I am occasionally using an extremely simple shortwave portable setup consisting of a battery, the transceiver, a short coax feedline and a vertical half-wavelength vertical antenna with a transforming ladder-type feedline.

                        | λ/2
                        H ladder
------  -----           H
------  -----

X .. Connector
H .. two-wire ladder
o .. coax

My goals are both to optimize the system's efficiency and to minimize uncontrolled radiation from the feedline and transceiver itself.

I would normally try to ground both the transceiver and the connection point between my arbitrary-length coax feedline and the antenna's connector. However, this resulted in bad performance, and indeed, the antenna's description encourages keeping the feedline mechanically far from the ground and "free-floating".

Should a setup like this be grounded using just ground spikes, should I rather provide a defined ground using wire or would it be best to not care and rely on the fact that the antenna is resonant?

  • 2
    $\begingroup$ Welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Sep 26 '19 at 15:50
  • $\begingroup$ How long is the ladder line? Also, it's not clear how the ladder line is connected at each end, both to the coax and to the antenna. Could you please edit your question with these details? $\endgroup$ – Mike Waters Sep 26 '19 at 17:55
  • $\begingroup$ Sorry for the late reply. English is not my first language, and I thought "transforming feedline" would make it clear -- it's a quarter-wavelength transformer type feedline that is used to symmetrize the connection, making it suitable for a coax connection. $\endgroup$ – jstarek Apr 5 at 8:41

An antenna design that requires grounding to suppress the common-mode or improve efficiency is a poor antenna design, unless significant effort is made to make a low-loss ground. For example, a quarter-wave monopole above a ground plane of radials or saltwater works well. A quarter-wave monopole with just a ground rod is as much a dummy load as it is an antenna.

Unfortunately, many amateur antennas are poorly designed in this way. The details of your antenna are not entirely clear, but if the "antenna's description encourages keeping the feedline mechanically far from the ground and "free-floating" that almost certainly means this design requires a radiating feedline to function.

If grounding is your solution to common-mode currents, some portion (perhaps a very large portion) of the transmitter's power is going towards warming the soil rather than radiating.

Since installing a low-loss ground is not feasible for most mobile setups, you should look for an antenna design that does not have a common-mode issue. It could be as simple as a dipole with a balun.

Having addressed the common-mode issue though an effective antenna design, there's no reason to ground the station because there are no common-mode currents that would use it.

Alternately, you can just accept that the common-mode currents are going to happen. If transmit powers are low and the receiver and feedline aren't in noisy environment compared to the rest of the antenna (because the feedline is the antenna), nothing especially terrible will happen, and you can just write it off as a compromise that's been made to make the antenna simpler, more portable, or easier to erect.

| improve this answer | |
  • $\begingroup$ Tested and can confirm. Thanks! $\endgroup$ – jstarek Apr 5 at 8:41

In your diagram you show 1/2 wavelength vertical and using a combination of ladder line and coax to do impedance matching.

This means usual advice about 1/4 wave vertical does not apply here. Imagine this antenna as a half wave dipole that has been flipped from horizontal to vertical. Dipoles do not need an RF ground. You do not need a ground spike or an RF ground system with this antenna.

The real problem with this antenna is that it would be very hard to match the impedance using just a cable and then you get high SWR on your coax and bad performance. I have never seen a vertical half wave antenna that did not have an impedance transformer at the feed point. This is because the impedance of end fed half wave is very high.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.