[time-nuts] Difference in antennas

Fri Nov 22 17:13:27 UTC 2019

I believe that's to avoid errors from phase center offset -- when the antenna calibrations are done, the measurements are taken with the antenna aligned that way so aiming it in the fields ensures releatability.

On Nov 22, 2019, 12:01 PM, at 12:01 PM, Bill Dailey <docdailey at gmail.com> wrote:
>I would like to tag on to this.  I have a large Leica L1 choke ring
>antenna.  It has an indicator for “N”.  Not sure why.  I placed it on
>the roof without respect to directionality.  I will rotate it with “N”
>facing north in a month or so to see if there is any effect.
>
>Bill Dailey
>
>Negativity always wins the short game. But positivity wins the long
>game. - Gary Vaynerchuk
>
>Don’t be easy to understand, 
>Be impossible to misunderstand 
>- Steve Sims
>
>> On Nov 21, 2019, at 6:00 PM, Dana Whitlow <k8yumdoober at gmail.com>
>wrote:
>> 
>> Most modern GPS receivers are very quiet even barefoot.  So, one
>could
>> argue that one should
>> not have much more LNA gain in the antenna than required to make up
>for
>> feedline loss, which
>> should be easily calculable.  While excess gain  in the antenna can
>improve
>> overall system noise
>> figure a small amount, it will degrade intermod performance, which is
>> likely to be a worse problem
>> than simple weak signals.  It's likely that the cure is worse than
>the
>> disease, as my doctor likes to
>> say.
>> 
>> Dana
>> 
>> 
>> 
>>> On Thu, Nov 21, 2019 at 4:00 PM Taka Kamiya via time-nuts <
>>> time-nuts at lists.febo.com> wrote:
>>> 
>>> So.... concensus is, 50dB gain antenna is too much gain, unless feed
>line
>>> is too long, reception is poor, or there are other circumstances
>extra gain
>>> is desired?
>>> 
>>> ---------------------------------------
>>> (Mr.) Taka Kamiya
>>> KB4EMF / ex JF2DKG
>>> 
>>> 
>>>    On Thursday, November 21, 2019, 3:00:14 PM EST, Bob kb8tq <
>>> kb8tq at n1k.org> wrote:
>>> 
>>> Hi
>>> 
>>> That is indeed the gotcha. Once you get past a certain amount of
>gain in
>>> the
>>> preamp, the C/N levels don’t change enough to notice. Looking today
>vs
>>> looking
>>> tomorrow is unlikely to be of any help if you are after a fraction
>of a
>>> db.
>>> 
>>> About the only way to check would be to fast switch an attenuator in
>and
>>> out of
>>> the signal path. Watch things for a minute at one setting and then
>do the
>>> same at
>>> another setting. Run for a while and log all the deltas. If you see
>a
>>> degradation of
>>> more than a few tenths of a db, you are getting towards the minimum
>gain
>>> point.
>>> 
>>> Indeed there are some receivers that have an AGC built in. *IF* your
>>> receiver has one
>>> and *IF* you can get at it, that would be a great way to work this
>out.
>>> Indeed anybody
>>> who makes it past both of those constraints has a pretty unique
>device.
>>> 
>>> ====
>>> 
>>> Simple answer for a 50 db antenna is to put an attenuator in after
>the DC
>>> has
>>> been eliminated from the circuit. It’s not idea, but it’s the best
>you can
>>> do. Running
>>> a great big splitter is one great way to come up with attenuation
>…..
>>> 
>>> Bob
>>> 
>>> 
>>>>> On Nov 21, 2019, at 10:29 AM, John Ackermann N8UR <jra at febo.com>
>wrote:
>>>> 
>>>> Bob, this is a great summary, thanks!
>>>> 
>>>> One related question, especially with mixed systems -- how do you
>tell
>>>> if you have optimum signal level at the receiver?
>>>> 
>>>> Most show some sort of SNR or Cn value.  What should we look for? 
>What
>>>> are the indication of *too much* signal?  One issue in particular
>is how
>>>> to handle a modern GPS that expects modest antenna gain when it's
>>>> plugged into a system with a 50dB gain antenna at the top.
>>>> 
>>>> Thanks!
>>>> John
>>>> ----
>>>> 
>>>> On 11/21/19 8:00 AM, Bob kb8tq wrote:
>>>>> Hi
>>>>> 
>>>>> Way back in time, the first gear out there to use what we now look
>at
>>> as “normal” antennas
