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Author Topic: Antenna Feed Length  (Read 640 times)
KB3FFH
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Posts: 194




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« on: June 15, 2006, 07:32:37 PM »

I have a Cobra center feed dipole with 65ft of ladder line than 100' of RG-213 to radio. My antenna is in the woods with the ladderline than the RG-213 goes underground than into the house. I am avoiding the HOA. Should I have a choke anywhere ? Maybe 10' of the RG-213 rolled up.Also I am going with a 1 to 1 current balun is this correct? Anything to make this better? I know its a long way and have not tried it yet. Thanks Bill
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K0ZN
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Posts: 1774




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« Reply #1 on: June 15, 2006, 07:48:53 PM »


Hi,

I am not familiar in detail with that antenna, but I can positively assure you that you CANNOT ( Should not!!) connect coax directly to ladderline... they are two totally incompatible types of transmission line. You should have a balun between the coax and the ladderline. In theory, if you do that, there should be a minimal need for a coax choke on the coax.  

You could certainly easily try a homebrew RF choke made of about 10-12 turns of coax and about 10 inches in diameter. If you go to the trouble of winding the "choke" carefully, by building it on some kind of form and making the space between the windings equal to the diameter of the coax, it will give a little higher "Q" and work a little better. You should not expect to see much difference, but that does not mean the choke is not working. It is not highly critical, but if you could place the choke nearer the balun it is slightly better.

FYI: I assume that system requires a tuner. If the SWR on some bands is quite high, your losses in the coax will be substantial. Coax, especially in longer lengths can be pretty lossy in high SWR situations....but sometimes you have no choice depending on your situation.

Good luck dealing with your HOA Nazi's.... It is always fun to beat them at their own game. Remember!!...it is NOT an antenna!  It is static electricity lightning sensor for your weather station !!

73,  K0ZN
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KZ1X
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Posts: 3330




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« Reply #2 on: June 15, 2006, 08:09:58 PM »

I don't know what a 'Cobra' antenna is, but if it's just a doublet fed with window line, do like I did:  phantom power out the coax (see the 7/04 QST article by AD5X) (build or buy the 'power inserters') and then run an autocoupler (such as the SGC239) at the end of the balanced line.  Voila!  instant, all-band antenna, works great, super low losses.  Feed with coax, just as you do now.
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KA5N
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« Reply #3 on: June 16, 2006, 03:38:00 AM »

The Cobra is a linear loaded dipole designed to be fed with 450 ohm line.  If you use it as you have outlined then you are going to have huge losses on some bands.  As previous responder said you can't just
connect 450 ohm lead to 50 ohm coax.  On some antennas 450 ohm line (of a particular length) is used as a matching section for the coax.
You would probably have better results using the antenna in the attic with 450 ohm line only.  A stealth location outside would be even better.  And the coax to an external tuner should work fine.  The limiting factor is that mismatch and high SWR on coax causes extreme losses.  450 ohm line has very low losses even with high SWR.
GL Allen
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W5DXP
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« Reply #4 on: June 16, 2006, 05:04:07 AM »

If you don't know the impedance at the coax to ladder-line junction, it's a very bad idea. One solution would be to locate a balanced tuner between the coax and ladder-line. It would certainly be possible to locate a fixed matching network per band at the coax to ladder-line junction and just plug in the specific one for the band you are presently using.
--
73, Cecil, W5DXP
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N3OX
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« Reply #5 on: June 16, 2006, 07:44:46 AM »

Regarding Cecil's suggestion of fixed networks, it seems like lengths of coax as caps and hand wound self-supporting inductors of #12 wire could be a cheap and easy way to make fixed-tuned networks for this kind of application.  

For an antenna like this, where the tuning just isn't going to change until you install it somewhere else, this might be a really worthwhile budget setup.  

It would be good to have remote switching but it seems like maybe a stout wafer switch driven by a motor could provide a double-pole many-throw switch to do it.

I've considered doing this here but my antenna is made of magnet wire and breaks a lot, and I can't put up the same lengths again, so I'd be forever adjusting the fixed networks... a fully adjustable remote tuner works well for me.

Anyway, KB3FFH, you do need to think about this... matching the ladderline to the coax at the junction will avoid horrendous losses on some bands.

73,
Dan
N3OX
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #6 on: June 16, 2006, 07:52:37 AM »

Though I guess you'd have to consider loss in the coax "capacitors" as they're not very high Q.

What's a good cheap way to make high-Q homebrew caps?

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K8AG
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Posts: 352




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« Reply #7 on: June 16, 2006, 09:50:42 AM »

My suggestion is to go to a ZS6BKW for this configuration.  You can probably modify your antenna.  With your setup, the SWR on the coax is likely to be excessive at times causing a lot of loss on both receive and transmit.  Your tuner simply matches whatever is on the feed.  Your transceiver says 1:1, but the line is still whatever was there before the tuner.

The ZS6BKW is a specially modified version of the G5RV and it has a particularly low SWR on most of the HF bands 80-10 (including WARC) except 30 and 75 meters.  My internal runer on my rig seems to have no problem matching the result meaning that the SWR on the cable, though not 1:1, is not terribly high and I won't lose that much in the coax.

http://www.anugency.bizhosting.com/rssl/news0100.htm

This link is a Russian club newsletter that has a particularly good article on these antennas.

Mine uses several turns of coax on a 2-liter bottle as a choke to keep RF off of the shield.  Seems to work well.

My 2 cents.

73, JP, K8AG
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W6OP
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Posts: 350




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« Reply #8 on: June 16, 2006, 04:24:49 PM »

There are two models of the Cobra Ultralite. I have the Senior model (for 160 meters, not because I am old). I have the full 100 feet ladder line it came with. I had to zig zag it through the trees to use it all up. That goes to a 4:1 current balun and then through a remote coax switch and 50 feet of coax. Works fine on all bands using the internal tuner on my TS-480.
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