Heliax question

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ccfd7031

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Ok so im in the process of setting up my first homebrew repeater. Not having a lot of experience with feed line i have a few questions. I plan on using LDF4 from duplexer to antenna. Not exactly sure on the length yetbut probably somewhere around 50ft. Of course i am wanting to keep losses to a minimum.

1. Can i have 100ft. Of LDF4 and have the excess rolled up in the shack? This is in case i ever move the site or antenna higher i will not have to make/buy a new longer section or put two sections together.

2. Will Times LMR connectors work on andrews LDF heliax or are they not interchangeable?

3. I need 3 jumpers. 2 from radios (mini uhf male) to duplexer (type N male). I cannot find these anywhere for a good price with decent coax (superflex , rg-213 or similar). I want to eliminate adapters if possible. Any ideas? The 3rd jumper should be easy from antenna to LDF.

Thanks for any advice!
 

WA0CBW

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here is my 2-cents worth........
1. Do the math on the db loss from 50 feet to 100 feet and see if you can afford the additional loss.
2. No. they are not interchangable.
3. From the duplexer to the radios I would use double shielded silver plated cables. Use something like RG-400 cable. This can be made or purchased with mini-uhf on one end and N on the other.
4. I would use 3/8 or 1/2" hardline from the main LDF4-50 line to the antenna. (Don't forget lightning and grounding protection).

Make sure the duplexer has sufficient isolation between TX and RX. You will probably need something around 75 to 95 dB to keep the desense less than 1 dB. If you are using a mobile radio for the transmitter be sure and provide enough cooling vs. power output. Remember repeaters are duplex and the transmitter should be rated for continuous duty.

That should get you started.

BB
 

ccfd7031

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Ok thank you. So if i go with 100 ft i can just coil the rest up and let it lay inside? I wasn't sure if this would cause a problem with swr or anything due to the coils.
 

gewecke

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Ok thank you. So if i go with 100 ft i can just coil the rest up and let it lay inside? I wasn't sure if this would cause a problem with swr or anything due to the coils.
Just so you know, heliax is not flexible by any means! Whatever route you choose to route your feedline should not have any sharp bends at all!

73,
n9zas
 

mpddigital

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Coax cable for repeater

If you want to run extra Heliax and coil it just plan for big loops :) As has been said, it is not very flexible. It IS very well shielded though!

Not only will standard LMR connectors Not work, there are different tools required to attach connectors for hardline. Recommend you purchase your cable with connectors attached.

RG-400 will work for your set-up but if you want LMR cables with Mini-UHF to whatever we make them in LMR-195, 240 and 400. Give Kim a call if you are interested. Mini-uhf can be a bit hard to locate in larger size jumpers and you are right in trying to reduce your need for adapters.

Good Luck!
 

kayn1n32008

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You are better off just getting the length you need rather than coiling it up
 

LtDoc

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If this is for VHF then LMR-600 would be 'as good' and much cheaper than the heliax, not to mention easier to work with. It can be coiled, not as tightly as you might think, but it can be bent. Heliax isn't very forgiving in that respect, and you are talking about large coils (doubt if you could do that inside).
With a run of 50 feet or so, the amount of loss difference between heliax and LMR-600 just isn't going to be a biggy.
Have fun.
- 'Doc
 

ccfd7031

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Ok, thanks guys! By coiling up i was just thinking keeping it in the coil how it was shipped and feed what i need up to antenna and to duplexer and just leave the rest as it was shipped. Doesn't look too big maybe two it three foot across..am i right?

Also i have read over and over to NOT use LRM for duplex as it will eventually cause noise due to the two different metals contracting and running against each other. That's why i want to go with ldf. I have a box of andrews connectors and lmr connectors so if the lmr wont work with andrews i will probably go ahead and sell them off if they are worth much. I see them on ebay for very cheap. Maybe I'll keep them just in case one day..
 

kayn1n32008

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You will be creating an inductor leaving the cable coiled up, this will cause some issues when you start putting RF through it. Again I would not recommend leaving it in a coil. Buy the correct length and keep the excess to a minimum
 

prcguy

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Unless you kink or damage the cable causing an impedance change the RF inside will have no clue whats happening between the connectors. Any inductance will only affect what's riding on the outside of the cable (common mode) and that's benificial.
prcguy





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You will be creating an inductor leaving the cable coiled up, this will cause some issues when you start putting RF through it. Again I would not recommend leaving it in a coil. Buy the correct length and keep the excess to a minimum
 

ccfd7031

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Ok bear with me here, what do you mean only effect what's riding on the outsideand it being critical?
 

prcguy

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Common mode currents induced on the outside of coax due to high VSWR, feeding an unsymmetrical dipole or feeding a wire with no counterpoise, etc, that's what I mean.

Making a coil with the coax of the appropriate inductance can affect the common mode currents on the outside of coax and the RF inside is unaffected.

Common mode can also happen on balanced line where instead of alternating currents between conductors which cancel out radiation, the currents are equal or nearly equal on both conductors in the same direction and the balanced line can then radiate.
prcguy



Ok bear with me here, what do you mean only effect what's riding on the outsideand it being critical?
 

ramal121

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Common mode currents induced on the outside of coax due to high VSWR, feeding an unsymmetrical dipole or feeding a wire with no counterpoise, etc, that's what I mean.

Making a coil with the coax of the appropriate inductance can affect the common mode currents on the outside of coax and the RF inside is unaffected.

Common mode can also happen on balanced line where instead of alternating currents between conductors which cancel out radiation, the currents are equal or nearly equal on both conductors in the same direction and the balanced line can then radiate.
prcguy
Yes. And to carry that further, if you use the proper antenna and it is tuned for very low or no VSWR there is no common mode current (flowing on the outside of the coax back towards the radio). So, coiling the coax has no effect other than the loss of the extra length.

LDF-50 will probably be shipped with a coil diameter of a little less than 3 feet. If you keep it to around that you'll be good.
 
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