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Re: Questions on practical aspects of coax and preamps.


I'll add my comments to those already given.

At 05:00 AM 10/25/2008, Curt Nixon wrote:
>Good Morning All:
>So, after this last week of ISS fun, I have the bug to upgrade into some
>simple satellite work.  I will be confined to simple FM untill I can
>upgrade to all-mode rigs.  My question is on coax and preamps.
>I will use a pair of vertically polarized yagis to begin with.  One for
>144MHz and one on 70cm.  I have some LMR 400 I got for the ISS project
>on 144 but it is so stiff, I didn't use it.
>1.  If you use LMR 400, do you add flexible sections at the antenna end
>and in the shack, using LMR400UF?
>Or, should I just use all UF and forget the additional connections?  The
>cost of the additional connectors (N) both in loss and $$ is probably
>advantage $ for UF.  Not sure about the loss potential for adding two
>sets of interface connections.

I use LMR-400 with LMR-400UF jumpers to go around my az-el 
rotors.  Losses in properly made connectors is negligible and the 
extra loss in a few feet of LMR-400UF is also negligible.  It does 
cost you a few more connectors.  BTW this is what I am using for the 
Rx side of me 2m-eme system.  I do use N-female connectors one end of 
the jumper to eliminate having to insert a "barrel" adapter between 
two male connectors.  N connectors are very good on all frequencies 
in use on satellites and have some wx-sealing properties (but one 
should still use two layers of electrical tape or coax seal or the like).

>2.  With a 5-7 element yagi on 144, is it really necessary to use low
>loss coax as a opposed to 8x or 8x mini.
>Runs here are 35' if munted at 12' and 60' if mounted to roof tower at
>about 20'

35-ft of mini-8 (RG-8x) still is lossy so it is better to use low 
loss cable.  The longer run it becomes more important.  Realise that 
in transmit on 145-MHz you will lose RF power in the coax.  If you 
only have a 5w HT that might become critical.  If you have a 10 or 25 
or 50w radio, you have power to throw away so on Tx the mini-8 would 
probably be fine.

>3.  On 70cm, I plan to add a mast preamp, but will use the yagi
>"barefoot" to begin with.  With a preamp at the antenna, is low loss
>coax still required or can the cost increment be put to better use in
>the preamp to begin with?   The preamp of course also gives a convenient
>place for a connector interface from stiff to flexible if desired.

Once you have a preamp at the antenna, then RG-58 is adequate between 
the preamp and radio.  If the preamp is used in the radio shack after 
a run of coax then low-loss cable will make a huge difference.  But 
seriously, the preamp belongs at the antenna or you are throwing away 
most of the advantatge that the low noise figure provides.

Without a preamp low loss coax is important for the same reasons as 
given above about using the preamp in the shack.  Using the LMR-400 
would make a lot of difference on the UHF Rx side.  Beyond a run of 
75 or 100 feet maximum, you will do better with 1/2 or 7/8 inch 
hardline.  This is another good reason to just mount the preamp at 
the antenna (you do not need "super" coax).

On my eme system Hardline is used on the Tx side to get my RF to the 
antennas with minimum loss.  The preamp is mounted close the antenna 
so I run 150-feet of LMR-400 back to the receiver.  EME requires much 
more attention to miniming losses and maximizing performance.

73, Ed - KL7UW              BP40iq, 6m - 3cm
144-EME: FT-847, mgf-1801, 4x-xp20, 185w
1296-EME: DEMI-Xvtr, 0.30 dBNF, 4.9m dish, 60W
http://www.kl7uw.com       AK VHF-Up Group
NA Rep. for DUBUS: dubususa@hotmail.com

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