I know, a dipole is the way to go, but cannot fit one, only a 'birdhouse' antenna like the 75m 80m Isotron antenna.

  • Is it better to put at 15 feet Above Ground Level outside, or 20 feet AGL inside my attic? Is there the chance of a fire from the high voltage on the Isotron in the attic? Does the outside location improve performance?
  • How about a giant decoupling coil (1 foot diameter x 12 turns) at the Isotron antenna from after the antenna feed line coax?
  • Also, if in the attic, what should I use to ground my mounting post?
  • Ground cable to top of 2nd floor copper water line, or go all the way to the ground? Ham shack is on 2nd floor.
  • Likewise, ground if outside on garage roof?

I have to compromise, but would like to get into my state ARES net.

  • $\begingroup$ The manual is on isotronantennas.com. Have you read the relevant manual? $\endgroup$
    – Mike Waters
    Oct 21, 2019 at 23:38
  • $\begingroup$ Hi, I didn't see in the manual about height advantage, and confuses me about the grounding of a 6' mounting pole to hold the antenna (in an attic or on a garage) as opposed to a tower? $\endgroup$
    – KMG365
    Oct 23, 2019 at 14:17
  • 1
    $\begingroup$ This is a lot of questions. You'll probably get better responses if you ask them separately. $\endgroup$ Oct 23, 2019 at 17:34
  • $\begingroup$ Your answer about grounding is on page 14 of the manual. $\endgroup$
    – Mike Waters
    Oct 26, 2019 at 22:08

2 Answers 2


CW 4ever. On 80 Meters, not much difference between 15 or 20 feet AGL. The resulting near-vertical radiation would be good for communications around your region. Experience will show beyond what distance the antenna can't reach.

When you're done with net business, you can get a couple of thousand additional miles by going on 7 MHz and up.

My first antenna in 1974 was an indoor end-fed wire, about 50 feet long stapled to the ceiling of a room in almost a loop-shape on the 2nd floor (the top floor). That was also where the radios were and the operator's seating position, so I guess I broke the law regarding the FCC's RF-exposure limits. Mostly 80 meters, I got up and down the east coast from NYC with 75 watts out, DX-60B, HG-10B NC-303, HT32-B.

Be warned: Today, my code speed is about 18 to 20 wpm. If it weren't for all that RF exposure, I might be up to 40 wpm.

That was a brick house, wooden beams. Foil backing on fiberglass insulation inside the ceiling I tried not to think about. In newer building construction with steel, there's a lot of attenuation of signals trying to get through the walls of the building. Moving from just a few inches inside the window to a few inches outside the window could pick you up 10 dB.

I don't know about the decoupling coil. In my experience, if you and your radio are inside the main RF field, a decoupling coil is like locking the barn doors after the horses have all run off. Coming from NYC, I don't know what that means, but I think it might be apropos.

For grounding in the attic, if you're pestered with RF voltage shocks when you touch your straight key, or if your antenna tuner is too squirrley when you bring your hand close to it, you can throw 1/4 wavelength wire around the floor to move the voltage node away from your equipment. MFJ also sells a "ground tuner," https://www.mfjenterprises.com/Product.php?productid=MFJ-931. Looks like their ground tuner would allow you to use a shorter length of wire for the same benefit. That's RF ground.

For DC ground, on the 2nd floor, all hope is lost. Get one of those dongles to check the quality of your electrical ground -- the third wire. That's all you can do. Search Amazon for "ac socket tester."

You'll find advice on the internet insisting that the Ham station must be on the ground floor, with just a few feet of heavy copper ground-wire drilled through the wall running between your radio and a ground rod (outside). Easier if you don't read the instructions.

For grounding on the roof, the antenna should provide the RF ground, whether it's the other half of a dipole or the ground plane of a vertical. Aluminum ground wire (Amazon item # B0751YB21D) should be run from the antenna's mounting hardware to a ground rod directly vertically straight down from the mounting hardware. (If the antenna is hit by lightning, it wants to go straight down; it's not polite to ask lightning to follow curves or zig-zags). You can put a lightning arrestor just below the coax-feedpoint of the antenna (if there is a coax feedpoint), Amazon item # B01LYK5TU5. The ground rod can accept both ground wires. Even better is two or more ground rods, so-many-feet apart (that's your homework), tied together with heavy copper wire.

Also, read up on grounding and bonding. Bonding ties the grounds of all of your electrical services (AC, phone, cable TV) together. Theory is that if one service is hit by lightning and goes up to 1000 volts, but if their grounds are not especially tied together, then for instance when your antenna is hit, the electrical ground (and Hot and Neutral) will go up to 1000 volts, but with the telephone and cable remaining at 0 volts, your TV could blow up, and your cable box, and your cordless telephone base, etc. Instead, by making a conscious effort to tie all the services' grounds together, if any of them are hit by lightning, then at least all the services will jump up to 1000 volts, which will at least save a lot of your electronics.

My favorite power strip surge protector is Amazon item # B000JE9LD4. In case of high voltage surprise on any of the three AC leads, the surge protection will short all three leads together. They may all go up to 1000 volts, but at least the voltage between the wires will be about 0 volts, which helps prevent a lot of damage. Even better to disconnect your transceiver from EVERYTHING before the thunderstorm gets here. On the other hand, if your antenna is indoors, then you might be able to stay on the air.

I almost forgot -- grounding the antenna mounting hardware will help bleed off static from the antenna, which will help keep from attracting static charge.

Reeling in the antenna before a thunderstorm arrives can help. Leave the antenna up, but disconnect the coax from the antenna before the storm arrives is a 2nd option. Lots of grounding strategies on the internet to read. https://www.polyphaser.com Polyphaser is a lightning protection company. They always have lots of lightning protection advice.

I confess my father had a CB antenna. It had a coax lightning arrestor, but the mounting hardware wasn't grounded. The lightning arrestor and a lot of the coax cable must have vaporized when the antenna was hit. But apparently, the lighting wasn't finished. Because the mounting hardware wasn't grounded, the lightning came into the wall, much to my sister's dismay, and a carpenter had to close up the hole.


Location and Height

The manual seems to suggest that it would be better mounted outside on a mast.


The manual (page 14) implies that no separate ground wire is necessary.

Choke decoupling

An Isotron antenna generally should not have any type of choke balun at the feedpoint, even though the feedline carries some common-mode current (and radiates along with the antenna).

I heard that directly from its inventor ("It makes the antenna extremely narrow-banded"). But you could certainly try it. :-)

Note that an air-wound choke works over a much narrower bandwidth than ferrite chokes do. See http://www.karinya.net/g3txq/chokes:

Air-wound vs. Ferrite common mode chokes

  • $\begingroup$ Thanks, it looks like an air choke is a waste of time. The dark green is the frequency of pass-thru, or blocked? So if I only want a wire antenna to radiate at 21MHz, then 7 Turns of RG58 on a 4.25" air core will ALLOW operation on 21MHz, or the opposite? $\endgroup$
    – KMG365
    Oct 28, 2019 at 14:16

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