The Official AVS Antenna and Related Hardware Topic! - Page 497

With digital you either receive it or you don't. When you don't receive a station you don't know what the problem is. Is it too weak? Is there interference? Is there multipath? The average person has no way to determine this unlike in the analog days when you could answer these questions with one look at the picture. So we play guessing games on here.

Low VHF is notoriously susceptible to noise while that's almost never a problem on UHF.

Chuck

Not necessarily. With the FM signals being so strong, they can overwhelm the tuner to the point where it cannot even see up the weaker channel 6 signal in the background. The key is attenuate the FM frequencies to the point where the tuner can lock in on the weaker channel 6.

It's like trying to hear someone across the room when someone closer to you is talking at the same time. You won't be able to hear the distant voice until the closer one is scaled back or stopped.

Antenna reviews by non-technical consumers are about as reliable as the Olympics judges from Bulgaria, East Germany, and the USSR in the 1970's....


FM harmonic distortion can reach into channel 4 in severe cases. Charles Rhodes did a series of articles regarding potential FM interference on TVTechnology.com back in 09 or 10.

Laying it directly on the driveway, no. Any antenna needs to be at least a few feet above the ground, the higher the better. On the ground, you're basically shorting it to earth.

Can you get WMAR (2.1 virtual, real 38) on the U75 when its pointed in the Baltimore direction ?

Also can you get channel 17 ? I notice that channel 6 and 17 are 2 edge stations for you while the rest of the Philly stations are 1 edge, and I really cant see why.

Was wondering if people measure the SWR on their home built antennas to see how well the build went? Buying an SWR meter seems a little pricey and I guess if it works then why sweat the details right - but I'm just wondering if there are simple ways of checking on the efficiency of the antenna and whether it is performing as it should. The GH/M8 models by Nikiml and Mclapp have swr and gain info, so I was wondering if a build of one of those antennas could be measured against that information to see if there were errors in the building of them.

+1 on not laying it on the driveway. Close to the ground you also pickup more man-made noise which interferes with RF6 reception. Higher up the antenna sees less of the ground so sees less man-made noise.

Good analogy.

Although stand-alone SWR Analyzers are readily available for Ham radio bands,
the test signal is ONLY output over fairly narrow bandwidths and is NOT
available over the TV & FM bands....probably due to FCC interference rules.

Unfortunately, the fol. wide-band IN-LINE Bird SWR Meters need quite a
few watts of INPUT power to measure SWR < 2.0 (Return Loss = 9.5 dB).
Which interferes with reception of TV & FM signals in your neighborhood.
LEGAL test signals need to be no more than about 10 mW (100 mW EIRP):
http://www.rfparts.com/bird.html

I did come across the fol. $695 AEA-140-525 Antenna Tester, which might
get the job done (I didn't see a manual available for download), except it
only covers the Hi-VHF and lower half of the UHF TV bands. With a very
low output of +5 dBm (3 mW), it might even be legal outdoors:
http://www.aeatechnology.com/products/swr/140-525
AEA also makes 600-999 MHz Cellmate EX and 100 kHz - 54 MHz VIA models:
http://www.aeatechnology.com/swr-meter/

Since it's a 50-ohm device, you'll also need a 50-to-75-ohm Low Loss Pad.
PS: Antenna Under Test needs to be mounted well away from other objects,
otherwise you'll be measuring reflections more than actual SWR.

Network Analyzers and some Spectrum Analyzers can measure Forward
Power and Reverse (Return) Loss, from which SWR can be calculated.
Unfortunately, they cost 10's of thousands of dollars new and several $1000 used.
Which puts them out of the reach of amateurs.....and even many pros,
who rely on electronic rental companies to bring the cost down....

From time to time I've see DIY projects for a Ham Radio "SWR Bridge",
which could be readily adapted for TV and FM Band if someone has the
requisite skilzzz....but I wouldn't undertake this redesign effort without
having access to the above expensive test equipment to verify the design:
http://ludens.cl/Electron/swr/swr.html
http://www.qsl.net/n9zia/wireless/appendixF.html#10
http://pe2er.nl/wifiswr/

As holl_ands said, there are SWR analyzers available, but they are mostly for 50 ohm lines. Using a minimum loss pad to match 50 to 75 ohms works, but you still can't read 75 ohm SWR directly, because the bridge is designed for 50 ohms.

It's OK, for example, to use the pad to connect a 75 ohm antenna to the 50 ohm input of a spectrum analyzer for field strength measurements, because you can allow for the loss of the pad. But, for SWR measurements the antenna must be connected directly to the bridge or network analyzer.

You can use a SWR analyzer that gives a reading for SWR and for impedance/resistance (like MFJ). The SWR reading will be off by a factor of the difference between 50 and 75 ohms, but the impedance might be helpful.

