretation of "reasonable" varies in different jurisdictions but locally, it seems almost random.
Incidentally, I've read CC&R's that were apparently written perhaps 50 years ago. They're full of restrictions based on race, color, ethnicity, and such, which are obviously unenforceable. The buyers were told that that the old contract is good enough because anything that's illegal won't be enforced. These contracts tend to also have very broad clauses blocking "anything with an aesthetic impact" and such, which is where the solar prohibition originated. One contract I've read itemized examples of prohibited installations, which included all forms of antennas and mentioned solar panels.
Locally, one planning department imposed some siting and support structure restrictions that effectively prevented installing of any solar panels. That apparently was accidental, but it took a year and an expensive legal action to get them to admit that they goofed.
One homeowner wanted a variance to install the solar panels very close to the property line. No CC&R's. The variance was denied because of neighbors protests. It went to court and he lost. Another tried to get his radio tower approved on the basis of it providing supports for his tower mounted solar panels. That also was denied.
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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS
Not sure about the higher frequencies but in the HF band we absolutely need an antenna coupler that matches the impedance to the selected frequency. If a coupler fails we can barely throw a signal a few miles whereas when the coupler does it's job we can bounce a signal off of the ionosphere at night for a few thousand miles.
That's certainly true at the transmitting end. A good impedance match is needed in order to enable the transmitter to deliver power effectively into the antenna, from whence it can be radiated.
It's rather less true at the receiving end, at least in the lower- frequency HF bands. In these bands, the ability to receive a usable signal is often dominated by the amount of natural and man-made noise in the band, and not by the receiver's own self-generated noise. Even with a serious impedance mismatch between the antenna and the receiver, enough signal reaches the receiver front-end to overcome the receiver's own internal noise.
If you happen to live in an area which is blessed by a very low background-noise level (e.g. out in the country, away from power lines) and you're DXing in the HF bands, then a good impedance-matched antenna will let your receiver take best advantage of the low noise level.
If you're SWLing in a city, surrounded by power lines and electric motors and neon lights and computers, the background noise level is going to be much higher, and the weak distant stations will be drowned out by the noise anyhow... and an inefficiently-matched antenna such as a whip or longwire will give you enough signal to hear the stations which are _not_ drowned out by the noise.
As an example - if your receiver has decent sensitivity, and a low internal noise level, you may find that you can hear a signal with decent audio quality all the way down to S0 or below (if there's no noise obscuring it). You then find that with an impedance-matched antenna the band's background noise is S6 or so. Assuming that you can make use of a signal which is somewhat below the broadband noise level, let's say that you decide you can copy stations whose own individual signals are S5 or better, and that lower-level signals are blanketed by the noise.
At this point, you realize that you can use an antenna which is 5 S-points (nominally, 30 dB) less efficient, and still receive the same set of stations. With a less efficient antenna, the stations' signals will be weaker... but so will the external band noise, by the same ratio, and thus the signal-to-noise ratio of each station will remain unchanged.
As an example, my ARES/RACES group has a multiband HF setup in our city's police and fire admin building, which is downtown near the main commercial-and-restaurant street and the light-rail system. We have a trap-dipole antenna strung up above the building's roof. On the
80-meter band, the broadband noise level across the whole band is rarely less than S9! We could probably receive the same set of stations using an antenna consisting of two coat-hangers and some damp string!
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Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
*** No, it says nothing about the matching. It only says that the signals coming down the antenna from the cosmos are greater than the receiver noise. If the antenna is matched to the receiver, whatever is picked up will be more efficiently fed into the receiver, resulting in a still higher level. If the antenna is not matched, well, there may be a heck of a lot of both noise and signal, and even unmatched the results are strong enough to override the rx noise..... One caveat , tho, ... in some conditionsm, a matched receiver input results in a higher receiver noise level.... not much, but enough for purists to argue the point incessantly :>))))) .
**** Of course, and a good point..... I was only talking about desired signal and atmospheric noise.... If there is a coherent interferer, then that's a whole 'nuther thang...... :>)))
The finals won't burn out as there are none and you won't be feeding radiation into your room.
However, as you probably noticed from the FM radio with the extendable antenna, you sometimes get a better signal when you extend the antenna nd move it to the right location.
A good (and matched) antenna may allow you to receive signals you could not have received with a random length antenna.
Most radios that I have seen usually have a matching network built in to match the antenna that comes with the radio. In most cases they aren't that elaborate, just a small coil and a trimmer capacitor.
My father learned that as great as his 50 foot long copper wire worked for listening to his favorite short wave broadcasts, the signals improved immensely when I installed an fan-dipole for his three favorite bands. Did he 'need' that antenna? no, he could hear his stations without it, but the signal strength was improved and he was able to pick up more stations.
Dad also ordered an antenna tuner kit, assembled it and attached it to his 50 foot wire. He found a great improvement in signal strength using the tuner, almost equal to the multi-band dipole I installed.
Signal to noise ratio are not synonymous with gain. One can have a lower-gain antenna with high s/n that outperforms a higher-gain antenna with low s/n. When it gets critical, the s/n can be the determining factor as to whether you receive intelligent communication or not.
You are probably digging for technical, theoretical information more than practical, but just in case...
If you look in stereo magazines, you seldom see radio ads bragging about how much better they receive than the competition. If you look in Ham magazines, that's a very important feature. Most consumers are looking for stereos that play music well and they listen to local stations. Most ham operators want to pull that weak signal out of the noise to make the contact.
Just some thoughts.
Hope this is helpful to someone. Buck N4PGW
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73 for now
Buck, N4PGW
www.lumpuckeroo.com
"Small - broadband - efficient: pick any two."
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