[TowerTalk] Fw: Importance of feed line length

Mon Sep 10 09:43:26 EDT 2012

----- Original Message ----- 
From: "Gene Fuller" <w2lu at rochester.rr.com>
To: "Jim Lux" <jimlux at earthlink.net>
Sent: Monday, September 10, 2012 9:33 AM
Subject: Re: [TowerTalk] Importance of feed line length


> Another way to appease your transmitter, if the transmission line SWR is 
> too high, is to vary the transmission line length by fractions of a 
> wavelength. This doesn't actually improve the SWR on the line but will 
> change the impedance where it terminates into the transmitter. I built a 
> box with 2,4, and 8 foot (physical, not electrical) lengths of RG-213 
> that, with relays,can be selected in 2 foot increments from 2 to 14 feet 
> that I use for various applications, including "line flattening" and phase 
> adjusting (e.g. with phased arrays).  The approach of changing line length 
> can also be helpful if the SWR is high enough so it's  outside the range 
> of an "antenna tuner". The lengths I use at too short to be much help on 
> the low frequency bands but good for 10-20 maters.(If your radio has a 
> built in tuner and the 80 meter SWR is too high you could try inserting 
> additional coax in  10-20 foot increments.)
> Gene / W2LU
>
> ----- Original Message ----- 
> From: "Jim Lux" <jimlux at earthlink.net>
> To: <towertalk at contesting.com>
> Sent: Sunday, September 09, 2012 11:20 PM
> Subject: Re: [TowerTalk] Importance of feed line length
>
>
>> On 9/9/12 6:05 PM, Justin Whitstine wrote:
>>> Hi everyone, I've been doing research to determine how important feed
>>> line length is.  I've been reading up on this off and on for the last
>>> few weeks.  I'm currently studying to get my ham license and am very
>>> excited about it.  I built my first antenna this weekend.  A 1/4 wave
>>> vertical on 2m, just to see what I could hear.  It was a rewarding
>>> experience!
>>
>> Very cool..
>>
>>
>>>
>>> I've seen much talk on feed line length and found some very
>>> convincing article, but then I seem to find others that contradict
>>> them.
>>
>>
>> Yes, indeed... it's an area of much discussion. There's theory and common 
>> practice and they differ, with a wide, wide array of explanations why. 
>> There's simplified theory and not so simplified theory. There's also 
>> "practical difference" and "immeasurable difference".
>>
>> You're just starting out, so the practical advice is "make it work, but 
>> don't agonize about the last half a dB or even a couple dB".
>>
>>
>>
>>
>>  I have read lots of posts from this forum and this seems to be
>>> one of the most knowledgeable places on the internet.  Once I get my
>>> general license I am planning on setting up an inverted v on 80m.
>>> Just when I thought I had figured out the feed line length issue, I
>>> learned about cable velocity factor and how common mode current
>>> travel at much higher speeds down the feed lines than the actual
>>> signals going to the antenna.  That really blew my mind!
>>
>> Sort of.. but that's really a "second order" effect. If you have decent 
>> RF chokes, then you won't have any common mode current (or, practically, 
>> it's small enough to not make a BIG difference).
>>
>>
>>
>>
>>>
>>> My 80m antenna will not be very high off the ground, so it will be a
>>> NVIS.
>>
>> How high is high?  10-20 ft is somewhat of a cloud and worm warmer, but 
>> hey, it works, and not everyone gets to throw a dipole up a wavelength 
>> above ground..
>>
>>
>>
>>   I will be using a common mode choke as per Jim Brown's
>>> document "A Ham's Guide to RFI, Ferrites, Baluns, and Audio
>>> Interfacing".
>>
>>
>> Most excellent.
>>
>>  Which was the most informative document on the
>>> internet about chokes, and what led me here.  I will go with a 50 ohm
>>> feed line because my antenna will be between 8 & 15 feet off the
>>> ground, and probably an inverted v.  I know 50 ohms will match an
>>> inverted v better.
>>>
>>
>> Maybe, maybe not.. but it really doesn't matter all that much.  A low 
>> antenna has strong interactions with the soil, and who knows what 
>> properties the soil (and any trees, plants, house, small children, grass, 
>> etc) have.  You just take what you get.  ANd because 50 ohm coax is 
>> common, you might as well use it.  75 ohm coax is also common, and I 
>> doubt you'd notice a huge difference either way.
>>
>> When you get to running a kilowatt, or ekeing out the last dB, agonizing 
>> about perfect match is important.  But for the start, 50 or 75 work 
>> equally well.
>>
>> CHoke it at the antenna with a couple ferrite cores.. Choke it where it 
>> comes into the shack with a couple cores.  That's what you want..
>>
>> Then either tune by pruning, or get a tuner at the shack end, and get on 
>> the air.
>>
>> the whole pruning thing requires a decent test set up and the ability to 
>> raise/lower the (ends) of the antenna.  You don't actually have to cut 
>> the wire. Just wad it up at the end to shorten it, or double it back and 
>> take a turn of PVC tape around it to hold it. (that way, you can lengthen 
>> it again)
>>
>> If you're reasonably in the ballpark, a tuner at the shack end (maybe in 
>> your radio, or an autotuner) will match it up nicely.  Sure, there's some 
>> loss in the coax from reflected power going back and forth, but unless 
>> it's truly hideous, it will still work.
>>
>>
>>
>>
>>> I'm trying my best here to not sound totally ignorant, but if any one
>>> knows the answer I would appreciate it greatly!  I look forward to
>>> your answer! Thank you, Justin
>>>
>>>  _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________ TowerTalk mailing
>>> list TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>