Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: Preamp and high power transmitter  (Read 6098 times)
AA4PB
Member

Posts: 13032




Ignore
« Reply #15 on: May 27, 2010, 12:45:28 PM »

I thought that If I hard keyed the relay and used a sequencer
that there would be NO current or voltage on the contacts during
switching.   2.8A vs 1.8A after switching doesn't seem like much
power difference to me at nearly 0V.

You are guessing that if the relay can handle 1.8A it can handle 2.8A. Until you look up the relay specs you don't know for sure.

In addition there is capacitance between the open contacts which can couple RF between the HOT contacts and the input to the preamp. How much voltage is present at the preamp input and can it handle it? These are the questions you need to answer OR you just try it and hope you don't blow the preamp. One of the problems though is that because it works once or twice doesn't mean that it'll work over the long term.

Logged
WB2WIK
Member

Posts: 20666




Ignore
« Reply #16 on: May 27, 2010, 01:39:22 PM »



Quote
One of the problems though is that because it works once or twice doesn't mean that it'll work over the long term.



That's for sure.  And another sure thing is that masthead preamps rarely work over the long term no matter what they are!  For one thing, they should always be switched to "bypass" mode (and not in line) when not in use, and keyed to actually put them in line.  That way, the preamp won't be connected to the antenna during lightning storms.  But even that small measure of protection doesn't guarantee much: Fact is, the antenna is still connected to something "near" the active stage of the preamp and that alone is enough to destroy sensitive GaAsFETs and similar devices which are very sensitive to ESD.

When I had my masthead preamps on 144 and 432 MHz (for about two years) I made more trips up and down the tower (to service the preamps) than I've made in the 22 years since I decided they weren't worth it. Tongue
Logged
K5MBV
Member

Posts: 265




Ignore
« Reply #17 on: May 27, 2010, 03:41:35 PM »

"You are guessing that if the relay can handle 1.8A it can handle 2.8A. Until you look up the relay specs you don't know for sure."

You're right, but I do know that there is practically no power difference between
the two because the resistance is very low. I believe that power is a cause of heating
which is what you must be assuming to be the cause of failures.
That should not be an issue here. Very low power and cold switching should not cause a problem. Hot switching however can results in arcing which could destroy the relay contacts. 

My old FT-736 Noise Figure at 2 meters  isn't very good, so a preamp makes a difference
for me. I would much rather not have to use one.

Ken  K5MBV
Logged
WB2WIK
Member

Posts: 20666




Ignore
« Reply #18 on: May 28, 2010, 08:23:26 AM »



My old FT-736 Noise Figure at 2 meters  isn't very good, so a preamp makes a difference
for me. I would much rather not have to use one.




I'd just modify the FT-736 to improve its noise figure, and use the lowest possible loss transmission line.  Funny yours isn't very good (they must vary), I have an FT-736R which I bought new in 1987 when it first came out and its NF is lower than atmospheric noise, so making it lower wouldn't do anything.  In fact, I do have several ARR MGF1402 GaAsFET preamps for the various bands around here, and one for 144 MHz is tagged by ARR at NF = 0.7 dB.  It has so much gain it can overload the receiver that follows, but experimentally I've installed this in line with my FT-736R receiver several times just to see if any very weak signals become more readable, and they don't.  My "weak signal" standards are various distant beacons that are just over the noise level.

I can't leave a preamp like that in line anyway (even if I installed it inside the transceiver, in the receive-only signal path) because as mentioned above, it causes receiver overload on strong signals, and I deal with plenty of those. Tongue 
Logged
K5MBV
Member

Posts: 265




Ignore
« Reply #19 on: May 30, 2010, 12:40:47 PM »

Have you tried the preamp at the antenna instead of in the shack? That
should be where you might see the difference. You may have a high ambient
noise level which would mask any improvement over the FT-736R. My noise
level here is very low compared to other places I've lived. We have underground
utilities and live in a small valley. The antenna has to be at least 45 ft to get
over the rim of the valley in most directions.

I've decided to try hard keying the preamp using a sequencer. I have two of
them with plenty of spare relays. The feed is 65' of new 1/2" Heliax and the
rotor loop is 20' of LMR400U.

