Category Archives: equipment

More about the Alligator Hat

The June 2009 edition of AR carries a small article I wrote about lowering the resonant frequency of a HF helical antenna to allow me to use it on a lower frequency than its design centre frequency. My 80m helical, for example, is resonant at 3585 and provides the lowest SWR at that frequency.  At the CW end of the band, say 3520, the same antenna has a SWR of over 3:1, sufficient for the IC706 to cut back its output power to less than 10w.  To operate on the CW end of the band with this antenna I therefore need to deal with the mismatch using an ATU, or change the resonant frequency of the antenna.

The method I used was to load the helical with a capacity hat formed by a short wire, actually an alligator lead I had in my field day accessories box.  I was operating from the car and was nowhere near home at the time.

Here is the SWR curve from the front panel of the IC706, with the radio tuned to 3585 kHz.  The SWR bar-graph is small but the general shape of the SWR curve can be appreciated.  The bars represent the SWR at frequencies below and above the centre frequency indicated by the main frequency dial.  The small dot below the bar in the centre of the graph reminds you that is the measurement corresponding to the dial frequency.  The increment per measurement is 10 KHz, as set using key M2.  The white rectangle drawn around the SWR bar graph was added to the photo by me.

SWR centred on 3585
SWR centred on 3585

After adding the loading and retuning the radio to 3521, the SWR curve moved down the band and here it is centred on 3521:

SWR of loaded helical, centred on 3521
SWR of loaded helical, centred on 3521

A nice application for the SWR indicator and sweep function in the IC706.

Myths and Legends

Recently I was reminded of a conversation I overheard between two hams discussing which type of beam antenna is best, a full size type or a multiband antenna with traps in the elements to enable it to operate on multiple bands.

One of these fellows was like the experts at the pub, who can advise you on anything from what is wrong with your car to what is wrong with your computer, TV, VCR and your dog.

His opinion was that multiband trapped antennas were nothing more than rotary RF chokes. His logic was that RF chokes have coils in them, so do traps, therefore a trap is an RF choke. His conversation partner agreed with him with a bit of a nervous laugh. This was a seemingly plausible argument but a bit worrying because most people use this type of antenna and they do seem to work.

The fact is that this is a silly conclusion to draw. Every radio has coils inside it to provide essential tuned circuits. That doesn’t mean they are RF chokes, preventing the transmitter signals from emerging on the antenna connector. How could it produce 100 watts at that point if the coils were choking all the RF?

However this kind of statement, if uttered with the right level of assurance and confidence, will feed silly ideas into the heads of less well informed listeners, asking them to suspend disbelief and accept such nonsense as fact.

A little thought about the traps in multiband antennas will reveal them to be specially selected sizes with a specific calculated inductance, to do their job and allow each element to exhibit multiple resonances. They are not RF chokes. RF chokes are coils with enough inductance that they present a very high impedance at the nominated operating frequency.  Perhaps our self styled expert thinks that is how traps work.  It isn’t.

Equipment sales: name your price

I have noticed some sellers of used radio equipment make life more difficult than necessary for themselves, by being quite unclear about what they really want for the gear.  ie. what is their asking price?

This is surprising because anyone who has bought equipment knows very well what they paid, knows what they got and usually has a good idea of whether they got a good deal.  You’d think anyone selling equipment would do a bit of basic research to find out what they are likely to get for a Bloggs XYZ120S transceiver.  But many people don’t do that.  You’d think their precious Bloggs transceiver was gold plated.  They should rethink what it is they are selling.

Radio equipment has a remarkably high resale value compared with most consumer electronics and certainly compared with motor cars we get huge prices for our old radio gear.  Why is that?  Before trying to answer that, let’s consider a few examples of unrealistic pricing or expectations.

One case is a Collins KWM2A transceiver with matching power supply.  These units were arguably the best you could get in their class for about 30 years, from the early 60s to the mid 80s and possibly longer.  They could barely be matched in even one respect by the best any other manufacturer produced.  The reasons – good design, simple operation, no frills, mechanically reliable and electrically hard to fault.  However this gear is now pretty old.  It uses tubes.  The manufacturer does not build these radios any more, parts are now harder to find and are becoming expensive.  Only the true fanatic will continue to maintain and use this type of equipment.  Modern equipment by other manufacturers offers much more in the way of operator conveniences apart from power and space efficiency.  So with this background we see a number of these radios being sold on the second hand market, sometimes with remarkably high pricing.  One ad even stated that since he had noticed similar equipment, together with a power amplifier being sold on Ebay for $US4000.  However he did not nominate an asking price and left it to the reader to work out what he would be prepared to accept. what should it be worth?  What alternatives are there that produce 100 watts on the bands from 80m thru 10m, with say 500 Hz dial readout and one VFO.  Well, almost none on the new market.  Even the cheapest Icom HF radio IC718 offers more facilities for around $800.  So to hope for any more than that is plainly based on the hope that there are people out there that want the equipment just for the pleasure of owning that particular brand or model.  Only the older operators will be at all interested in that thought, yet their numbers are diminishing.  So what chance does our seller have of receiving the $2500 he may think this is worth.  My feeling is, not much at all.

Another example is a portable multimode portable transceiver for the 420-450 MHz band.  This is a Yaesu FT790R.  3 watts output and modes SSB, CW and FM.  This is a curious gadget type radio, with insufficient power to make many contacts at all, unless you are located on the top of a tall hill or connected to a power amplifier.  OR it may be used as an intermediate frequency (IF) radio for higher bands such as microwave bands from say 2 to 10 GHz.  On those bands, using a higher IF is preferable to using 144 or lower frequencies as the IF.   So what is this type of radio worth?  Well, the latest version of the portable low power radio from Yaesu is the FT817.  It operates on all bands from 1.8 MHz to 450 MHz and produces 5 watts of any mode on all those bands.  It has digital readout and tons of memories, an inbuilt keyer and so on.  It is available new for just under $900 and is available on the second hand market for around $600 to $750 depending on age (5 years or 5 days).  So what kind of price should the seller hope to receive for his 20+ year old radio without warranty and spare parts availability.  Would you think $250 excessive?  So would I, but the seller in this example wanted $600 for his old FT790R (including a 10w power amplifier). In my view this is at least double its maximum value.

Another example was someone who should have known better.  A rare piece of Icom VHF/UHF equipment was advertised with a list of all the extras that had been loaded into it.   As my eyes ran over the listing, I searched for the asking price.  I found no clues given except for the amazing phrase “don’t make silly offers”.  Well, what is not silly?  How can a buyer read the seller’s mind and know what he has in mind for an unusual item like that?

In other consumer goods no-one would expect anything for a 20 year old transistor radio, or even a 2 year old MP3 player.  They get thrown on the scrap heap once the next model arrives.  Even more expensive goods like televisions and stereo gear is almost thrown out as worthless at the ripe age of 10 years.

In the case of used amateur radio equipment, you can be lucky and find equipment that has been looked after an used carefully.  The case is not damaged, the electronics inside it are all in good order.  No-one has made non-standard modifications (“improvements”) that reduce its quality of reception or transmission.  You can also be unlucky and find a radio is being sold because someone has discovered it does not work well at all.  With 20 to 30 year old equipment, the work needed to fix some problems can be quite expensive.   However there is currently a high demand for used equipment, which is artificially propping up prices for old equipment that should really be worth $50, not $300.

But what if you do think your Bloggs radio is a fabulous piece of equipment someone will love to use, and get heaps of service from?  Well, the least you can do is to look around first and see what they are selling for.  Be realistic, especially for 20 to 30 year old equipment.

But please, name an asking price and don’t expect buyers to read your mind.