The least expensive way for an antenna experimenter would be to construct a return loss bridge for 75 ohms and use a signal generator for the signal source to drive the bridge. I have thought of that as a project for myself, but it is way down on the list of things I need to do.

The last 3 links in the post above by holl_ands show some examples, but you must modify any 50 ohm designs for 75 ohms.

Your construction can be checked by substituting a type F 75 ohm terminating resistor for the antenna under test.

Google return loss bridge:
www.vk2zay.net/article/179
w7zoi.net/testgear/rlb.pdf
http://www.qsl.net/n9zia/rlb/index.html

The setup would consist of a signal source, the bridge, a detector, and your antenna. The SWR can easily be derived from the return loss figure.

http://unicornelex.com/black-forest-...ridge-kit.aspx
unicornelex.com/widebanddbmmeterkit.aspx

Keep in mind that the SWR is going to vary a lot as you tune through the frequency range. It will change much faster for a yagi antenna than for an inherently broadband antenna like the bowtie. In both cases it will only behave well at its design frequency, and will increase as you move away from the design frequency. When you look at the SWR curves from modeling by holl_ands, 300ohm, and others, you can see how much the SWR changes through the frequency range.

I have avoided needing to use a 75 ohm SWR bridge by setting up two antennas with an A/B switch. One antenna is my reference antenna with known performance, and the other antenna is the one under test. Using the switch and a SLM (signal level meter) I can tell right away which gives more signal output on any particular channel.

Isn't that what you really want to know?

http://www.digitalhome.ca/forum/show...&postcount=221

You would also want to know at least one more thing, the waveform of the signal from a scope. Even though the signal output from one antenna is greater than another, its waveform could be so distorted as to be unusable. High SWR antennas can distort the clean waveform needed.

I'm not sure what kind of scope you have in mind. If you mean an oscilloscope that has enough bandwidth at that RF frequency, I don't understand what it would tell me. Please enlighten me.

If you mean the display on a spectrum analyzer, then it might tell me something about multipath if the waveform didn't have a flat top. Trip discovered that his meter display showed a good waveform at his new location, but he had a serious multipath problem that prevented good reception of his local channels.

The best way to aim an antenna in a serious multipath situation is to monitor signal quality using BER (bit error rate/ratio) rather than signal strength, which brycenesbitt found out.

I respectfully disagree. High SWR only causes loss, which you are aware of from modeling.

High...and even moderate SWR can distort the digital ATSC waveform, as signals
mimic multipath bouncing up and down the coax downlead (Preamp's not so much).
EVM (Error Vector Magnitude) is a measurement of the amount of distortion:
http://www.digitalhome.ca/forum/show....php?p=1303957

Technically speaking, the digital decision "eye pattern" becomes very noisy,
as the 8 VSB amplitude levels become very indistinct, since the SWR reflections
are mis-aligned in time to the primary signal. Dr. O Bendov describes this as
so many dB of Noise Figure degradation, which is somewhat of a misnomer....

Bear in mind that, in a perfect world, ATSC needs more than 15 dB SNR
if the Noise is truly Random, which corresponds to an SWR of about 1.4.
Fortunately the "multipath" bouncing up and down the coax isn't Random,
and the Tuner isn't affected until seeing the TWICE reflected signal (down,
up and back down), which is about 7.5 dB each way or SWR of about 2.5.
So ATSC actually tolerates a significantly higher SWR....I try for under 2.7.

I didn't know it was that fragile.

Can we please go back to analog!

Thanks, holl_ands.

Like the kind of scope this guy uses:
http://www.hdtvmagazine.com/columns/...ruary-2009.php

Keep in mind, NONE of the waveforms on that page are perfect. But hey, its NYC on indoor antennas, heh.

Ah, yes.

Peter Putnam is using arxaw's favorite RS indoor antenna and a BK Precision 2625 spectrum analyzer, which shows the amplitude of the signal across the channel. The ragged traces don't prove to me that the problem is high SWR, it looks more like what I would expect from multipath reflections when using an indoor antenna in an urban location.

So how can the average guy use that information to test and aim his antenna if he can't afford expensive test equipment?

We get a lot of people coming here with reception problems. Is there any advice that we can give them that we are not already doing?

I still think that using an A/B switch to compare two antennas is a valid, relatively inexpensive, test to see which antenna is better. If the output of the switch is connected to a tuner, it will tell you which antenna is better no matter whether the problem is gain, location, beamwidth between 3dB points, multipath reflections, SNR, F to B/R, or high SWR.

I think you have to be very careful about drawing any conclusions about the performance of an antenna based on the waveforms seen at any one location. The waveform seen is the result of the transmitter antenna response (often not flat if you're not in the main lobe), what obstacles may lie in the path to you, and the response of the receive antenna at the location it is placed to the many reflections inside a building (or outside). I'll bet those displays would change a lot if the antenna was moved.

I have a spectrum analyzer and what I've found out is that non-LOS signals are constantly changing on all time scales.

