[Top] [All Lists]

[TowerTalk] Matching 75 ohm hardline for 3 bands

To: <>
Subject: [TowerTalk] Matching 75 ohm hardline for 3 bands
From: (Tom Rauch)
Date: Sun, 1 Apr 2001 16:04:11 -0400
I agree with the conclusions but it is important to remember a few 

> I wish you had included more info on what these antennas are and how
> you intend to use them.  I'm tempted to say, as do others, to just run
> the 75 ohm cable with the theoretical 1.5:1 SWR and let it be. 
> However, to that 1.5:1 theory you also end up seeing the results of
> other impedance bumps caused by your connectors. With hardline I've
> seen some interesting homebrew ones that, on UHF, would cause me to
> change the discussion from one involving feedline impedance to one
> talking about the impact of connector design at UHF.  So, while your
> "floor" SWR is 1.5:1, your "ceiling" SWR, and therefore overall
> feedline loss, is unknown.  Short of measuring the loss in the
> resulting cable, from Tx to ant, you're in the dark.

The actual mismatch normalized to a 50 ohm system ranges from 
1:1 to 2:1 at the transmitter. The transmission line sees a 1.5:1 
mismatch, but NOT the radio.

It is incorrect to say the SWR will be 1.5:1 because the radio is not 
a 75 ohm radio. Transmission line losses can be calculated based 
on a 1.5:1 SWR but things at the radio can get ugly.

It is also virtually impossible to cut a line for multiples of 1/2 wl 
when the line is dozens of wavelengths long. The slightest 
frequency change will result in a SWR increase, because the error 
of multiple 1/2wl sections in series adds. He has OVER 100 half-
waves in series at 450 MHz, so a 1% change in frequency is a 
100% change in the impedance cycle at the end of the line.

A 100 foot line at 70cm is like using a 9300 foot line at 75 meters! 
The bandwidth of the SWR minimum would be very narrow at 450 
MHz because of all those 1/2 wl sections in series.

> Finally, do you have antenna mounted preamps or are you accepting the
> noise figure degradation introduced by the line losses?  Even if the
> loss introduced by a 1.5:1 SWR on UHF ends up being only a dB, that is
> significant in terms of noise figure for SOME types of operation.

One thing we should keep in mind...on receiving the SWR is set at 
the shack end of the line...not the antenna end. You have a 
mismatch loss between the feedline and antenna, but the 
transmission line loss increase due to VSWR is determined by the 
input impedance of the load in the shack.

The antenna is the source, the radio is the load on receiving.  

73, Tom W8JI 

<Prev in Thread] Current Thread [Next in Thread>