Like many things, price does not necessarily reflect a better antenna,
however there are differences between antennas.

One difference is in the quality of the filters in the antenna itself.
This matters more when one is mounting an antenna at a communications site
than say at a home timing lab.  If you're at a site with a lot of RF noise,
if the antenna can pre-filter this out this will help the GPS receiver
dramatically.  I just pulled the spec sheet for one that I'm familiar with
and the filter in this one drops signals outside the GPS band by 65dB at
the band edges.

Another difference is the robustness of the higher end units, including
surviving nearby lightning strikes, operating at a wider voltage range, etc.

I'm not enough of a time-nut (yet) to be able to tell you if there is some
subtle difference between the antennas which would matter once you get to
some level of precision I don't have to deal with yet.

I've personally had good luck with the PCTEL/maxrad timing
reference antennas (GPS-TMG series).  I usually have been able to pick them
up off of ebay for not a lot of money.  These also seem to be often
re-sold with a lucent or symmetricom label or some other company's label on
them, mainly because they seem to be a supplier to all of these providers.
I don't seem to recall ever having one of this type fail.    That can't be
said of other ones I've bought from random no-name suppliers.

On Thu, Nov 21, 2019 at 12:00 AM Taka Kamiya via time-nuts <
time-nuts@lists.febo.com> wrote:

--

• Forrest

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

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

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

John Ackermann N8UR writes:

I don't think you're going to figure out an optimum gain without
proper measurements along the whole signal chain.  As a proxy, the
reported SNR should stay fairly constant with changing constellation and
environmental conditions.

That one is easy to solve with either a too long / too low quality
cable.  :-) Otherwise you may just have to add a 20..30dB damping plug
on the receiver side.  Keep in mind that a 50dB antenna typically either
would not work at all with the supply coming from that GPS module (12V
vs. 5V or even just 3.3V) or actually has lower gain because of the
lower supply voltage.  So you may need a DC block on the receiver and a
bias-Tee at the antenna as well.

Regards,
Achim.

+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Factory and User Sound Singles for Waldorf Blofeld:
http://Synth.Stromeko.net/Downloads.html#WaldorfSounds

I need another project like a hole in the head — but curiosity continues to nag me.

I moved to Boulder CO a few months ago. The “curiosity” is to determine the position of two antennas at either end of my house and monitor it over time, with the idea that one could see plate movement in 3 dimensions plus rotation around the axes.

What hardware/software would be suitable for a TimeNuts (PositionNuts?) project like this?

— Eric K3NA

John Ackermann N8UR jra@febo.com writes:

Too much gain can manifest in at least two different ways:

1. intermodulation distortion in the preamp

2. distortion/overload in the GPS receiver

Adding an attenuator or cable as someone suggested can help you
determine if the preamp gain is excessive given your cabling and GPSr
frontend.  If you add 10 dB of loss, and the C/N0 doesn't change,
arguably you have gain you didn't need, and which therefore has elevated
risk of IMD.  If it goes up, you (mostly) know you are overdriving your
receiver (which would be surprising to me).  If it drops, then you
probably need most of the gain.

This is tricky, because a system with too much preamp gain will be prone
to IMD if other signals appear but may operate just fine when they
don't.

That said, I am unclear on:

typical filtering before the antenna preamp (very little in a
dual-frequency antenna?)

3rd-order IMD dynamic range in these preamps

strength of non-GNSS signals that appear in the filter passband

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@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

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Hi

The “cheap” approach these days is to use a uBlox ZED-F9P

https://www.digikey.com/products/en/rf-if-and-rfid/rf-evaluation-and-development-kits-boards/859?k=F9P https://www.digikey.com/products/en/rf-if-and-rfid/rf-evaluation-and-development-kits-boards/859?k=F9P

along with one (or more) the <$100 eBay antennas. You take 24 hour data
runs and ship them off to any of several free analysis outfits. One is NRCan.
There are a lot of others. They all do a fine job.

Bob