[Elecraft] Transmission Line Losses (Was: Baluns)

[Bill Coleman]

On 4/24/02 4:50 PM, Ron D' Eau Claire at rondec@easystreet.com wrote:

>Bill, I'm looking at losses for high SWR's, not "matched" conditions. This
>data is published by the ARRL and a variety of others although the charts in
>current ARRL publications require some extrapolation for really low loss
>lines like open wire line.

Ok, I'm looking at two charts in my 2000 ARRL Handbook.

The first reads "Cable Attentuation dB per Hundred Feet". The caption 
starts: "Figure 19.4 - This graph displays the matched line attenuation 
in dB per 100 feet.

Taking the 10 MHz example, this table yeilds the following values:

RG-58     - 1.4 dB
RG-8      - 0.66 dB
Window    - < 0.1 dB (probably 0.07 dB)
Open Wire - < 0.1 dB (probably 0.04 dB)

Note, these values are for the matched condition.

>For example, using 100 feet of feeder with SWR of 20:1 at 10 MHz, the losses
>are approximately:
>
>RG-58  = 6.5 dB
>
>RG-8   = 5 dB
>
>"Window" line = < 1 dB
>
>Open Wire line = < 0.1 dB

The second chart has the caption - "Figure 19.4 - Increase in line loss 
because of standing waves" Note that this chart is completely frequency 
independant. The only requirements are knowing the matched line loss, and 
the SWR present on the line. The chart STARTS at 0.2 dB for the matched 
condition, so estimating the additional loss is really hard.

Using your 20:1 SWR figure (more on that in a moment), I read the 
additional loss as follows:

RG-58     - 5 dB
RG-8      - 3.3 dB
Window    - 0.6 dB (estimated)
Open Wire - 0.3 dB (estimated)

Which gives a total of

RG-58     - 6.4 dB
RG-8      - 4 dB
Window    - 0.7 dB 
Open Wire - 0.3 dB 

The first figure is pretty close to yours, the others are much further 
afield. You estimated far too little loss for Open Wire.

--

Let's talk about this 20:1 SWR issue. You realise that by specifying a 
20:1 SWR for all four feedlines, you made it such that only two of them 
are connected to the same antenna? (eg the two Coaxes) This is because 
each type of feedline has a different characteristic impedance. In order 
to establish a 20:1 SWR, we'd have to modify the antenna as well as 
changing the feedline.

Let's take an antenna with an impedance of 22.5 ohms at 10 MHz. If you 
conect this to RG-8, we'll see an SWR of 2.2:1. 100 feet of that would 
have a total loss of about 0.8 dB.

Now, take the SAME antenna and connect it with 450 ohm window line. 
You'll see a 20:1 SWR, and the loss we computed as 0.7 dB above. Gosh, 
that's a mighty small difference! Much different than the figures given 
above!

--

If you look at these charts -- something becomes obvious. The 
transmission line losses generally fall into place based on a) the size 
of their smallest conductors, and b) their impedance. Since the table in 
19.4 is for the MATCHED condition, it makes sense -- higher impedances 
will show less loss for the same sized conductors, since the currents are 
smaller. 

Note that RG-59 has lower losses at higher frequencies (VHF/UHF) than 
RG-58. The two coaxes are very similar, but RG-59 has a smaller center 
conductor. However, since the impedance is higher, the losses are reduced 
some what. The resistive losses dominate at low frequencies, so below 
about 7 MHz, RG-58 has lower loss. Above that, dielectric losses come 
more into play, so the RG-59 gains a substantial advantage at 300 MHz.

But, at HF, dielectric losses aren't usually an issue.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901