I made a couple of animations of KGO on RF 7 which is 110 miles from here.

The first one shows how the display changes as the signal gets stronger.

http://images.aa6g.org/AVSForums/KGO_Animation.gif

The second is a series of images over a couple minutes when the signal is weak (most unstable) and what happens when a plane causes multipath.

http://images.aa6g.org/AVSForums/KGO-Animation-3.gif

Lastly, attached is an image of KSCO on RF 3 which is LOS here but I'm off the back of their antenna. This is an extreme example of what often happens when you're not in the main lobe of a station's transmit antenna. That still manages to produce an SNR of 26 dB. It's very common for the signal to vary with frequency.

Chuck

Chuck:

Thanks for the screen shots and neat animation.

Very helpful.

I agree.

http://images.aa6g.org/AVSForums/KGO-Animation-3.gif

What is that pulse of energy on the left edge of the display that appears and disappears?

Any idea what multi-path looks like?

Thanks

Thanks for the link to the DHC thread, and in particular to your links in post #716. I have looked at those references and they seem to focus on the SWR between the transmitter PA and antenna which can reduce the SNR/MER below the 27 dB required by the FCC for a transmitted signal.

I think it is a mistake to use the antenna reciprocity theorm to assume that what goes on the in receiver's feedline is equivalent to that in the transmitter's feedline. The signal that comes from the transmitting antenna is sinusoidal (analog.....like NTSC).

An increase in the SWR in the receiving feedline causes an additional lineloss that is of the same nature as the inherent coax loss at that frequency and is added to that loss. Both increase the system NF and cause a reduction in SNR which requires a stronger signal to maintain the minimum required SNR.

A multipath problem that shows on the spectrum analyzer as a ragged signal also requires a stronger signal to maintain the minimum SNR between the dips and the noise floor in order for the equalizer to compensate, if it can.

OK, you've got me hooked, holl_ands.

What kind of a test can I devise to prove Dr. Bendov's theory?

Would it be sufficient to insert a section of coax in the feedline with an impedance much higher or lower than 75 ohms to give a SWR mismatch of 4 to 1, or is there a better way to do it?

Agree with the screen shots. Based on your experience, if my TV shows a signal is present because of the AGC levels but I get no display, does that indicate that if I move my antenna to different locations, there is a good chance I will get a display? The AGC level is about the same as stations I can receive.

It looks like this:
http://www.prism.gatech.edu/~wn17/

Note that there are two pages.

Yes, probably. It sounds like the multipath reflections are too much for your tuner to handle for that particular channel.

It is possible to have a strong signal that can not be decoded by the tuner if the errors caused by the multipath reflections exceed the capability of the error correction system (FEC). Trip proved that at his location in TN.

What are your AGC numbers? Does it give signal strength derived from AGC or does it give SNR? You need at least 15.5 dB SNR to decode a digital signal.

Thanks, very educational and I got a good laugh out of page 2.

"Honey, would you get upset if I mounted a garbage can to the chimney?"

I listened to it on my ham transceiver and it sounds like packet radio. It's a bunch of different transmitters that make short transmissions. It's on about 173.3 MHz.

Chuck

What were the SNR numbers for the unshielded antenna and the two shielded versions?

Chuck

That would be my experience. Are you using a Sony? That's the only TV I know of that displays AGC numbers. I've found it to be a fairly reliable indicator of the minimum signal strength required to decode a signal as long as there is no co-channel or adjacent channel interference.

Chuck

Chuck:

It's only implied by the divisions on the screen.

I'm having a hard time reading the numbers on Bill Naivar's TEK 2715 SA display. He claims 2 dB more gain with the can, but I can't make it out. I guess you would have to e-mail him.


I really like the Diagnostics Screen on my SONY KDL22L5000.

AA6G.....neat call. Did you know Mark, W6AKG?

@Calaveras, Yes my TV is a Sony. @rabbit73, don't know how my AGC numbers are derived. I do know 100% is shown when I tune an empty channel, and it goes down from there. Most of the stations I receive show 35% to 50%.

Thanks to you both for your replies.

I don't think you can reliably see multipath on a spectrum analyzer. One of my strongest stations has a very flat analyzer trace but ranks among the lowest for SNR. Variance in amplitude across the analyzer trace does not automatically mean multipath.

There are DTV analyzers that will show multipath.

Chuck

There's no doubt that the appearance of the signal on the analyzer improved with the shielding but unless that translates into an improved SNR then the shielding is not doing what you think it is doing. Without knowing the SNR in the 3 configurations it's not possible to draw any SNR conclusions which is really only what counts in DTV reception. Can you run those tests with your Sony connected?


The diagnostics are very helpful when you know what the numbers mean. I wish all TVs had that. It would sure help us troubleshoot the many reception problems that appear here.

I didn't know W6AKG.

Chuck