[time-nuts] Difference in antennas
Bob kb8tq
kb8tq at n1k.org
Fri Nov 22 17:30:54 UTC 2019
Hi
If your antenna has test data associated with it (or even if it does not) there may / will
be differences in the phase response for various angles both in the vertical and horizontal
planes. Indeed some antennas have very minor changes vs angle and others move
quite a bit.
To keep this somewhat under control, survey antennas get a mark on them to tell you which
direction to point north. The idea being that if orientation is known, post processing corrections
to eliminate ( or at least reduce) the phase issues are possible. If you are not doing post
processing, there really isn’t much value in the magic “this way north” marking.
Bob
> On Nov 22, 2019, at 11:07 AM, 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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