>>>>> was survey gear. For various reasons they decided on a 12V power
>supply
>>> and 40 to 50 db
>>>>> of gain in the preamp mounted in the antenna. They also got into
>L1 /
>>> L2 pretty quickly.
>>>>> 
>>>>> A bit later the cell phone (and later broadcast) guys got into
>this. In
>>> a location with a lot of
>>>>> RF (like a cell site) having a lot of gain at the antenna didn’t
>work
>>> all that well. IMD issues
>>>>> got into the act pretty quickly. In addition, front end filtering
>was
>>> required to reduce overload
>>>>> issues. The focus was on L1 only so filtering was relatively easy.
>>>>> 
>>>>> There is a whole separate set of antennas that put a big chunk of
>the
>>> RF portion of the radio
>>>>> in the antenna. Those still survive here and there. I have one of
>them
>>> and probably a couple
>>>>> of dozen of the more “normal” antennas.
>>>>> 
>>>>> As time marched on, supplying 12V to antennas became a bit less
>>> popular. Most of the cell
>>>>> guys went over to a 5V antenna supply. The net result was 12V 50
>db
>>> survey antennas that did
>>>>> L1/L2 and much smaller 5V 25 db antennas for “timing”. The timing
>>> antennas didn’t do L1/L2 so
>>>>> not going to work for survey. The survey antennas had way to much
>gain
>>> and no filtering so
>>>>> not going to work for a cell site.
>>>>> 
>>>>> Indeed things did and do get crossed up in various pro and
>basement
>>> systems. With care and
>>>>> the right set of circumstances things may work. In other cases the
>>> result can be an ongoing set
>>>>> of systems issues over an entire network of stations.
>>>>> 
>>>>> Prices for a good new survey antenna are up in the many thousands
>of
>>> dollars range. They have
>>>>> very stable phase centers and (usually) test results to allow
>>> correction of any residual phase
>>>>> issues. This is part of what lets you get into the “couple of mm”
>range
>>> on a survey.
>>>>> 
>>>>> For timing, you have to dig a bit and answer a few questions. Is
>your
>>> concern how close you
>>>>> are to BIH? If so you will need to know all the delays in your
>system.
>>> This includes the delays
>>>>> in the antenna filters and the preamp. Is your concern (or
>measure) the
>>> ADEV at 1 second?
>>>>> If so the delays are not a concern. Your antenna choice may be a
>bit
>>> different depending on
>>>>> this focus.
>>>>> 
>>>>> Bob
>>>>> 
>>>>> 
>>>>> 
>>>>>> On Nov 21, 2019, at 1:25 AM, Taka Kamiya via time-nuts <
>>> time-nuts at lists.febo.com> wrote:
>>>>>> 
>>>>>> I have been looking antennas.  Prices seem to range less than 30
>>> dollars to more than 500 dollars.  Some are 20db gain and some are
>40 db
>>> gain.  Some are specified as marine use only.  Some are specified as
>timing
>>> use.  Some doesn't say anything at all.  Power supplies are
>different.
>>>>>> Other than obvious, antenna is an antenna, isn't it?  It captures
>L1
>>> signal, amplify it and send it down the coax.  What makes one more
>costly
>>> than others?  What makes one timing antenna and one navigation
>antenna?  It
>>> doesn't make sense to me.
>>>>>> 
>>>>>> I did some simple experiment with 26db, 40db, and magnetic stick
>on
>>> type.  I didn't really see significant difference.  Signal level
>itself
>>> even wasn't all that different.  I have nearly a clear sky view 360
>degrees
>>> above 30 degrees above horizon.  In some directions, clear view to
>>> horizon.  My feed is Timewave type.  So It may not be the best but
>nearly
>>> ideal.
>>>>>> 
>>>>>> Can someone shed light on this topic?  (of course, I know some
>antenna
>>> has integrated receiver.  I am not talking about those)
>>>>>> 
>>>>>> ---------------------------------------
>>>>>> (Mr.) Taka Kamiya
>>>>>> KB4EMF / ex JF2DKG
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>>>>> 
>>>>> 
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>>>> 
>>>> 
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