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 <[email protected]>
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 <[email protected]> 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
>>> <[email protected]> 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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