[time-nuts] Lucent KS-24361/Thunderbolt antenna advice

Bob Camp kb8tq at n1k.org
Wed May 6 18:09:12 EDT 2015


Hi

> On May 6, 2015, at 3:53 AM, John Marsden <oghma at live.co.uk> wrote:
> 
> 
> On Tue, 5 May 2015 17:58:06 -0400
> 
> Bob Camp said (inter alia):
> 
> 
>> Ok, all of these receivers are designed to work with an amplified antenna. The typical antennas have between 20 and 30 db of gain.
>> They allow a cable loss of ~ 10 db between the antenna and the receiver. A 3 db splitter would come out of the “cable loss” budget.
>> 
>> The receivers put out 5V on the coax. You can find antennas that only work with> 7V. You can also find 3.3V antennas that will be
>> damaged by 5V. Most of the low cost antennas you come across will work at 5V.
> 
> OK, Bob,  that's made things a bit clearer - at least I have some pointers now on what to look for :)
> 
>> They (mag-mount antennas) also are a bit tough to mount solidly. You will spend weeks letting the GPSDO’s stabilize and many days doing
>> surveys. Throwing that all away each time the antenna moves is no fun.
> 
> That's an important point - one which I hadn't really considered until now...
> 
> 
>> The auction sites will sell you a “timing” antenna for < $40 delivered. With some shopping, you can get a very good antenna for < $150
>> delivered. You already have $300 to $500 invested. Skimping on the antenna does not make a lot of sense.
> 
> OK, so one last question before I head off to scour FleaBay for something suitable:  What's the difference between the $40 and $150 ones?  That is to say, what is it about the $40 ones that makes them a bad choice, that isn't an issue in the $150 ones?

You are after an antenna with:

1) Consistent delay over temperature.
2) Consistent phase center versus signal arrival angle
3) Good rejection of improperly polarized signals
4) Good rejection of signals reflected to the back side of the antenna
5) Good rejection of signals arriving at very low angles.
6) Adequate bandwidth for the bands you are after
7) Good filtering of the adjacent bands that you are not after
8) Good waterproofing, and rational mounting structure
9) Some way to keep birds from nesting on it

I’m sure there are more things to look for. The list is in no particular order.


> 
> 
>> There are a number of different
>> antenna designs out there. You can make a good one any number of different ways. There is nothing magic about a quad helix style.
>> 
> 
> Ok, I only ask becuse there seemed to be a big thing about LHCP quad helix antennas - even to the point of seein an article showing how to 'unwrap. a RHCP Q-H, and rewrap it 'inside-out' to change the polarisation to LHCP.
> I'm seriously considering making an active Q H if I can't find anything that looks promising - pending the answer to the question above, of course - I don't want to spend $100 making a '$40' one ;)

GPS helix antennas were a really big deal in about 1982. Once people started to get experience with GPS and a variety of designs, they became less of a big deal. I do not know of any modern 
precision antennas that use a helix. 

> 
> I take your point about not skimping, given the investment I already have, and obviously am keen not to limit the equipment's capabilities in order to save a few quid.

Far more important is not skimping on the antenna location. A crummy antenna in a great location will outperform the best antenna you can find located in the basement. 

Bob

> 
> Many thanks for your continued advice/help
> 
> John
> 
> 
> 		 	   		  
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