Considering an amp, and worried about RF in the shack, house, computer, etc...

Discussion in 'Amateur Radio Amplifiers' started by K3JGT, May 9, 2020.

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  1. K3JGT

    K3JGT Ham Member QRZ Page

    Good afternoon, everyone...

    I'm just getting back into Ham radio after a long absence. On HF, I'm running 100 watts into an AH4 tuner and long wire. From my rig to the tuner (which is outside) is about 70' of coax. I have no RF issues in the shack and a really, really good ground system, with a very high water table under the house. The Ah4 only handles 100 watts.

    I'm considering buying an amp (likely the 811h) and thinking about new antennas for the amp. With any kind of dipole the run to the antenna feed point will be about 150'. I really don't want any RF in the shack but am also concerned about feed line loss (even with high quality LMR400 or BuryFlex coax).

    My question... as i consider antenna options with low loss feedlines, 450 ohm ladder line with an in-shack tuner seems likely. I've never used ladder line before. All things being equal, will the ladder line (with maybe only 5' of ladder line in the shack before it exits the house through an 8' high concrete foundation) be more or less likely to introduce RF in the shack as compared to an all coax option?

    Thanks to all in advance.... Jim

    .
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That shouldn't be any kind of concern, and on HF, IMO LMR400 is a waste of money. RG-213/U is fine for this and even on 10 meters will have very little loss; on 160, 80, 60, 40, 30 and 20m the loss is almost exactly as low as LMR400 and should be <2 dB for 150 feet. My "shortest run" of coax here to any of my HF antennas is 175 feet...the ones up higher have 200' runs. RG-213/U all the way, loss is too small to think about.
    Unless you want to run a fixed-length doublet on several bands, IMO there's no need for ladder line. Ladder line can provide better matching to antennas having a higher native feedpoint impedance, but it's so much nicer to use coax and build (or buy) antennas that are nearly 50 Ohms on the bands you want to use. Advantages of coax are too numerous to list, but important ones are it won't radiate unless you have a lot of common mode current (which is avoidable); you can run it anywhere you want to including underground or laying on a roof, or really just anywhere; its performance isn't impacted by ice (ladder or window line absolutely is); and it's just so easy to work with.

    Balanced line has the edge when an antenna is severely mismatched, like native VSWR >10:1, but for me it's too easy to build antennas that don't have that problem.

    Using a balanced antenna like a center-fed doublet (parallel dipoles, trap dipoles or whatever to cover multiple bands) as far away from the house as possible prevents most RFI generation problems -- especially when a good current balun is used at the feedpoint, and possibly a ferrite line isolator just outside the house where the transmission line comes in.
     
  3. WG7X

    WG7X Platinum Subscriber Platinum Subscriber QRZ Page

    Jim,

    Anecdotal evidence here: I have two amps. One is a full legal amp the other is a 700 watt amp. Due to antenna problems last winter I had to run the ladder line directly from the antenna to the tuner. I have very little RF in the shack problems and no interference to any home electronics, including the XYL's TV in the living room.

    The secret is to make sure that you use a current balun, not a voltage one. Using a current balun, the current nodes on the feed line will balance out and the feed line will not radiate. With a voltage balun, the opposite is true and the feed line actually becomes part of the antenna, and it will radiate, causing all sorts of problems.

    Having said that, you need to change to something other than an end-fed antenna, these are the worst to use with an amplifier. A dipole at the distance that you indicated, with either window line feed or coax will probably not be a problem...
     
  4. K3JGT

    K3JGT Ham Member QRZ Page


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    WB2WIK.... Thank you very much for the perspective. I'm slowly coming back to the position of just going with coax and throwing up individual or fan dipoles. I'm especially pleased to hear about your success with the 200' runs of RG-213.
     
    Last edited: May 10, 2020
  5. K3JGT

    K3JGT Ham Member QRZ Page

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    WG7X. Thank you. I really appreciate the real world experience.

    And, Yes ! I definitely need to get off the end fed antenna before powering up. After getting some opinions, I'll probably go with coax fed single band dipoles or fan dipoles. That's what I grew up on!

    Jim
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You didn't discuss how much room you have to play with, but if you can live with 80/40/20/15/10 meters, a pre-made "fan" (parallel) dipole that is time-proven to work very well is the Alpha-Delta model DX-CC. It covers "all" of 40-20-15-10m, and a decent portion of 75m or 80m and just works. It can't cover the whole 80m band, as it's inductively loaded on that band (only) and is only 82' long; but you can adjust its length to cover about 100 kHz of either 75m or 80m, and for some, that's enough.

    Of course, a "longer" (130' long) homebrew parallel dipole could cover at least half the band on 80m, along with other bands having elements. 1/2" PVC tubing from Home Depot works well as "spreaders," and a very good parallel dipole can be homebrewed using all-new materials for less than $100 (plus the cost of coax), even including a good homebrew 1:1 current balun for the feedpoint.
     
    WA4SIX likes this.
  7. N7WR

    N7WR Premium Subscriber QRZ Page

    I run an amp (1KW). The amp sits next to the exciter and about 4 feet from the in shack computer. I have 3 different antennas each of which has a separate feedline, through lightening arrestors, to a remote switch in the shack. The coax run to each antenna is LMR 400 and each antenna is 100 feet from the shack (in 3 different directions). Each antenna is resonant on the designed bands so I do not have an antenna tuner. Absolutely no RFI into the house at all
     
  8. N4UP

    N4UP Premium Subscriber QRZ Page

    Echo the comments regarding coax and distance from shack to antenna. My TX antennas are between 100-400 feet from the radio room.

    I have no appreciable losses on any of my feedlines. Some of my antennas I feed with RG-213, but I have a preference for direct-burial DXE-400-MAX. Not that I could tell much difference.

    I routinely run legal limit 160-10 meters ( 1500 watts except 200 on 30 ) with no RFI issues from any of my antennas.

    Bottom line, if done properly you should have no RFI issues running an amp.
     
  9. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

    JIM,

    THE QRZ FOLKS HAVE PROVIDED YOU WITH A LOT OF TOP NOTCH SCOOP.....BRAVO.....

    WE ARE HAVING LOADS OF FUN HELPING YOU SPEND YOUR HARD EARNED MONEY.

    IF YOU ARE GOING TO USE COMPUTERS IN YOUR SHACK.......BE PREPARED FOR JUST ABOUT ANYTHING
    AS IT RELATES TO RF GETTING INTO THE EQUIPMENT / COMPUTERS.

    YOU MIGHT HAVE TO SPEND A LITTLE MORE FOCUS ON GROUNDING AND THE ACTUAL LAYOUT OF YOUR
    EQUIPMENT AT YOUR OPERATING POSITION. ..... THREE CHEERS FOR SNAP ON FERRITE CORE SPLIT BEADS.

    HAVE FUN AND ENJOY YOUR AL 811H

    73 - K1LKP
     
  10. K3JGT

    K3JGT Ham Member QRZ Page

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    WB2WIK.... I like the idea of the spreaders. In the past I just sort of ran my wire legs in slightly different directions or drooped them under one another. Also, concur on the home-brew approach. The only antenna I ever had that really disappointed was one I bought new off the shelf.

    Thanks again... Jim
     

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