QST - September 2010 - (Page 48)
THE DOCTOR IS IN
Figure 1 — Interconnection diagram showing connections between radio, ampliﬁer and antenna.
Tom, K4YAZ, asks: I find that I need to use more power to consistently reach my local D-Star repeaters in the Tampa Bay Area. I live in a condo and my VHF/ UHF antenna is on a second floor porch. Moving it higher or into the clear is out of the question, due to my condo association. I am using a dual band transceiver with 50 W output on high power. This is not enough to consistently reach the local repeaters.
Since I am mainly using the 70 cm band, I have purchased an amplifier that has an output of 100 W with 30 W drive on 70 cm. The amplifier also has a useful receive preamp. To obtain the full 100 W output from the linear, one must input 30 W. My 50 W radio would be over the input rating of my linear, and the approximately 15 W output from the radio on medium power will not drive the linear to the full 100 W output. Is there an easy solution to this issue, or do I have to get a different amplifier? If you are right on the edge of the coverage area, the 3 dB increase in power may make the difference. Also consider any other losses, such as in the coax run to your antenna — at 70 cm, coax loss can be a big factor, and if the length is more than a few feet, better coax may reduce loss by almost as much as the amplifier gain — get every decibel you can. Your amplifier drive problem turns out to have an incredibly simple solution. If you have 2.2 dB loss between the radio and the amplifier, your 50 W radio output will be just 30 W at the amplifier. This only makes sense because of the preamp in the amplifier; otherwise, the loss would also reduce receiver sensitivity. With a preamp that has a reasonable gain, the noise figure and thus signal to noise ratio is determined largely by the noise figure of the preamp. On HF making a reasonably accurate attenuator using power resistors can be easy. [See the article by Phil Salas, AD5X, in the August 2010 issue. — Ed.] At UHF, component, lead and wiring inductance can make it a very difficult job — but all is not lost! Now for the really simple part. You need coax cable between the radio and amplifier anyway (see Figure 1). If you use coax with a loss of 2.2 dB, you’re done. On 440 MHz, it takes just 16 feet of Belden 8259 RG-58 coax
to get the loss — hopefully emphasizing my earlier point! If you select a different cable, make sure you have the cable attenuation specs available. The extra coax can be neatly coiled out of the way — or better yet, move the amplifier closer to the antenna to reduce the coax length and attenuation between the amplifier and antenna. Note that this all assumes that the amplifier input is a good match to 50 ; if not, the coax loss will be higher. If you have a wattmeter, it will be a good idea to confirm all the power levels when you’re done — and life being what it is — plan to do a little trimming.
Jerry, K2QJB, asks: I just bought a surplus military transmitter that works very well but uses cathode keying for CW. This GRC-109 has two tubes, a 6AC7 Pierce oscillator and a 2E26 as the final amplifier stage. I have an old electronic keyer that was designed for grid blocked keying. What is the difference between grid block and cathode keying and can I use my keyer with this transmitter? The radio does have a built in straight key but I prefer an electronic keyer.
Grid block and cathode keying were the most popular CW keying methods in the vacuum tube days, with the popularity edge going to cathode keying — in terms of number of radios. The two are quite different and only if your keyer has relay contacts at the output might it be able to handle either from the same circuit. Cathode keying involved opening the connection between one or more RF stage cathodes and ground. Use caution, because the typical keying line has around +300 V on it with the key up. [How we survived so many years with this condition may be one
of life’s mysteries. — Ed.]. The current, with your 2E26 output stage, could be in the 100 to 150 mA range. You can easily measure both with a VOM across the key contacts, set for voltage and then for current. Note that the transmitter will key while measuring current, so make sure it is tuned and properly loaded before you do this. Grid block keying applies a negative cutoff bias to one or more RF stages to keep them turned off while the key is up. The typical voltage is –100 V or so, with a much smaller keying current of a few tens of a mA or less. Some keyers of the period had two output connectors, one for positive and one for negative voltages, or sometimes a switch to select the keying polarity. Most project keyers in magazine or handbook articles had output options so you could make it for either. Be careful, though — most recent keyers are designed to key the positive voltages of modern solid state keyers. They may not be able to handle the voltage or current of an older cathode keyed tube radio even if both key positive voltages. If you have a keyer designed for positive voltages, check its specs against what you measure at the GRC-109 key contacts — both in terms of voltage and current. If your keyer is not compatible with the radio, the simple answer is to get a small 12 V dc relay with contacts that will handle the required current and voltage of the GRC-109 and wire it for the appropriate polarity that can be keyed by your keyer.
Bob, NF5Y, asks: Would you explain the benefit of a 5⁄8 wavelength vertical monopole antenna compared to one of a height of 3⁄4 wavelength? The 5⁄8 wave firstname.lastname@example.org
Joel R. Hallas, W1ZR
48 September 2010
QST Technical Editor
Table of Contents for the Digital Edition of QST - September 2010
QST - September 2010
It Seems to Us: Disaster Drills: The Last Word?
This Just In
Guide to ARRL Member Services
Up Front in QST
A Solar Powered Repeater for Emergency Communications
A Portable Antenna Mast and Support for Your RV
One Ham’s DC Power Connector Preference
Attic Antenna Experiments
The Doctor is IN
Getting on the Air
Hints & Kinks
The Flight of PBH-8
Ham Radio Manufacturing: An Inside Look
JOTA 2010 is October 16-17
2009-2010 School Club Roundup Results
ARES®: 75 and Counting
2010 Simulated Emergency Test
This Month in Contesting
2010 ARRL DX Phone Contest Results
2008 ARRL International DX Contest (Phone) Scores
The World Above 50 MHz
Amateur Radio World
At the Foundation
Convention and Hamfest Calendar
ARRL VEC Volunteer Examiner Honor Roll
Field Organization Reports
75, 50 and 25 Years Ago
Index of Advertisers
QST - September 2010