Ken  K5MBV
Logged
WB2WIK
Member

Posts: 20666




Ignore
« Reply #20 on: May 30, 2010, 07:36:06 PM »

Have you tried the preamp at the antenna instead of in the shack? That
should be where you might see the difference.

Ken  K5MBV

Of course I've tried that: Virtually no difference, since my transmission line loss is very low.  If you have substantial line loss, then of course a masthead preamp can make a difference.  My line is 185' of 7/8" Heliax and it doesn't matter if the preamp is at the antenna or in the shack: The line loss is so low, it really makes no difference (measured, and confirmed).
Logged
K5MBV
Member

Posts: 265




Ignore
« Reply #21 on: June 04, 2010, 08:28:21 PM »

I'm told that an additional 1/2 dB added to the NF is noticeable
for EME.  No need to have that much more NF if it's not
necessary. 7/8" Heliax loss per 100' at 150 MHz is about .44 dB.

Ken  K5MBV
Logged
WB2WIK
Member

Posts: 20666




Ignore
« Reply #22 on: June 05, 2010, 02:59:31 PM »

I'd concentrate more on antenna gain and reducing losses everywhere in the system.

I can't be concerned about 1/2 dB.  My e.m.e. margin is much higher than that, and "aiming" towards population centers for low elevation work raises noise levels a whole lot more than 1/2 dB.  If you plug in your antenna and the noise level shows an obvious increase, a "zero dB" noise figure with an infinite gain preamp generally won't help. Tongue

Logged
KM3F
Member

Posts: 526




Ignore
« Reply #23 on: June 05, 2010, 11:40:40 PM »

Many prople can't get their thoughts around the concept of noise figure in total.
Reciever noise floor.
Transmission line loss,
Antenna gain,
imediediate area noise level dictate the final level you have, then if anything can improve on it.
.
A reciever with low sensitivity/high noise figure usually can be improved with a pre-amp of a lower noise figure.
Transmission line with as low a loss as is practical so little of the weak signal is lost.
Antenna gain sometimes is a hinderence if pointed at a source of noise. The antenna gain can't seperate the desired signal from the noise just like a signal amplifier usually can't.
From these items you can see if any area can be improved in your setup.
Certian special cases can use low NF amps but are the exception in general use.
For example,what is the total round trip of the signal path loss for EME operation on 2m?
Well over 250 db loss. This would be an example of how the loss is made up by both the transmitt end and the recieve end.
Looking at this sitution should be very educational where the use of amplifiers, feedlines antenna array/gain, noise figures and transmit powers all come into play.
Once you see this, the understanding of simple weak signal work with the usual setup becomes
easy to understand.
Logged
KQ7W
Member

Posts: 13




Ignore
« Reply #24 on: June 07, 2010, 02:24:44 PM »

Ken -

not sure 100% but this might help

http://www.dxengineering.com/Products.asp?ID={0F7EACFC-C37D-49DF-B413-E5241597BA41}&SecID=116&DeptID=12

0ms to 2ms might be good enough for QSK almost?
Logged
K5MBV
Member

Posts: 265




Ignore
« Reply #25 on: June 10, 2010, 09:15:11 PM »

I'm using as low loss coax as I can handle, but don't have the
space or money for more antenna gain. I'm using a single M2
18 el Yagi. The thing is 36 ft long and the trees are going to have
to be trimmed to rotate it 360 deg.The preamp actually doesn't
help with the S/N as much as it just adds noise. The ambient
noise is showing S0 on the FT-736R and with the preamp on the
meter reads S1. We are lucky to live in an underground utility
area and the background noise is very low here.

The preamp is easy to reach when the yagi is pointed straight
up. I don't believe it's working as well as it should and plan to
send it to ARR to be checked out.

MY EME station is on 2304 with a 3 meter BUD, a 200W SSPA,
and a G5DDK LNA. It's not hooked up to the dish feed yet because
I've been playing with the sequencer and relays.
Logged
K5MBV
Member

Posts: 265




Ignore
« Reply #26 on: June 10, 2010, 09:21:53 PM »

Correction: G4DDK LNA.

I looked at the sequencer in the URL above and wish I had seen
it before buying the one from ARR. It's a beautiful piece of Ham gear
and it would be hard to make a sequencing mistake using it.
Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!