[time-nuts] Difference in antennas

[Bob kb8tq]

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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