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Author Topic: quick question, need help  (Read 736 times)
KJ4YZI
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Posts: 38




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« on: November 04, 2012, 08:11:32 AM »

if i am in a hotel room 6th floor, and i make a 10 meter delta loop cut to size, and im using a balun, but i get swr like full scale, could it be because i am using only a 3 foot piece of rg8 coax from radio to balun? i didnt have any longer piece when i took this trip so i figured this would work. why can i not tune any size antenna whether its 20 meters or 10 i cannot get one to come anywhere near 3.1 swr with my internal antenna tuner on my ft450. im guessing its because of the construction of this building but i mean its suspended with some insulating wire from ceiling and walls, but really, i could always make any kind of wire and recieve a bunch of strong signals on 40 and 20 even tho i cannot talk to them.. can me balun really be bad? i mean i used it a couple months ago at home. could i narrow it down by cutting the pl259 off the end and attaching the delta loop to the sheild and center conductor? but then would it not be right for a delta loop because my delta loop attaches to both posts on the balun like a closed loop, without the balun would it be like a short? i have only today off so im just wondering if i can get this working today while im on this trip the next 2 weeks, thanks and this page will remain on my screen so i can read the responses. thanks a million.
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W1NK
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« Reply #1 on: November 04, 2012, 08:42:29 AM »

I think you are forgetting one important step in delta loop construction/use... Namely a quarter wave matching section of 75 ohm coax.   You should be able to leave the balun connected to the antenna, but, you'll have to add that matching section between the balun and your RG8.

Frank, W1NK

NB. I am assuming you are using a 1:1 balun.  If you are using a 4:1 balun, then the matching section isn't necessary.

« Last Edit: November 04, 2012, 09:16:02 AM by W1NK » Logged
WB6BYU
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Posts: 12974




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« Reply #2 on: November 04, 2012, 09:19:56 AM »

The first problem is that you are inside the hotel room.  That is going to throw the resonant
frequency off - you don't know how much duct work, foil-lined insulation, or wiring is in the
walls closely coupled to the antenna.  Even stringing a loop around a wood-shingled roof I've
found that I need to shorten the antenna by about 10% over the formula length, and in
your case there will be even more interaction with the building.

You have two choices:  use a wide-range tuner, or try adjusting the wire length to get it
closer to resonance.  A dip meter or SWR analyzer will help, or you can just try different
lengths of wire to see if you can find one that your tuner can handle.  (But don't expect
that same length of wire to work the same way in the next hotel room.)


It is, of course, possible that there is something wrong with your balun.  Or there might
be a problem with your coax or tuner.  You can try connecting the loop wires directly to
the coax, or right to the connector on the back of the radio without any coax (tie one
end under the ground post and stick the bare wires on the other end into the center
of the coax connector).   

You did test out the antenna beforehand, didn't you?

But the most likely problem is detuning due to operating inside the hotel room - this is
totally unpredictable, and it isn't unexpected that you would have problems trying to
use a tuner with a limited matching range.
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KJ4YZI
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Posts: 38




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« Reply #3 on: November 04, 2012, 11:16:44 AM »

well after i wrote this i tried cutting the end off my 3 foot jumpper, and tied directly to the shield and center, and after cutting some off, i am almost 1.5-1 at 28.400.... so something is wrong with that balun i guess, yes it is a 4:1 balun.  although i have only heard one contact, and after listening to him, ten meters is open pretty good right now, plus the logs online, but i still hear nothing throughout the band. but anyways im gonna leave it up and see if maybe its just not open in my area. cant hurt anything.
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AD6KA
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Posts: 2232




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« Reply #4 on: November 04, 2012, 09:40:10 PM »

Quote
, i am almost 1.5-1 at 28.400....
so something is wrong with that balun i guess,

Not necessarily.
Is 28.400 the "center frequency" you are shooting for?
At an SWR of 1.5:1 you are only losing %4 of your Forward Power
power, (less feedling loss) nothing I would get too hinky about.

73, Ken  AD6KA
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WB6BYU
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Posts: 12974




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« Reply #5 on: November 05, 2012, 10:53:51 AM »

Quote from: KI4MQJ
i am almost 1.5-1 at 28.400....
so something is wrong with that balun i guess.



Possibly, but not necessarily.  A 4 : 1 balun will change the impedance that the
tuner has to match.  It is possible that it shifts the impedance out of range,
especially if you have also changed the wire length.

You'll have to test the balun with a 200 ohm load and see what SWR you get.
You can't count on the impedance of the loop inside the building to tell you
anything about the balun.


Even though you can match the antenna, a typical steel and concrete building
can have very high signal attenuation.  It isn't surprising that you can't hear
signals very well, even if the band is open.



Quote from: AD6KA

At an SWR of 1.5:1 you are only losing %4 of your Forward Power
power...



While I agree that an SWR of 1.5 : 1 is nothing to worry about, the assumption
that it represents a loss of 4% of power is inaccurate and misleading.  You can't
equate reflected power with power lost in spite of the number of such tables
prepared for CBers over the years.
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