I'm a new ham and would like to start working HF with my mobile (my only) rig, an FT-857d. I have a handful of antennas, all of the variety that has a fiberglass base, extended by a stainless steel whip.

  • IHF Iron Horse 75, 40
  • Pro-Am mobile antenna 75
  • Antron Hamwhips 20

My question is, how do I determine the correct length to set the whip extension for a given frequency? Is it a simple calculation of the total physical length of the antenna as some fraction of a wavelength? Or is the fiberglass base electrically lengthened by the wire coil evident beneath the black covering? Note: I have little other equipment besides the FT-857d; no SWR meter besides the qualitative one built in to the transceiver. While I do have an LDG YT 100 auto tuner, I don't yet have the cables necessary to connect it and I still want to set the antenna's physical length as close to optimal as possible for the SSB portion of the band.

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    $\begingroup$ It's not too difficult to build a directional coupler, which is the heart of an SWR meter. Just takes a few ferrite cores and some wire. $\endgroup$ Sep 1, 2017 at 13:46

2 Answers 2


Congratulations on getting your ham license!

Mobile antenna installations are much less predictable than installations made for fixed locations. The mobile antenna installation is affected by the poor ground that the vehicle presents and the (typically) offset location of the antenna on the vehicle. As a result, most mobile antenna installations are compromises that require more trial and adjustment than a typical home antenna.

A good SWR meter or, even better, an antenna analyzer would be a good addition to your toolkit. It will receive a lot of use throughout your ham "career". If you cannot invest in one now, you probably could borrow one from a nearby ham or ham radio club. Sometimes an elmer (mentor) comes along free for the ride!

Start by adjusting your antennas per the manufacturer's instructions. This gets it into the ballpark. For most mobile antennas, you cannot directly calculate the length of the antenna as a function of wavelength because the manufacturer has added inductors or capacitors to the antenna that will throw off normal antenna calculations. Instead the manufacturer will include a chart or graph to help range in the adjustment.

From there you will need to use the SWR meter or antenna analyzer to adjust the antenna to the optimum setting. The advantage of using an antenna analyzer over an SWR meter is that it gives you much more information to help optimize your antenna system.

Once you get your antenna adjusted and you are ready to begin mobile operations, be prepared for the unexpected. Generally a mobile antenna is much less efficient than a home antenna. So at times you will be frustrated that you can hear stations that you cannot work. When I add the "mobile" suffix to my call, I have found that other stations are more likely to look past the more challenging conditions in order to make the QSO. On the other hand, I am often surprised to have a European station answer my mobile CQ - that's the magic of amateur radio. Enjoy!

  • $\begingroup$ I think I'll bite the bullet and get an analyzer. My whips were gifts from a local ham and I can't find instructions or a manual for them anywhere online, so I don't have an easy way to get in the ballpark. I suppose trial and error with the transceiver's built-in SWR meter will have to do until I get something better. $\endgroup$
    – Gregory
    Aug 31, 2017 at 17:04
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    $\begingroup$ @Gregory You will enjoy an antenna analyzer. They have really changed the hobby when it comes to antennas and understanding what is going on. For your manuals, I have had success in the past emailing people who have done a review or posting as an owner of the product and asking if they could email me a copy of the manual. In one case, I needed to send an SASE with a "green stamp" to get a copy. $\endgroup$
    – Glenn W9IQ
    Aug 31, 2017 at 17:45

Congratulations both for earning your ticket and for obtaining one of the most versatile radios available these days.

I recommend you go back to that local ham who gave you those gift antennas (if they are available) and ask them for advice. Anybody willing to gift equipment is very likely to want to offer advice also.

Plus cabling, etc. Maybe even loan you an SWR meter.

Don't discount the value of personal networking in amateur radio. You may find your best resource is you local ham club.


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